* «."■• c>V^ 'T. ,'Y "••". v;;-wV:hl;

^4l§fe':
:&&&;■;'*&'*£■:.< •.:.■:•::■

 
NATIONAL LIBRARY OF MEDICINE
        Bethesda, Maryland

              Gift of
       Edward B. Schlesinger
 
   X
INST-

^

:<.-. "'?  .'.'

' r: 
 
 
 
 
 
COMPARATIVE  ZOOLOGY,
STRUCTURAL AND SYSTEMATIC.
FOR USE IN SCHOOLS AND COLLEGES.
        By JAMES ORTON, Ph.D.,

PEOFEBBOR OK NATURAL HISTORY IN VA8BAR COLLEGE; CORRESPONDING MEMBER OF
    TUB ACADEMY OF NATURAL BOIENOE8, PHILADELPHIA, AND OF THE
      LYCEUM OF NATURAL HISTORY, NICW YORK; AUTHOR
         OF "THE ANDES AND THE AMAZON," ETC.
 " The education of a naturalist now consists chiefly in learning how to
compare."—Agassiz.
        NEW YORK:

HARPER & BROTHERS, PUBLISHERS,
       FRANKLIN  SQUARE.
            18 76. 
 Entered according to Act of Congress, in the year 1876, by




            HARPER  &  BROTHERS,



In the Office of the Librarian of Congress, at Washington.
 
PREFACE.
  The distinctive  character of this  work consists in the
treatment of the whole Animal Kingdom as a unit; in
the comparative study of the development and variations
of organs and  their functions, from the  simplest to the
most complex state; in withholding Systematic Zoology
until the student has mastered those structural affinities
upon which true classification is  founded; and in  being
fitted for High Schools and Mixed Schools by its language
and illustrations, yet going far enough to constitute a com-
plete grammar of the science for the under-graduate course
of any College.
  It is designed solely as a manual for  instruction.   It is
not a work of reference, nor a treatise.  So far as a book
is encyclopediac, it is unfit for a  text-book.  This is pre-
pared on the principle of "just enough, and no more."  It
aims to present clearly, and in a somewhat new form, the
established facts and principles of Zoology.  All  theoretical
and debatable points, and every fact or statement, however
valuable, which is  not absolutely necessary to a clear and
adequate conception of the leading principles, are omitted.
It is written in the light of the most recent phase of the
science, but not in the interest of any  particular theory.
To have  given  an  exhaustive survey  of  animal  life would
not only have  been undesirable, but impossible.   Even
Cuvier's  great work  must be supplemented by museums, 
6
PREFACE.
monographs, and microscopes.  Natural History has out-
grown the limits of a single book.  Trial has proved the
folly of giving the student so many things to learn that he
has no time to understand, and the error of  condemning
the student to  expend  his strength upon  the details of
classification, which  may  change in the coming decade,
instead of upon structure, which is permanent. Of course,
specialists will miss many things, and find  abundant room
for criticism in what they regard as deficiencies; but the
work should be judged by what it does contain, rather than
by what it does not.
  What is  claimed, in  the language of inventors, is the
selection and arrangement of essential principles and typ-
ical illustrations from the stand-point of the teacher.  The
synthetic method is employed, as being the most natural:
to begin with complex Man, instead of the simplest forms,
would give a false idea.   Man is not a model, but a mon-
strosity, the most modified of Vertebrates.   But these out-
lines must be filled up, on  the part of the teacher, by lect-
ures, and by the exhibition of specimens, and, on the part
of the student, by  observation (noting, above all, the char-
acteristic habits  of animals),  and by personal work  with
the knife and microscope.   No text-book can  take the
place  of nature, or supersede  oral instruction  from a  com-
petent teacher.
   Suggestions and corrections from naturalists and teach-
ers will be  thankfully received.
   In a work of this character, which is but a compend of
the labors  of  all  naturalists, it  would be  superfluous to
make acknowledgments.  The works  referred to on  page
385 have been specially consulted. 
CONTENTS.
                     INTRODUCTION.                   pAGE
Definition of Zoology, and its Place among the Sciences.................... 11
Historical Sketch....................................................................... 14
           PART  I.—STRUCTURAL ZOOLOGY.

                       CHAPTER  I.
Minerals and Organized Bodies distinguished........................ 19

                       CHAPTER II.
Plants and Animals distinguished........................................... 21

                      CHAPTER III.
Relation between Minerals, Plants, and Animals................... 27

                      CHAPTER IV.
Nature of Life...................................................................... 28

                       CHAPTER V.
Organization.............................................................*............ 30
    1. Cells............................................................................. 31
    2. Tissues.......................................................................... 32
    3. Organs, and their Functions.............................................. 41

                      CHAPTER VI.
Nutrition............................................................................... 44

                      CHAPTER VII.
The Food of Animals............................................................. 46 
8                       CONTENTS.


                      CHAPTER VIII.                    page
How Animals Eat..................................................................  49
    1. The Prehension of Food..................................................  49
    2. The Mouths of Animals...................................................  54
    3. The Teeth of Animals.....................................................  62
    4. Deglutition, or how Animals swallow.................................  71

                       CHAPTER IX.
The Alimentary  Canal..........................................................  73

                       CHAPTER X.
How Animals Digest.............................................................  90

                       CHAPTER XI.
The Absorbent System..........................................................  93

                      CHAPTER XII.
The Blood of Animals..........................................................  96

                      CHAPTER XIII.
The Circulation  of the Blood.............................................. 102

                      CHAPTER XIV.
How Animals Breathe.......................................................... 110

                      CHAPTER XV.
Secretion and Excretion....................................................... 120

                      CHAPTER XVI.
The Skin and Skeleton......................................................... 125

                     CHAPTER XVII.
How Animals Move............................................................... 152
    1. Muscle......................................................................... 152
    2. Locomotion................................................................... 155

                     CHAPTER XVIII.
The Nervous System............................................................. 264
    1. The Senses................................................................... 173
    2. Instinct and Intelligence.................................................. 181
    3. The Voices  of Animals.................................................... 185 
                         CONTENTS.                        9


                       CHAPTER XIX.                    PAGE
Reproduction........................................................................  188

                       CHAPTER XX.
Development.........................................................................  193
    1. Metamorphosis........................................................'.......  203
    2. Alternate Generation.......................................................  206
    3. Growth and Repair.........................................................  207
    4. Likeness and Variation....................................................  209
    5. Homology, Analogy, and Correlation..................................  211
    6. Relations of Number, Size, Form, and Rank........................  214
    7. The Struggle for Life......................................................  219
           PAET II.—SYSTEMATIC ZOOLOGY.

                       CHAPTER XXI.
The Classification of Animals.............................................. 223
    Protozoans........................................................................ 231
    Coelenterates...................................................................... 237
    Echinoderms..................................................................... 247
    Mollusks........................................................................... 254
    Articulates........................................................................ 269
    Vertebrates....................................................................... 295

                      CHAPTER XXII.
Systematic Arrangement of Representative Forms................ 349

                      CHAPTER XXIII.
The Distribution of  Animals................................................ 357
NOTES................................................................................. 367

THE NATURALIST'S LIBRARY........................................... 385

INDEX................................................................................. 387 
 
INTRODUCTION.
   1. Definition of Zoology, and its Place  among the
  Sciences.—The province of Natural History is to describe,
  compare, and  classify natural  objects.  These  objects have
  been divided into the "organic" and the " inorganic," or those
  which are, and those which are not, the products of life.  Bi-
  ology is the science of the former, and Mineralogy the science
  of the latter.  Biology again separates  into Botany, or the
  Natural History of Plants, and Zoology, or the  Natural His-
  tory of Animals; while Mineralogy divides into Mineralogy
  proper, the science  of  mineral  species, and Lithology, the
  science of mineral aggregates or rocks.   Geology is that com-
 /prehensive knowledge of  the earth's structure  and develop-
(, ment which rests on the whole doctrine of Natural History.
   If we examine a piece of chalk, and determine its physical
  and chemical characters, its mode of occurrence and its uses,
  so as to distinguish it from all other forms of matter, we have
  its  Mineralogy.   But chalk occurs in vast  natural beds: the
  examination of these masses—their origin, structure, position,
  and relation to other  rocks—is the work of the Lithologist.
  Further, we observe that  while  chalk and marble are chem-
  ically alike, they widely differ in another respect. Grinding a
  piece of chalk so thin that we can see through it, and putting
  it under a microscope,  we find imbedded in it innumerable
  bodies, about the hundredth of an inch in diameter, having a
  well-defined, symmetrical shape, and chambered like a Nautilus.
  We can not say these are accidental aggregations, nor are they
  crystals: if the oyster-shell is formed  by  an Oyster, these also
  must be the products of life.  Indeed, the dredge brings up
  similar microscopic skeletons from the bottom of the Atlantic.
  So  we conclude that chalk is but the dried mud  of an ancient 
12
INTRODUCTION.
sea, the cemetery of countless animalcules that lived and died
long ago.   The consideration of their fossil remains belongs
to Paleontology, or that part of Biology which describes the
relics of extinct forms of  life.   To study the stratigraphical
position of the chalk-bed,  and by the aid of its Paleontology
to determine its  age  and part in the world's history, is the
business of Geology.
  Of all the sciences, Zoology is the most extensive.  Its field
is a world of varied forms—hundreds of thousands in number.
To determine their origin  and development, their structure,
habits, distribution, and mutual relations, is the work of the
Zoologist.   But  so many and far-reaching are the aspects
under which the  animal creation may be contemplated, that
the general science is  beyond the grasp of any single person.
Special  departments have, therefore, arisen; and Zoology, in
its comprehensive sense, is the combined result of the labors
of many workers, each in his own line of  research.
  Structural Zoology  treats of the organization of animals.
There are two main branches: Anatomy, which considers the
constitution and construction of the animal frame; and Physi-
ology, which is the study of the  apparatus in action.  The
former is  separated into Embryology, or an account of the
successive modifications through which an  animal passes in
its development from  the  egg to the  adult state; and Mor-
phology, which includes all inquiries concerning the form of
mature animals, or the form and arrangement of their organs.
The microscopical examination of any part, especially the tis-
sues, belongs to Histology.  Comparative Zoology is the com-
parison of the anatomy and physiology of all animals, existing
and extinct, to discover the fundamental likeness underneath
the superficial differences, and to trace the adaptation of or-
gans to the habits and spheres of life.   It is this comparative
science  which has led to  such grand generalizations as the
unity of structure amidst  the diversity  of  form in  the ani-
mal creation, and by revealing the degrees of affinity between
species has enabled us to classify them in natural groups, and
thus laid the foundation of Systematic Zoology.   When the
study of structure is  limited to a particular class or species
of animals,  or to a particular organ or part, monographic
sciences are  created,  as Ornithotomy, or anatomy of birds; 
INTRODUCTION.                   13
Osteology, or the science of bones;  and Odontography, or
the natural history of teeth.
  Systematic Zoology is the classification of animals, or the
study of animals, as to their kinds, giving to each a distinctive
name and description.  The systematic knowledge of the sev-
eral classes, as Insects, Reptiles, and Birds, has given rise to
subordinate sciences, like Entomology, Herpetology, and Or-
nithology.**
  Distributive Zoology is the  knowledge of  the successive
appearance of animals in the order of time  (Paleontology in
part), and of  the geographical  and physical distribution of
animals, living or extinct, over the surface of the earth.
  Theoretical Zoology includes those provisional modes of
grouping facts, and interpreting them, which still stand wait-
ing at the gate of science. They may be true, but we can not
say that they are true.  The evidence is incomplete.  Such are
the theories which attempt to explain the origin of life and
the origin of  species.
  Suppose we wish to understand all about  the Horse.   Our
first object is to study its structure.  The whole body is in-
closed within  a  hide, a skin  covered with hair;  and if this
hide be taken off, we find a great mass of flesh or muscle, the,
substance which, by its power of contraction, enables the ani-
mal to move.  On removing this, we have a series of  bones,
bound together with ligaments, and forming the skeleton.
Pursuing  our researches, we find within this frame-work two
main cavities: one, beginning in the skull and running through
the spine, containing the brain and spinal marrow; the other,
commencing  with the mouth,  contains the gullet, stomach,
intestines, and the rest of the apparatus for digestion, and
also the  heart and lungs.   Examinations of this character
would give us the Anatomy of the Horse, or, more precisely,
Hippotomy.   The study of the bones alone would  be its
Osteology; the knowledge of the nerves would belong to
Neurotomy.   If we examined, under the microscope, the
minute structure of the hair, skin, flesh,  blood, and bone, we
would learn its Histology.  The consideration of the manifold
changes undergone in developing from  the egg  to  the full-
* Numbers like this refer to the Notes at the end of the volume. 
14
INTRODUCTION.
grown animal, would be the Embryology of the Horse; and
its Morphology, the special  study of the form of the adult
animal and of its internal organs.
  Thus far we  have been  looking, as it were,  at  a steam-
engine, with  the fires out, and nothing  in the boiler; but
the body of the living Horse is a beautifully formed, active
machine, and every part has its different work to do in the
working of that machine, which is what we call its life.  The
science of such  operations as the grinding of the food in the
complex mill of the mouth; its digestion  in the laboratory of
the stomach; the pumping of the blood through a vast system
of pipes over the whole body; its purification in the lungs;
the process of growth, waste, and repair; and that wondrous
telegraph, the brain, receiving impressions,  and sending mes-
sages to the muscles, by which the animal is endowed with
voluntary locomotion—this is Physiology.   But Horses are
not the only living  creatures in the world; and  if we compare
the structures of various animals, as  the  Horse, Zebra, Dog,
Monkey, Eagle, and Codfish, we  shall find more or less re-
semblances and differences, enough to enable  us to classify
them, and give  to each a description which will distinguish it
from  all others.  This is the work  of Systematic Zoology.
Moreover, the Horses now living  are not the only kinds that
have ever lived; for the examination of the earth's crust—the
great burial-ground of past ages—reveals  the bones of numer-
ous horse-like  animals: the study of this  pre-adamite race
belongs to Paleontology.  The chronological and geographical
distribution of  species  is the department of Distributive Zo-
ology.  Speculations about the origin of  the modern Horse,
whether by special creation, or by development from some
allied form now extinct, are kept aloof from  demonstrative
science, under the head of Theoretical Zoology.
  2. History.—The Greek philosopher  Aristotle (b.c. 384-
322) is called the "Father of Zoology."  Certainly, he is the
only great representative in ancient times, though his frequent
allusions to familiar works on anatomy show that something
had been done before  him.   His "History of Animals," in
nine books, displays a wonderful  knowledge of external and
internal structure,  habits, instincts, and  uses.   His descrip-
tions  are  incomplete, but generally exact, so far as they o-o. 
INTRODUCTION.
15
Alexander, it is said, gave him nine hundred talents to collect
materials, and put at his disposal several thousand men, for
hunting specimens and procuring information.
  The Romans accomplished little in natural science, though
their  military expeditions furnished  unrivaled opportunities.
Nearly three centuries and  a half after Aristotle, Pliny (a.d.
23-79) wrote his " Natural History."  He was a voluminous
compiler, not an observer: he added hardly one new fact.  He
states that his work was extracted from over two thousand
volumes, most of which are now lost.
  During the Middle Ages, Natural History was studied in
the books  of the ancients; and at the close of  the fifteenth
century it was found where Pliny had left it, with the addi-
tion of  many vague hypotheses and silly fancies.  Albertus
Magnus, of the thirteenth  century, and  Conrad Gesner and
Aldrovandus,  of  the sixteenth, were  voluminous writers, not
naturalists.  In the latter half of the sixteenth century, men
began to  observe nature for themselves.   The earliest note-
worthy  researches were made on Fishes,  by Rondelet  (1507-
1566) and Belon  (1517-1564), of France,  and Salviani  (1514-
1572), of Italy.  They were followed by valuable  observations
upon  Insects,  by Redi (1626-1698), of Italy, and Swammer-
dam (1637-1680), of Holland; and toward  the end of the
same  century, the Dutch  naturalist, Leeuwenhoeck  (1632-
1723), opened a new world of  life by the use of the micro-
scope.
  But there was no real advance of Systematic Zoology till
the advent of the illustrious John Ray (1628-1705), of  En-
gland.  His " Synopsis," published in 1693, contained the first
attempt to classify animals  according to structure.  Ray was
the forerunner of "the  immortal Swede," Linnaeus  (1707-
1778), " the great framer of precise and definite  ideas of nat-
ural objects, and terse teacher of the briefest and clearest ex-
pressions of their differences."   His chief merit  was in defin-
ing generic groups, and inventing specific names.2  Scarcely
less important, however, was the impulse which he gave to
the pursuit of Natural History.   The spirit of inquiry, which
his genius infused among the great,  produced voyages of re-
search, which led to the formation of  national museums.  The
first expedition was sent forth by George III. of England, in 
16
INTRODUCTION.
1765.   Reaumur  (1683-1757) made the earliest zoological
collection in France;  and the West Indian collections of Sir
Hans Sloane (1660-1752) were  the nucleus  of  the  British
Museum.   The accumulation of  specimens suggested com-
parisons, which eventually resulted in the highest advance of
the science.
  The brilliant  style  of Buffon (1707-1788) made Zoology
popular not only in France, but throughout Europe.  While
the genius of Linnaeus led to  classification, that of Buffon lay
in description.   He was the first to call  attention  to the sub-
ject of  Distribution.   Lamarck (1745-1829), of Paris, was
the next great light.  The publication of his "Animaux sans
Vertebres,"  in 1801, was an epoch in  the history of the lower
animals.   He was also the first prominent advocate of the
transmutation of species.
  But the brightest luminary in Zoology was George Cuvier
(1769-1832), a German, born  on French  soil.   Before his
time, " there was no great principle  of  classification.   Facts
were accumulated, and  more or less  systematized, but they
were not yet arranged according to law; the principle was
still wanting by which to generalize  them and give meaning
and vitality to the whole."  It was Cuvier who found the key.
He  was the first to so interpret structure as to be able from the
inspection of one bone to reconstruct the entire animal, and
assign its position.   His anatomical investigations revealed
the natural affinities of animals, and led  to the  grand general-
ization, that  the  most  comprehensive groups in the kingdom
were based,  not  on special characters, but on different plans
of structure.  Palissy had long ago (1580) asserted that pet-
rified shells  were of  animal  origin;   but the  publication of
Cuvier's  "Memoir on Fossil Elephants," in  1800, was the
beginning of those profound researches on the  remains of
ancient  life  which  created Paleontology.  The discovery of
the true relation between all animals, living and extinct, open-
ed a boundless field of inquiry; and from that day the advance
of Zoology has been unparalleled.  Special studies of particu-
lar  parts or classes  of  animals have so rapidly developed, that
the history of Zoology during the last fifty years is the history
of many sciences.3 
      PART I.



STRUCTURAL  ZOOLOGY. 
  The first thing to be determined about a new specimen is not its name,
but its most prominent character.  Until you  know an animal, care not for
its name.—Agassiz.
  The great benefit which a scientific education bestows, whether as  train-
ing or as knowledge, is dependent upon the extent to which the mind of the
student is brought into immediate contact with facts—upon the degree to
which he learns the habit of appealing directly to Nature.—Huxley. 
COMPARATIVE   ZOOLOGY.
                   CHAPTER I.
    MINERALS  AND ORGANIZED BODIES DISTINGUISHED.
  Nature may be separated into two great kingdoms—
that of mere dead matter, and that  of matter under the
influence of life.4  These differ in the following points:
  (l) Composition.—Fewer elements are used in the organic
world than in  the mineral.  Over sixty are found in the
latter, while four or five make  up the former.  Organic
bodies  have a striking  uniformity  of composition: the
majority being ternary compounds, consisting of carbon,
hydrogen, and oxygen; or quaternary, adding nitrogen;
while a few  only contain also sulphur or iron.  But min-
erals exhibit a greater variety in their constitution, being
made of one element alone, or two or more.  In organic
substances each ingredient is equally united with all the
rest, while mineral compounds can  be resolved into bi-
nary combinations.  Thus, fibrine, which  is composed of
carbon, hydrogen, oxygen, and nitrogen, is called a quater-
nary compound; and carbonate of ammonia, made of the
same materials, is considered a binary union of two other
binary  compounds, carbonic acid and ammonia.  But this
distinction is shaken by the probability that binary princi-
ples exist in living bodies.  It  is true, however, that the
constitution of  organic substances is  more complex than
that of minerals; for, though composed of few elements, 
20
COMPARATIVE ZOOLOGY.
a large number of atoms of those elements enter into com-
bination.  Thus, one molecule of fibrine contains 216 atoms
of carbon, 169 of hydrogen, 68 of oxygen, 27 of nitrogen,
and 2 of sulphur.  Organic molecules are therefore larger
than the molecules of minerals.  Furthermore, combina-
tions  formed under the influence of life are  invariably
characterized by  the presence of carbon and water.  An
animal  always  contains a mixture of  solids, liquids, and
gases.
  (2) Structure. — A mineral is homogeneous, while an or-
ganized body is heterogeneous; i. e., it is composed of dif-
ferent parts, called tissues and organs, having peculiar uses
and  definite relations  to one another.   The  tissues and
organs,  again, are heterogeneous, consisting mainly of mi-
croscopic cells,  a  structure developed only by vital action.
All the  parts of an organism are mutually dependent,
and  reciprocally  means and ends, while each  part of a
mineral exists for itself. The smallest fragment of marble
is as much marble as a mountain-mass; but the fragment
of a plant or animal is  not an individual.  The particles of
a mineral are held together by cohesion; the exact nature
of the force, commonly called " life," which presides over
an organized body remains to be discovered.
  (3) Size and: Shape. — Living bodies  gradually  acquire
determinate dimensions; so do minerals in their perfect
or crystal condition.  But uncrystallized, inorganic bodies
have an  indefinite bulk.  Most minerals are amorphous;
crystals have regular forms, bounded,  as a rule, by  plane
surfaces and straight lines; plants and animals are cir-
cumscribed by curved surfaces, but never assume accurate
geometrical forms.6
  (4) Phenomena.—Minerals  remain internally at rest, and
increase by external additions.  In contrast, organisms are
all in motion: they are constantly in a state of composi-
tion and decomposition, incorporating foreign particles and 
         PLANTS AND ANIMALS DISTINGUISHED.      21

giving out their own.   Organized bodies, moreover, pass
through a cycle of changes—growth, development, and re-
production.  Of more complex constitution, they are more
unstable, and are more liable to decomposition than most
inorganic compounds.   The  action of heat is invariably
destructive.  At the end of a certain period, fixed for each
species, living bodies infallibly perish.
                   CHAPTER  II.
          PLANTS AND ANIMALS DISTINGUISHED.
  It may seem  an easy matter to draw a line between
plants and animals.   Who can not  tell a Cow  from a
Cabbage ?  Who would confound a Coral with a Mush-
room?   Yet it is impossible to assign any absolute, dis-
tinctive  character which will divide the one mode of life
from the other.  The difficulty of defining an animal in-
creases with our knowledge of its nature.   Linnaeus  de-
fined it in three words; a century later, Owen declared
that a  definition of plants which would exclude all ani-
mals, or of animals which would not let in a single plant,
was impossible.  Each different character used in drawing
the boundary will bisect the debatable ground in a differ-
ent latitude of the organic world.   Between the higher
animals and higher plants the difference is apparent; but
when we  reflect how many  characters the  two  have  in
common,  and  especially when we descend to  the lower
and minuter forms, we discover that  the two "kingdoms"
touch, and even dissolve into, each other.   This border-
land has  been as hotly contested  among naturalists  as
many a disputed frontier  between adjacent  nations.  Its
inhabitants have been taken and retaken several times by 
22
COMPARATIVE ZOOLOGY.
botanists and zoologists;  for they have  characters that
lead on the one side to  plants, and on the other to ani-
mals.  To  solve the difficulty, some  eminent naturalists,
as Hackel and Owen, propose a fourth " kingdom," to  re-
ceive those living  beings which are organic, but not dis-
tinctively vegetal or animal.   But a greater difficulty arises
in attempting to fix its precise limits.
   The drift of modern research points to  this:  that there
are but two kingdoms of  nature, the mineral and the  or-
ganized, and  these closely linked together; that the lat-
ter must be taken as one whole, from which  two great
branches rise and  diverge.  " There is at  bottom but one
life, which is  the  whole  life of some creatures, and the
common basis of the life of all; a life of simplest moving
and feeling, of feeding and breathing, of producing  its
kind and  lasting its day; a life which, so far as we at
present know1, has no need of such parts as we call organs.
Upon  this  general foundation are built up the manifold
special characters of animal  and vegetable existence; but
the tendency, the endeavor, so to speak, of the plant is one,
of the animal is another, and  the unlikeness between them
widens the higher  the building is carried up.  As we pass
along the series of either  [branch] from low to high, the
plant becomes more vegetative, the animal more animal."8
   In general, we may say that a rooted organism, retaining
carbon and exhaling  oxygen, feeding on mineral matter
by absorption, and  having  cellulose tissues, is a plant; that
an irritable or locomotive organism, retaining oxygen and
exhaling carbonic  acid, feeding on organic matter by a
mouth and stomach, and having albuminous tissues, is  an
animal. But Nature knows no such line of demarkation;
for it is bridged at numerous points.
   (1) Origin.—Both branches of the tree of life start alike:
the lowest of  plants and animals, as Protococcus and Che-
garina, consist of a single cell.  In fact, the cycle of life in 
         PLANTS 'AND ANIMALS DISTINGUISHED.       23

 all living beings, high or low, begins in a small round dot
 of matter—in plants called an ovule, in animals, an ovum.
 This cell, or dot, contains a fluid, called protoplasm, iden-
 tical in composition and in function.  In the very simplest
 forms  the protoplasm is not inclosed by a membrane; but
 generally there is a^celhw^all.  In plants, with few excep-
 tions, this wall is of cellulose, or something akin to starch;
 in animals, with  few exceptions, the wall is a pellicle of
 firmer protoplasm, i. <?., albuminous.
   (2) Composition.—Modern research has broken down the
 partition between plants and animals, so far as chemical
 nature is concerned.  The vegetable fabric and secretions
 may be ternary or binary compounds; but the  essential
 living  parts of plants, like animals, are quaternary, con-
 sisting of four elements—carbon, hydrogen, oxygen, and
 nitrogen.  Cellulose (woody fibre), starch, and chlorophyl
 (green coloring matter) are eminently vegetable products,
 but not distinctive; for cellulose is wanting in some plants,
 as Fungi, and present  in  some animals, as Tunicates;
 starch, under the name of glycogen, is found in the liver
 and brains of Mammals, and chlorophyl gives color to the
 fresh-water Polyp.   Still, it  holds  good,  generally, that
 plants  consist mainly of cellulose,  dextrine, and starch;
 while animals are mainly  made up of albumen, fibrine,
 and gelatine; that nitrogen is more abundant  in animal
 tissues, while in plants carbon is predominant.
   (3) Form.—No outline can be  drawn  which  shall be
 common to all animals or all plants.  The lowest members
 of both have no fixed shape.   The seeds of Conferva? can
 hardly be distinguished from animalcules; the compound
 animals, Sea-mat  and Sea-moss (Polyzoa), are often taken
 for sea-weeds; the trees  mimic the branching Coral, and
the Coral buds and blossoms like the  Rose.   The ideal
form of a plant — trunk, branches,  twigs, special organs
— is a form  natural to all living matter enjoying rapid 
24             COMPARATIVE ZOOLOGY.
growth.  Ascending the scale of animal creation, we find
the primitive form gradually obscured by new and power-
ful molding influences.
  (4) Structure.—A plant is  the multiplication of the unit
—a cell with a cellulose wall.  Some simple animals have
a similar cellular structure,  and all animal tissues, while
forming, are cellular.  But this character, which is perma-
nent in plants, is generally  transitory in animals.  Their
tissues are composed of little plates, or laminae, intersect-
ing each other  in  such a  way as to constitute  spongy
masses, or  membranes,  which are not made up of cells
independent of each other,  as in vegetables.   Excepting
the very lowest forms, animals are more  composite than
plants, i. e., their organs are  more complex and numerous,
and more specially devoted to particular purposes.  Repe-
tition of similar parts is a characteristic of plants; of ani-
mals, it is differentiation.  Most animals,  moreover, have
fore-and-aft polarity; in contrast, plants are up-and-down
structures, though,  in this respect,  they are  imitated  by
radiated animals, like the Star-fish.
  (5) Physiology.—In their modes of nutrition, plants and
animals stand widest apart.  Yet " the difference" (says
Owen) " is, at  most, one of form and proportion of the  in-
ternal cavities, and of their external openings: they are
the same as to function."  As a rule, plants absorb and
assimilate food by the external surface, which is therefore
greatly extended by leaves and roots.   As a rule, animals
receive nourishment through a mouth into an internal
permanent  or  temporary stomach.   But there are certain
molds which  seem  to  swallow their food,7 and Tape-
worms have neither mouth nor stomach.
  A plant in the seed and an animal in the  egg exist in
similar conditions:  in both cases a mass of organic matter
accompanies the germ.  When this supply of food is  ex-
hausted, both seek nourishment from without.   But here 
         PLANTS AND ANIMALS DISTINGUISHED.       25

analogy ends: the plant feeds on mineral matter, the ani-
mal on organic.  The former only has the power to  con-
vert  the inorganic elements (carbon, oxygen, hydrogen,
and nitrogen) into organic substance;  the  latter is de-
pendent upon the organic  substance thus prepared.   But
the Fungi  also live on  organic compounds (in a state of
decomposition); and it is probable that some animals have
the power of elaborating tissue directly from the mineral
kingdom.  The materials of nutrition are absorbed by all
living  bodies in a fluid state: the food of plants is gas-
eous or liquid; that of animals is received in a form more
or less solid, but afterward dissolved.   While the absorb-
ents of plants (roots and leaves) are external, those of ani-
mals are distributed along the  walls of a digestive cavity.
Plants exhale less water, and  animals more water, than
they imbibe.
  In general, plants receive nourishment to grow, animals
to repair waste.  The  fabric of the former, once completed,
remains unchanged; while the  tissues of the latter require
constant  renewal.   Plants are continually  receiving ad-
ditional members; animals are born perfect.   In other
words:  in the nutrition  of  the former, addition is the
prominent  idea; in the latter, substitution.  In the for-
mer, the  result is a straggling outline, with almost unlim-
ited growth; in the latter, a finished, compact form.8
  Plants decompose, and animals  recompose, carbonic
acid;  the one yielding the oxygen and fixing the  carbon,
the other exhaling carbonic acid and retaining some  oxy-
gen.  Animals, therefore, inspire what plants set free : the
food of the former undergoes oxidation; that of the latter,
deoxidation.  This chemical antagonism, however, will not
serve  as  a  rigorous  definition.   Mushrooms and leafless
parasites, all plants in  the dark,  germinating seeds and
opening flowers, give off carbonic acid.'  Every plant be-
gins life as an animal—a consumer, not producer:  not till 
26
COMPARATIVE ZOOLOGY.
the young shoot rises above the soil, and unfolds itself to
the light of the sun, at  the touch of whose mystic  rays
chlorophyl is  created,  does real,  constructive  vegetation
begin; then its  mode of life  is reversed — carbon is re-
tained and oxygen set free.
  Most plants, and  many  animals, multiply by  budding
and division;  on both  we  practice grafting; in  both the
cycle  of life  comes round again  to  the ovule or ovum.
Do annuals flower but to die  ?  Insects lay their eggs in
their old age.  But in all cases of  true ovulation, the ani-
mal embryo absorbs its yolk from  the inside, while that of
a seed is itself surrounded  by  the albumen.
  Animals generally possess sensation, consciousness, and
volition.  Yet volition is wanting in some lower forms;
and many plants show  excitability, if  not sensibility.  For
aught we know, irritability is  the  birthright of all living
matter.  In plants, the protoplasm is scattered  and buried
in rigid structures: feeling is,  therefore, dull.  In animals,
the protoplasm is concentrated into special  organs, and so
feeling, like electricity rammed into Ley den jars, goes off
with a flash.10
  The self-motion of animals and the  rooted state of plants
is a very general distinction; but it fails where we need it
most.  It is a  characteristic of living things  to move.  The
protoplasm of all  organisms is unceasingly active.11   Be-
sides  this internal  movement, myriads of  plants, as well
as animals, are locomotive. Rambling Diatoms, writhing
Oscillaria, and the agile spores of Cryptogams crowd our
waters, their  instruments  of  motion (cilia) being of the
very  same  character  as in microscopic animals;  while
Sponges, Corals, Oysters,  and Barnacles  are stationary.
A  contractile vesicle is  not exclusively an animal prop-
erty,  for the  fresh-water Yolvox and  Gonium have it.
The act of muscular contraction in the highest  animal is
due to the same kind of  change in the form of the cells of 
      RELATION BETWEEN MINERALS, PLANTS, ETC.    27

the ultimate fibrillse, as that which produces the sensible
motions of plants.   The ciliary movements of animalcules
and  of microscopic plants  are precisely  similar, and in
neither case indicate consciousness  or self - determining
power.
  Plants, as well  as animals, need a season of repose.
Both have their epidemics.   On both, narcotic and acrid
poisons produce analogous results.   Both have circulatory
systems.   Are some animals warm - blooded ?  In germi-
nation and flowering, plants evolve heat—the stamens of
the Arum, e. g., showing a rise of 20°.  In a  sense, an Oak
has just  as  much  heat as an Elephant, only the miserly
tree  locks up the sunlight  in solid carbon.
  At present, any boundary of the  Animal Kingdom is
arbitrary.  " Probably life is essentially the same in the
two  kingdoms;  and to vegetable life, faculties are super-
added in the lower animals, some of which  are,  here and
there, not indistinctly foreshadowed in plants."   " It must
be said that there are organisms which at one period of
their life  exhibit an aggregate of  phenomena such as to
justify us in speaking of them as animals, while at another
they  appear to be as distinctly vegetable."12
                   CHAPTER  III.
   RELATION BETWEEN MINERALS, PLANTS, AND ANIMALS.
  There are no  independent members of creation: all
things touch upon one another.  The matter of the living
world is identical with that of the inorganic.  The plant,
feeding on the minerals, carbonic acid, water, and am-
monia, builds them  up into complex, organic compounds,
as starch, sugar, gum, cellulose, albumen, fibrine, caseine, 
28              COMPARATIVE ZOOLOGY.

and gluten.   When the plant is eaten by the animal, the
ternary products, as starch, sugar, gum, and cellulose, are
simply carried by the blood to the lungs, and decomposed
by oxidation  back to carbonic  acid and  water, in which
process heat is generated.   The other vegetable products,
as albumen, etc., containing  nitrogen, go  to sustain the
animal, forming blood, muscle, cartilage, etc.  These, giv-
ing way to fresh material  (for there is constant renewal),
are resolved into carbonic acid, water, and  ammonia, and
returned to the earth and  air through the kidneys.  Both
plant and animal end their life by going back to the min-
eral world: and thus the circle is complete—from dust to
dust.  Carbonate of ammonia and water, a  blade of grass
and a horse, are but  the same  elements differently com-
bined and arranged.   Plants  compress the  forces  of inor-
ganic nature  into chemical compounds; animals  liberate
them.  Plants produce; animals consume.   The work of
plants is synthesis, a building-up; the work of animals is
analysis, or destruction.  The tendency in plants is deoxi-
dation; the tendency in animals  is oxidation.  Without
plants, animals would perish;  without animals, plants had
no need to be.13
                   CHAPTER IY.
                         LD7E.
  An impenetrable veil hides the nature of life.  We
know nothing of it except  by the phenomena  it mani-
fests; and  as  these manifestations differ from  those  of
any known physical force, they have been attributed  to
a " special principle."   But  the existence of this assumed
cause has never been  demonstrated.   The biologist can 
LIFE.
29
take  no cognizance  of  an independent principle whose
mysterious  union with  the  body  causes  it to live, and
whose separation leaves it to die.   In the modern view,
life is a system  of functions, or the sum total of living
phenomena.   Many  of  these  phenomena are evidently
manifestations of the common  force of nature, acting
either mechanically,  chemically, electrically, or osmotic-
ally.
   Still, the combination of elements into organic cells,
the arrangement of these cells into tissues, the grouping
of these tissues into  organs, and the marshaling of these
organs into plans of structure, call  for some further shap-
ing, controlling power to effect such wonderful co-ordina-
tion.   Moreover, the manifestation of  feeling and  con-
sciousness is a mystery which no physical hypothesis has
cleared up.   The simplest  vital phenomenon has in it
something over and above the  known forces of the labo-
ratory.14
   Life is exhibited only under certain  conditions.   One
condition is the presence of a physical basis called proto-
plasm.  This substance  is found in all living bodies, and,
so far as we know, is identical in all—a viscid, transpar-
ent, homogeneous, minutely granular,  albuminoid matter.
Life is inseparable from this protoplasm; but it is dormant
unless excited by some  external  stimulants, such  as heat,
light, electricity,  food, water, and oxygen.  Thus, a certain
temperature is essential  to  growth and motion;  taste is
induced by chemical  action, and sight by luminous vibra-
tions.
   The essential manifestations of  animal life may be re-
duced to three: contractility, or the property possessed by
certain parts of the body, especially the muscular fibres, of
shortening themselves; sensibility, or  the peculiar power,
confined to  nervous matter, of receiving and transmitting
impressions; and the power of assimilating food.   All 
30               COMPARATIVE ZOOLOGY.

animals move, feel, and grow.  But some of the lowest
forms are without the slightest trace of organs; they seem
to be as perfectly homogeneous and structureless as a drop
of jelly.   They could not be more simple.  They are de-
void of muscles,  nerves, and stomach; yet they have all
the fundamental  attributes of life—moving, feeling, and
eating.  It has been supposed that the muscular and nerv-
ous matter is diffused in a molecular form; but all we can
say is, that the highest power of the microscope reveals no
organized structure whatever, i. e., there are no parts set
apart for a particular purpose; but a fragment is as  good
as the whole to  perform all the functions of life.  The
animal series, therefore, begins with forms that feel with-
out nerves, move without muscles, and  digest without a
stomach:  in  other words, life is  the cause of organiza-
tion, not the  result of it.  Animals do  not  live because
they are organized, but are  organized because they are
alive.
                    CHAPTER Y.
                  ORGANIZATION.
  We have seen that the simplest life is a formless speck
of protoplasm, without distinctions of structure, and there-
fore without distinctions of function, all parts serving all
purposes—mouth, stomach, limb, and lung—indiscrimi-
nately.  There is no separate digestive cavity, no separate
respiratory, muscular,  or nervous systems.  Every part
will successively  feed, feel, move, and breathe.  Just as
in the earliest state of society, all do every thing, each does
all.   Every man  is  his own tailor, architect,  and lawyer.
But in the progress  of social development the principle of
the  division of labor emerges.   First comes a distinction 
ORGANIZATION.
31
between the governing and governed classes; then follow
and multiply the various civil, military, ecclesiastical, and
industrial occupations.
   In like manner, as we advance in the animal series, we
find the body more and more heterogeneous and  complex
by a process of differentiation, i. e., setting apart certain
portions of the body for special duty.  In the lowest forms,
the work of life is carried on by very simple apparatus.16
But in  the higher organisms, every function is performed
by a special organ.  For example, contractility, at first the
property of the entire  animal, becomes centred  in  mus-
cular tissue; respiration, which  in simple  beings is effect-
ed by the whole surface, is specialized  in lungs or gills;
sensibility, from being  common to the whole organism, is
handed over to the nerves.  An animal, then, whose body,
instead  of being uniform throughout, is made up of differ-
ent  parts for the  performance of particular functions, is
said to  be organized.   And the term is  as applicable to
the  slightly differentiated cell as to complex Man.   Or-
ganization is expressed  by single cells, or by their combi-
nation into tissues and organs.
   1. Cells.—A cell is the simplest form of organized  life.
In general, it is a microscopic globule, consisting of a deli-
cate membrane inclosing a minute  por-
tion of  protoplasm.  The very simplest
kinds are without  granules  or signs of
circulation; but usually the protoplasm
is  granular, and contains a defined sep-
arate mass  called  the  nucleus, within
which are sometimes seen one  or two,
                                      ' Fig. 1.—Parts of a Cell:
rarely more, dark, round specks, named   a,v,y,ce\\-wa\\; P,rxa-
    7  , .  mi       i  •       i      •    cleus; io, nucleolus.
nucleoli.  Ihe enveloping membrane  is
extremely thin and transparent, structureless and minutely
perforated:  it is only an excretion of dead matter acting
as  a boundary to the cell-contents."  The nucleus is gen-
 
32              COMPARATIVE ZOOLOGY.
erally attached to the inside of the membrane, and is the
centre of activity.
  Cells vary greatly in size, but are generally invisible to
the naked eye, ranging from -gbr to Tw^nr of an inch in
diameter.  About 4000 of the smallest would be necessary
to cover the dot of this letter i.  The natural form of iso-
lated cells is spherical;  but when they crowd each other,
as seen  in bone, cartilage, and muscle, their outlines be-
come angular, either hexagonal or irregular.
  Within  the narrow boundary of a simple  circle, the
cell-membrane, are  exhibited all the essential phenomena
of life — growth, development, and  reproduction.   The
physiology  of  these  minute organisms is of peculiar in-
terest, since all animal structure is  but the multiplication
of the cell  as  a unit, and the  whole life of an  animal is
that of the cells which compose it:  in them and by them
all its vital processes are carried on.1'
  The structure of  the cell is  well  illustrated by an egg,
in which the  shell  represents  the  outer membrane, the
white the cell - contents,  the  yolk  the nucleus, and the
germinal spot  of the yolk the  nucleolus.   It  is also seen
in blood - corpuscles, by diluting with a weak solution of
salt or sugar a drop of blood from  a Frog, and  placing it
under the microscope.  (See Figs. 62 and 158.)
  2. Tissues.—There are organisms of  the lowest grade
(as Gregarina) which consist of a single cell, living for and
by itself.  In  this case, the animal  and cell are  identical:
the Gregarina has as much individuality as the  Elephant.
But all  animals, save these unicellular beings, are mainly
aggregations of cells.   For the various parts of a body
are not  only separable by the  knife, into bones, muscles,
nerves, etc., but these are susceptible of  a finer analysis
by the microscope,  which shows that they arise from the
development and union of cells.   These cellular fabrics,
called tissues, differ  from one another  both chemically 
ORGANIZATION.                  33
and structurally, but agree in being permeable to liquids
— a  property which secures  that flexibility of the organs
so essential to animal life.  Every part of the human body,
for example,  is moist: even the hairs, nails, and cuticle
contain water.  The contents as well  as the shape of the
cells are usually modified according  to the tissue which
they form:  thus, we find  cells  containing earthy matter,
iron, fat, mucus, etc.
  In plants, the cell always retains the characters of the
cell; but in animals (after the embryonic period) the cell
usually undergoes such  modifications that the  cellular
form  disappears.   The cells are  connected together or
enveloped by an intercellular substance {blastema), which
may be watery, soft, and gelatinous, firmer and tenacious,
still more solid and hyaline, or hard and opaque.   In the
fluids of the body, as the blood, the cells are separate; but
in the solid tissues they coalesce, being simply connected
together, as  in  the  epidermis,  or  united into  fibres and
tubes.
  In the lowest forms of  life, and in all the  higher ani-
mals in their  embryonic state, the cells of which  they are
composed are not transformed into tissues: definite tissues
make their first appearance in the  Polyps, and  they differ
from one another more and more widely as we  ascend the
scale of being.   In other  words, the  bodies of the lower
and immature animals are more uniform in composition
than the higher, adult forms.   In  the latter only are all
the following tissues found represented:
  (1) Epithelial Tissue.—This is the  simplest form  of cellu-
lar structure.   It covers all the  free surfaces of the body,
internal and external, so that an animal may be said  to be
contained between the walls of a double bag.  That which
is internal, lining the mouth, windpipe, lungs, blood-vessels,
gullet, stomach,  intestines—in fact,  every cavity and canal
—is called epithelium.  It is a very delicate skin, formed of
                           3 
34
COMPARATIVE ZOOLOGY.
flat or cylindrical cells, and in some parts (as in the wind-
pipe of  air-breathing animals, and along the gills of the
                                      Oyster) is  covered
                                      with cilia, or minute
                                      hairs, about  60\6 of
                                      an inch long, which
                                      are incessantly mov-
                                      ing.     Continuous
                                      with this inner  lin-
                                      ing of  the body (as
                                      seen on the lip), and
Fig. 2.—Various kinds of Epithelium Cells: a, colum-  Covering the OUtSlde,
  liar, from small intestine; 3, a single cell, showing  :Q  +y.a  />v\i/Jwvnic. t\r
  nucleus;  b, ciliated, from  one of the small air-  lb  ^  epiaermZS, Or
  tubes; d, the same, from the windpipe, with single  cuticle.   It  is  the
  cell magnified about 200 times; c, squamous, from
  eyelid of a calf, showing changes of form, from the  Ollter  layer  of  the
  deep to superficial cells, 1 being the scurf.         ,,  , .  ,,   ,  . ,
                                      " skin,   which  we
can remove by  a blister, and in Man varies in thickness
from -g^y of an inch on the cheek to tV on the sole of the
foot.   It is constantly wearing  off at the surface, and as
constantly growing in the deeper portion; and in the proc-
ess of growth and passage outward, the cells  change from
the spherical form to dead horny scales (seen in scurf and
dandruff).  In the lower layer of the cuticle we find  the
pigment cells, characteristic of colored races.   Neither the
epidermis nor the corresponding  tissue within (epithelium)
has any blood-vessels or nerves.  The epithelial tissue, then.
is simply a superficial covering, bloodless and insensible
protecting the  more  delicate  parts underneath.   Hairs,
horns, hoofs, nails, claws, corns, beaks, scales,  tortoise-
shell, the wings of Insects, etc., are modifications of  the
epidermis.
   (2) Connective Tissue.—This is  the most extensive  tissue
in animals,  as it is  the great connecting medium by which
the different  parts are held together.  Could it be  taken
out entire, it would be a complete mold of all the organs. 
ORGANIZATION.
35
It surrounds the bones, muscles, blood-vessels, nerves, and
glands, and is the  substance of the  ligaments, tendons,
" true skin," mucous mem-
branes, etc.   It  varies  in
character, being  soft, ten-
der, and  elastic, or  dense,  /TV^^pp^wi^w f^r
tough,  and  generally un- /y^\(\  V"wVsHf\1^\
yielding.    In the  former fm^MJ^^l^
state, it consists of  innu- ll
merable fine white and yel-
low fibres, which interlace
in  all  directions, leaving
irregular spaces, and form-
ing a loose, spongy, moist Fig. 3.—Connective Tissue, showing areolar
   u    T    i.1   1 i.    j.1.            structure, X 26.
web.    In  the latter, the
fibres are condensed into sheets or parallel cords, having
Fig. 4.—Connective Tissue from human peritoneum; highly magnified; a, containing
                         blood-vessel. 
36
COMPARATIVE ZOOLOGY.
Fig. 5.—Adipose Tissue, a;
 with fibres of connective
 tissue, b.
a wavy, glistening appearance.   Connective tissue is not
very  sensitive.  It contains gelatine — the matter which
tans when hide is made into leather.18
  (3) Adipose Tissue. — This is simply  an aggregation  of
large cells  (averaging -g^rr inch  in diameter) filled with
                   fat. They are distributed through the
                   connective tissue (as in the "blubber"
                   of Whales),  or united  into  distinct
                   masses (as about the kidneys in Ru-
                   minants).   The marrow of bones is a
                   good example.  Globules of fat occur
                   in  many Molluscs  and  Insects;  but
                   true adipose  tissue  is found  only  in
                   back-boned animals, particularly the
                   herbivorous.  In  the average Man, it
                   constitutes about -^V part of his weight,
                   and a single  Whale has  yielded 120
tons of oil.   The fat of animals has the different names  of
oil, lard, tallow, suet, spermaceti, etc.   It is  a  reserve  of
nutriment in excess of consumption,
serving also as a packing material, and
as a protection against cold.
  (4) Cartilaginous Tissue. — This  is a
deep-seated structure, better known as
" gristle," which is dense, elastic, blu-
ish white, and translucent, composed of
cells imbedded in a granular or hyaline
substance. It is found where strength,
elasticity, and insensibility are wanted,
as at the joints.  It also takes the place
of bones in the embryo.  When carti-
lage is mixed with connective tissue, as
in the ear, it is called  itbro-cartilaae.   Fl,°- 6--0f>sifying carti-
                     ^ #         *      lage, x 10; a, cartilage
  (5) Osseous Tissue.—This hard, opaque   cells> passing into com-
tissue, called " bone,"  differs from the   spongy bone,' e.
 
ORGANIZATION.
37
former in having the intercellular spaces or meshes filled
with phosphate of lime and other earths, instead of a hya-
line substance.  It may be called petrified cartilage—the
quantity of earthy matter, and therefore the brittleuess of
the  bone, increasing  with the age of  the  animal.   If a
chicken-bone be left in
dilute muriatic acid sev-
eral days, it may be tied
into a  knot, since the
acid has  dissolved the
lime, leaving  nothing
but  cartilage  and  con-
nective tissue.  If a bone
be  burned,  it becomes
light, porous,  and brit-
tle,  the lime alone re-
maining.ia
  Bone is a very vascu-
lar tissue, that is, it is traversed  by minute blood-vessels
and nerves, which pass through a net-work of tubes, called
                                  Haversian canals.  The
                                  canals average -nrVo- of
                                  an  inch, being finest
                                  near the surface of the
                                  bone, and larger further
                                  in, where they  form  a
                                  cancellated  or  spongy
                                  structure,  and  finally
                                  merge  (in   the  long
                                  bones) into  the  central
                                  cavity,  containing  the
                                  marrow.   Under   the
Fig. 8.—Frontal Bone of Human Skull under the micrOSCOpe, each Canal
  microscope, showing lacunae and canaliculi.              .    .
                                  appears to be the  cen-
tre of a multitude of lamince, or plates, arranged around it.
Fro. 7.—Transverse section of a Bone (Human
   Femur), X 50, showing Haversian canals. 
38
COMPARATIVE ZOOLOGY.
Lying between these plates are little cavities, called lacuna?,,
from which radiate exceedingly fine pores, or canaliculi.
The form of the  lacunae  differs in the bones of different
animals, being angular in Fishes, and oval in other ani-
mals.  The size is related, not to the  size of the animal,
but to the  size of the blood - corpuscles, and is therefore
greatest in Reptiles.
  True bone is found only in Yertebrates, or back-boned
animals.
  (6) Dental Tissue.—Like bone, a tooth is a combination
of earthy and animal  matter.   It may be called petrified
epithelium.   In the  higher animals, it  consists  of three
parts: dentine, forming the body of the tooth, and  always
present; enamel, capping the crown; and cement, covering
the fangs (Fig. 29).  The  last  is true  bone, or osseous tis-
sue.   Dentine resembles bone, but differs  in having neither
Fig. 9.—Highly magnified section of Dentine and Cement, from the fang of a Human
   Molar: o, 6, marks of the original dentinal pulp; d, dentinal tubes, terminating
   in the very sensitive, modified layer, g; h, cement.
lacunae nor (save in Shark's teeth) canaliculi.  It shows, in-
stead, under the miscroscope, innumerable parallel  tubes,
reaching from the outside to the pulp-cavity within.  The
"ivory" of Elephants  consists of dentine.  Enamel  is the
hardest substance in the body, and is  composed of minute
six-sided fibres, set closely together.   It is wanting in the
teeth  of most Fishes,  Snakes,  Sloths,  Armadillos, Sperm-
whales, etc.
   True dental tissue is confined to Yertebrates.
   (7)  Muscular Tissue.—If we examine  a piece of lean meat, 
ORGANIZATION.
39
Fig. 10—Striated Muscular Fibre (of the Pig),
 X 200.  The constituent cells are seen at a;
 c is a fasciculus, or bundle.
we find it is made up of a number of fasciculi, or bundles
of fibres, placed side  by side, and bound together by con-
nective tissue.  The mi-
croscope informs us that
each fibre is itself a bun-
dle of smaller fibres; and
when  one of  these  is
more closely examined,
it is found to be inclosed
in a delicate, glossy tube,
called  the sarcolemma.
This tube is filled with
very minute,   parallel
fibrils, averaging -nr^
of an inch in diameter,
and  having a beaded as-
pect, each fibril being a row of cells.  Tissue of this descrip-
              tion  constitutes all ordinary muscle, or "lean
              meat," and is marked  by regular cross-lines,
              or striae.
                Besides this striated muscular tissue, there
              exist, in the coats of the stomach, blood-ves-
              sels,  and  some  other  parts of Yertebrates,
              smooth muscular fibres, or membranes, which
              show a  nucleus under the microscope, and
              do not break up into  fibrils.  The gizzards
              of fowls exhibit this form.
                All muscle  has the property of shorten-
             ing itself when excited; but the contraction
             of the striated kind is under the  control of
             the will, while the movement of the smooth
             fibres is  involuntary.30  Muscles are well sup-
Sbn/dfyided1^ P^d with arteries, veins, and nerves; but the
transverse septa color is due to a  peculiar pigment, not to the
into separate nu- ...
cleated portions, blood.
Fig. 11.—Striated
 Muscular Fibres, 
40
COMPARATIVE ZOOLOGY.
  Muscular tissue is found in all animals, from the Coral
to Man.
  (8) Nervous Tissue.—Nervous  matter exists under three
forms: First—the cellular, consisting of nucleated cells,
              varying from -g-oVo- to -^^  of an  inch in
              diameter, and distributed through the body,
              particularly in the gray portion of the brain,
              spinal cord,  and  nerve-centres, or ganglia.
              Second—the fibrous, consisting of pale, flat,
              extremely  fine filaments.   They  abound in
the  sympathetic  nerves,  the
only  nerves
Third —the
Fig. 12.—Structure
 of a Nerve:  1,
 sheath, or neuri- foun(i  jn  the Invertebrates.
 lemma; 2, med-
 ullary substance tubular.   These are much  larger than the
 axis cylinder, or fibrous, the coarsest being two" of an inch
             in diameter.
primitive band.
                            They consist of tubes inclos-
ing a transparent fibre and a viscid fluid called the nerve-
marrow.41  The delicate tube itself is called neurilemma,
        Fig. 13.—A Ganglion of the Sympathetic Nerve of a Mouse.

analogous to the sarcolemma of muscular tissue.  Nerve-
tubes are found only in back-boned animals, and chiefly in
the white substance of the brain and spinal cord.
  A bundle of fibrous or tubular nervous matter, surround-
ed by connective tissue, constitutes a nerve. 
ORGANIZATION.
41
   3. Organs, and their Functions.—Animals, like Plants,
 grow, reproduce, and die:  these three are the capital facts
 of every organism.  Out of these may issue some peculiar
 phenomena, as Motion and Will.
   Life is manifested in certain special acts, called func-
 tions, performed by certain special parts, called  organs.
 Thus, the stomach is an organ, whose function is digestion.
 A single organ may manifest vitality, but it does not (save
 in the very lowest forms) show forth the whole life of the
 animal.  For, in being set apart for a special purpose, an
 organ takes upon itself, so to speak, to do something foi
 the benefit of the whole animal, in return for which it k
 absolved from doing many  things.   The stomach is not
 called upon to circulate or  purify the blood.
   There may be functions without organs, as the organless
 Amoeba digests, respires, moves, and reproduces by  its gen-
 eral mass.   But as we ascend the scale of animal life, we
 pass from  the simple to the complex: groups of cells or
 tissues, instead of being repetitions of each other, take  on
 a  difference, and become  distinguished as special  parts
 with specific duties.   The higher the  rank of the animal,
 the more numerous the organs.   The more complicated
 the structure, the more complicated the functions.  But in
 all animals, the functions are performed under conditions
 essentially  the same.  Thus, respiration in the Sponge, the
 Fish, and in Man has one  object and  one means,  though
 the methods differ.   A  function, therefore, is a group of
 similar phenomena produced by like causes.
   The life  of an  animal consists in the accumulation and
 expenditure of force.  The  tissues are store-houses of pow-
 er, which, as they waste and decay, is  given off in various
 forms. Thus, the nervous tissue generates nerve-force; the
muscles, motion ;  and the fatty elements, heat.  If we con-
template the phenomena presented by a Dog,  the  most
obvious fact is his power of moving from place to place. 
42
COMPARATIVE ZOOLOGY.
a power produced by the interplay of muscles and bones.
We observe, also, that his motions are neither mechanical
nor irregular; there is method in his movement.   He has
the power of  willing, seeing, hearing,  feeling, etc.; and
these functions are accomplished by a delicate apparatus
of nerves.
  But the Dog does not exhibit perpetual motion.  Sooner
or later  he becomes exhausted, and rest is necessary. Sleep
gives only temporary relief.  In every exercise of the mus-
cles and nerves there is a consumption or waste of their
substance.  The blood restores the organs, but in time the
blood itself needs renewal.   If not renewed, the animal
becomes emaciated, for the whole body is laid under con-
tribution to furnish a supply.  Hence the feelings  of hun-
ger and thirst, impelling the creature to seek food.  This
alone will maintain the balance between waste and repair.
We notice, therefore, an entirely different set of functions,
involving, however, the use of motion and will.   The Dog
seizes a  piece of meat, grinds it between its teeth,  sends it
into the stomach, where  it is digested, and  then  into the
intestine, where it is further changed; there the nourish-
ing part  is absorbed, and  carried to  the  heart, which
propels  it  through little tubes, called  blood - vessels, all
over the body.  In this process of digestion, certain fluids
are required, as saliva, gastric juice, and bile: these are
secreted by special organs, called  glands. Moreover, since
all the food eaten is not fitted to make blood, and as the
blood itself, in going  around the  body, acts  like a scaven-
ger, picking up worn-out particles, we have  another func-
tion, that  of excretion, or removal of useless matter from
the system.  The kidneys and lungs do much of this; but
the lungs do something  else.   They expose the blood  to
the air,  and introduce oxygen, which, we shall find, is es-
sential to the life of every animal.
  These centripetal  and centrifugal movements in the 
ORGANIZATION.
43
body—throwing in and throwing out—are so related and
involved, especially in  the lower forms, that they can not
be sharply defined and classified.  It has been said that ev-
ery Dog has two lives—a vegetative and an animal.  The
former includes the processes of digestion, circulation, res-
piration, secretion, etc., which are common to all life; the
functions of the other, as motion, sensation, and will, are
peculiar to animals.  The heart is the centre of the vege-
tative life,  and the brain is the centre  of  the animal life.
The aim of the vegetative organs  is to nourish the individ-
ual, and reproduce its kind; the organs of locomotion and
sense  establish relations between the individual  and the
world without.  The former  maintain life; the others ex-
press it.  The former develop,  and afterward sustain the
latter.  The vegetative organs, however, are not independ-
ent of the animal;  for without  muscles and nerves we
could not procure, masticate,  and  digest food.  The closer
the connection and dependence between these two sets of
organs, the higher the rank.93
  All the  apparatus and phenomena of  life may be in
eluded under the heads of
                      Nutrition,
                      Motion,
                      Sensation.
  These three are possessed by all animals, but in a varie-
ty of ways.  No two species  have exactly the same mech-
anism and  method of life.  We must learn to distinguish
between what is  vital  and what  is only accessory.   That
only is essential to life which is  common to  all forms of
life.   Our brains,  stomachs, livers, hands, and  feet are
luxuries.   They are necessary to make us human, but not
living, beings.  Half of our body is taken up with a com-
plicated system of digestion; but the Amoeba has neithei
mouth nor stomach.   We have an elaborate  apparatus oi
motion ; the Oyster can not stir an inch. 
44
COMPARATIVE ZOOLOGY.
  Nutrition, Motion, and Sensation indicate three steps
up the grade of life.   Thus, the first is the prominent
function in the Coral, which simply " vegetates," the pow-
ers  of moving and  feeling being very  feeble.   In the
higher Insect, as the Bee, there is great activity with low
organs of nutrition.   In the still higher Mammal, as Man,
there is less power of locomotion, though  the most perfect
nutritive  system; but  both  functions are subordinate  to
Sensation, which is the crowning development.
  In studying the comparative anatomy and physiology of
the animal kingdom, our plan will be to trace the various
organs and functions, from their  simplest expression up-
ward  to the  highest complexity.   Thus, Nutrition will
begin with absorption, which is the simplest  method  of
taking food; going higher, we find  digestion, but in no
particular spot in the body; next, we see it confined to a
tube; then to a tube with  a sac, or stomach; and, finally,
we reach the  complex arrangement in Man.
                   CHAPTER VI.
                      NUTRITION.

  Nutrition is  the  earliest and most  constant of vital
operations.  While  the  organs of motion and sensation
seem to be wanting  in some lower forms, the means of
vegetative life are always present.  So  prominent is the
nutritive apparatus, that an animal has been likened to a
moving sac, organized to convert foreign  matter  into its
own likeness, to  which the complex organs of animal life
are but auxiliaries.  Thus, the bones and  muscles are le-
vers and cords to carry the body about, while the nervous
system directs its motions in  quest of food. 
NUTRITION.
45
  The objects of nutrition are growth, repair, and propa-
gation.   The first object of life is to grow, for no animal
is born finished.  Some animals, like plants, grow as long
as they live ;33 but the majority soon attain a fixed size.  In
all animals, however, without exception, food is wanted for
another purpose than growth, namely, to repair the waste
which is constantly going on.  For every exercise of the
muscles and nerves involves the death and decay of  those
tissues, as shown by the excretions.   The amount of mat-
ter  expelled from the body, and the amount of nourish-
ment needed to make good the loss, increase with the  ac-
tivity of the animal.  The supply must equal the  demand,
in order to maintain the life of the individual; and  as an
organism can make nothing, it must seek it  from without.
Not only are  the muscles and nerves wasted by use, but
every organ in the body; so that the whole structure needs
constant renewal.  An animal begins to die the moment it
begins to live.  The function of nutrition, therefore, is con-
structive, while motion and sensation are destructive.
  Another source of demand for food is the production of
germs, to propagate the race, and the nourishment of such
offspring in the egg and  infantile state.  This reproduc-
tion and development of parts which can maintain an in-
dependent existence is a vegetative phenomenon (for plants
have it), and is a part of the general process of Nutrition.
But it will be more convenient to consider  it  hereafter
(chapters xix., xx.).  Still, another  necessity for aliment
among  the  higher  animals is the maintenance of bodily
heat.  This will be treated under the head of  Respira-
tion.
  For the present, we will study Nutrition, as manifested
in maintaining the life of an adult individual.
  In all animals, this  process essentially  consists in  the
introduction of food, its conversion into tissue, and  the
removal of worn-out material. 
46
COMPARATIVE ZOOLOGY.
  1. The food must be procured, and swallowed.  (Inges-
tion.)
  2. The food must be dissolved, and the nutritious parts
separated into a fluid.  (Digestion.)
  3. The nutritive fluid must be carefully taken up, and
then distributed all over the body.   (Absorption and Cir-
culation.)
  4. The nutritive fluid, now called blood, must be ex-
posed to the air, to absorb oxygen and liberate carbonic
acid.  (Respiration.)
  5. The tissues must repair their parts wasted by use,
by transforming particles of blood into living matter like
themselves.   (Assimilation.)
  6. Certain matters  must  be strained  from  the  blood,
some to serve a purpose, others to be cast out of the sys-
tem.  (Secretion and Excretion.)
  The mechanism  to accomplish all  this in the lowest
forms of life is exceedingly simple, a single cavity per-
forming  all the functions.  But in the  majority  of ani-
mals the apparatus is very complicated:  there  is a set of
organs for the prehension of food; another, for digestion ;
a third, for absorption; a fourth, for distribution; and a
fifth, for purification.
                   CHAPTER VII.
               THE FOOD OF  ANIMALS.

  The term food includes all substances which contribute
to nutrition, whether they simply assist in the process, or
are  actually appropriated, and  become tissue.   With  the
food is usually combined more or less indigestible matter,
which is separated in digestion. 
THE FOOD OF ANIMALS.              47
  Food is derived from the mineral, vegetable, and animal
kingdoms.   Water  and salt, for example, are inorganic.
The former is the most abundant, and a very essential ar-
ticle of food.  Most of the lower forms of aquatic life seem
to live by  drinking: their real nourishment, however, is
present in the water in  the state of solution.  The Earth-
worm, some Beetles, and  certain savage  tribes of Men
swallow earth; but this, likewise, is for the organic matter
which the earth contains.   As no animal, so far as we now
know, is  produced immediately from inorganic matter, so
no animal can be sustained by it.
  Nutritious or tissue-forming food comes from the or-
ganic world, and is either albuminous, as the lean meat
of animals and the gluten of wheat; oleaginous, as animal
fat and vegetable oil;  or saccharine, as starch and sugar.
The first is the essential food-stuff; no substance can serve
permanently for food—that is, can prevent loss of weight
and change in the body—unless it contains  albuminous
matter.   The other  two  are not absolutely vital.  Albumen
contains  nitrogen, which is necessary to the formation of
tissue; fats and sugars  are rich in carbon, and therefore
serve to maintain the heat of the  body.  Warm-blooded
animals feed largely on farinaceous or starchy substances,
which in digestion are converted into sugar. But any ani-
mal, of the higher orders certainly, whether herbivorous or
carnivorous, would  starve, if fed on pure albumen, oil, or
sugar. Nature insists upon a mixed diet,  and so we find
in all the staple articles of food, as milk, meat, and bread,
at least two of these principles present.   As a rule,  the
nutritive principles in vegetables are less  abundant than
in animal food, and the indigestible residue is consequent-
ly greater.   The nutriment in flesh increases as we ascend
the animal  scale; thus, Oysters are less nourishing than
Fish; Fish, less than Fowl; and Fowl, less than the flesh
of Quadrupeds. 
48             COMPARATIVE ZOOLOGY.
  Many animals, as most Insects and Mammals, live solely
on vegetable food, and some species are restricted to par-
ticular plants, as the Silk-worm to the white mulberry.34
But the majority of animals feed on one another; such are
hosts of the microscopic forms, and nearly all the radiated
species, marine Shells, Crustaceans, Beetles, Flies, Spiders,
Fishes, Reptiles, Birds, and clawed Quadrupeds.
  A few, as Man himself, are omnivorous, i. e., are main-
tained on a mixture of animal and vegetable food.  The
use of fire in the preparation of food is peculiar to Man,
who has been called " the cooking animal."  A few of the
strictly herbivorous and carnivorous animals have shown
a capacity for changing their  diet.  Thus, the Horse and
Cow may be brought to eat fish  and flesh; the  Sea-birds
can be habituated  to grain; Cats are fond of  alligator-
pears ; and Dogs  take naturally to the plantain.  Certain
animals, in passing from the  young to the mature state,
make a remarkable change of food.  Thus, the Tadpole
feeds upon vegetable matter; but when it becomes a Frog
it lives on Insects.
  Many tribes,  especially of Reptiles and Insects, are able
to go without food  for months, or even years.  Insects in
the larval,  or  caterpillar, state are very voracious;  but
upon  reaching  the  perfect, or  winged, state, they eat lit-
tle—some species taking no food at all, the mouth being
actually closed.  The  male of the minute Notommata
takes  in no nutriment from the time it  quits its egg till its
death.
  In  general, the greater the  facility with which an ani-
mal obtains its food, the more  dependent is it  upon a con-
stant  supply.  Thus, carnivores endure abstinence better
than herbivores, and wild animals than domesticated ones. 
HOW ANIMALS EAT.
49
                  CHAPTER  VIII.
                 HOW ANIMALS  EAT.
   1. The Prehension of Food. —(l) Liquids.—The sim-
plest  method of  taking  nourishment  is  by absorption
through  the skin.   The  Tape-worm,  for example, has
neither mouth nor stomach, but imbibes the juices of the
animal it infests.   Many other animals,  especially Insects,
live upon liquid food, but obtain it by suction through a
special orifice or tube.  Thus, we find a mouth, or sucker,
furnished with minute teeth for lancing the skin of ani-
mals, as  in  the Leech;  a bristle-like  tube  fitted for pier-
cing, as in the Mosquito; a sharp sucker armed with  barbs,
to fix  it securely during the act of sucking, as in the
Louse; and a long, flexible  proboscis, as in the Butterfly.
Bees have a hairy, channeled tongue, and Flies have one
terminating in  a  large fleshy knob, with or without little
" knives " at the base for cutting the skin : both lap, rather
than suck, their food.
   Most animals drink by suction, as  the Ox; and a few
by lapping, as the Dog; the  Elephant  pumps the water up
with its trunk, and then pours it  into its throat; and Birds
(excepting Doves) fill the beak, and then, raising the head,
allow the water to run down.
   Many aquatic animals, whose food consists of small par-
ticles  diffused through the water, have  an apparatus for
creating currents, so as to bring such particles within their
reach.  This is particularly true of low, fixed forms, which
are unable to go in search of their food.  Thus, the Sponge
draws  nourishment from the water, which is made to cir-
culate, through the system of canals traversing its  body,
                          4 
50
COMPARATIVE ZOOLOGY.
by  the vibration of  minute hairs, or cilia, lining the ca-
nals.  The microscopic Infusoria  have cilia surrounding
the mouth, with which they draw or drive into the body
little  currents  containing  nutritious particles.   Bivalve
shells, as the Oyster and  Clam, are likewise dependent
upon  this  method  of  procuring food, the  gills being
fringed with cilia.   So the singular  fish, Amphioxus (the
only example among Vertebrates), employs ciliary action
to  obtain the  infusorial  organisms on which it feeds.
The Greenland Whale has a mode of ingestion somewhat
unique, gulping great  volumes of water into its mouth,
and then straining out, through its  whalebone sieve,  the
small animals which the water may contain.
  (2)  Solids.—When  the food is in solid masses, whether
floating in  water or not, the animal is  usually provided
                          with prehensile  appendages for
                          taking  hold  of  it.   The jelly-
                          like Amoeba has neither mouth
                          nor  stomach, but extemporizes
                          them, seizing its food by merely
                          applying  its soft body to it, and
                          then wrapping itself around it.
Fio.ii.-AmizoPoi(Rotaiiareneta),  0ther minute creatures (Fora-
 with pseudopodia extended, x 30.   minifera)  extemporize  arms
by throwing out thread-like prolongations of their bodies
(pseudopodia), which adhere  to their prey, and then con-
tract.
  A higher type is seen in Polyps and Jelly-fishes, which
have hollow tentacles around the entrance to the stomach
(Fig.  194).   These  tentacles are  contractile, and, more-
over, are covered with an immense number of minute sacs,
in which a  highly elastic  filament  is coiled  up spirally.
When the tentacles  are touched  by a passing animal, as
a Crab, they seize  it, and at  the same moment throw  out
their myriad filaments, like so many  lassos, which increase
 
HOW ANIMALS EAT.
51
the adhesive  power  of the tentacles, and  probably also
emit  a fluid, which  paralyzes  the victim;  the mouth,
meanwhile, expands to an extraordinary size, and the creat-
ure is soon ingulfed in the digestive bag.
  In the next stage, we find no tentacles, but the food is
brought to  the mouth by the flexible lobes of the  body,
commonly called "arms,"  which  are  covered with hun-
dreds of minute suckers; and if the prey, as an Oyster, is
too large  to be swallowed, the stomach protrudes, like  a
proboscis, and sucks it out of its shell.  This is seen in the
Star-fish (Fig. 207).
  A great  advance is shown  by  the  Sea-urchin, whose
mouth  is  provided with five sharp teeth, set in  as  many
jaws, and  capable of being projected so as to grasp, as well
as to masticate, its food.
  In Mollusks having a single shell, as the Snail,  the chief
organ of prehension is  a  strap-
like tongue,  covered with mi-
nute recurved teeth,  or spines,
with which the animal rasps its
food, while the upper lip is arm-
ed  with a sharp, horny  plate
(Fig. 27).   In many marine spe-
cies, as  the Whelk, the tongue  is
situated at the end of a retractile
proboscis, or muscular tube.  In
the Cuttle-fish, we see the sudden
development of an elaborate sys-
tem of  prehensile organs.   Be-
sides a  spinous tongue, it has a
pair of hard  mandibles, resem- Fig. 15.— Suckers on the Tentacles
,,.    .iii   r   -r>        Jofa Cuttle-fish: a, hollow axis of
Dlmg the  beak OI a Parrot, and   the arm, containing nerve and ar-
working vertically; and around   fii^fZSX&S^
the mouth are eiffht or ten  pow-   di8k  around. tne *&**** /. 9,
                °          r      which contains a retractile mem-
erf ul arms furnished with numer-   brane, or "piston," i.
 
52
COMPARATIVE ZOOLOGY.
 ous cup-like suckers.  So perfect is the adhesion of these
 suckers, it is easier to tear  away a limb than to detach it
 from its hold.
   The  Articulated  animals  exhibit  a great  variety of
 means for procuring nourishment, in addition to the suc-
 torial  contrivances  already mentioned, the  innumerable
 modifications of the mouth corresponding to the diversity
 of food.  The Earth-worm  swallows earthy matter, which
 it secures with its lips, the  upper  one being  prolonged.
 Other worms (as Laodicea) are so  constructed that the
 gullet, which is frequently  armed with teeth  and forceps,
 can be  turned inside out, to form a proboscis for seizing
 prey.  Millepedes, Caterpillars, and  Grubs have a pair of
 horny jaws moving horizontally.   The Centipede has a
 second  pair of jaws, which are really modified feet, ter-
 minated by curved fangs containing a poison-duct.   The
 Horse-shoe  Crab uses its  feet for  prehension, and the
 thighs,  or basal joints, of its  legs to masticate the food
 and force it into the stomach.   The first six pairs  of legs
 in the Lobster and Crab are likewise appropriated  to con-
 veying  food  into  the  mouth, the sixth being enormously
 developed, and  furnished with powerful  pincers,  one of
 which serves as an anchor,  enabling the creature to hold
                             fast to  some  fixed  object,
                             while the other is an instru-
                             ment for seizing  or  cutting
                             its prey.  Scorpions have a
                             similar  pair of  claws for
                             prehension,  and also a pair
Pig. 16.—One of the Fangs, or Perforated  of Small forceps  for  hold-
      Mandibles, of the Spider.           .1    r  -, .
                             ing the food in contact with
the mouth.   In their relatives, the Spiders, the claws are
wanting, and the forceps end  in a fang, or hook, which is
perforated to convey venom.36
  The biting Insects, as  Beetles  and Locusts,  have two
 
HOW ANIMALS EAT.
53
pairs of horny jaws, which open sidewise, one above and
the other below the oral orifice.  The upper pair are called
mandibles;  the lower, maxillae.   The former are armed
with sharp teeth, or with cutting edges, and sometimes are
fitted, like the molars of quadrupeds, to grind the  food.
The maxillae are similar, but  smaller, and in some Insects
have appendages called  palpi, or  feelers, which not only
select, but hold, the food steady while they are divided by
the mandibles and maxillae:  such appendages represent a
third pair of jaws.  The Mantis  seizes  its prey with its
long  fore legs, crushes it between  its thighs, which  are
armed with spines, and then  delivers it up to the jaws for
mastication.  All Articulates  move their jaws horizontally.
  The back-boned  animals generally apprehend food by
means of  their jaws, of which there are two, moving ver-
tically.  The toothless Sturgeon draws in its prey by pow-
erful suction.  The Hag-fish has a  single tooth, which it
plunges into the sides of its  victim, and, thus securing a
firm hold, bores its way into the flesh by means of its saw-
like tongue.   But Fishes are usually well provided with
teeth, which, being sharp and curving inward, are strictly
prehensile.   The fins and tongue are  not prehensile.  A
mouth with horny jaws, as in the Turtles, or bristling with
teeth, as in  the Crocodile, is  the only means possessed by
nearly all Reptiles for securing food.   The  Toad, Frog,
and Chameleon capture insects by darting out the tongue,
which is tipped with glutinous saliva.  The constricting
serpents (Boas) crush their prey in their coils before swal-
lowing; and the venomous  Snakes have a  poison-fang.
Xo Reptile has prehensile lips.  All Birds use their tooth-
less beaks in procuring food, but  birds of prey also seize
with their talons, and Woodpeckers, Hummers, and Par-
rots with their tongues.  The beak varies greatly in shape,
being a hook in the Eagle, a probe in the Woodpecker, and
a shovel in the Duck. 
54
COMPARATIVE ZOOLOGY.
   Among the Quadrupeds we find a few special contriv-
ances, as the trunk of the Elephant, and the long tongues
of the Giraffe and Ant-eater; but, as a rule, the  teeth are
the chief organs of prehension, always  aided more or less
by the  lips.   Ruminants, like the Ox, having  hoofs on
their  feet, and no upper front teeth, employ the lips and
tongue.  Such as can stand erect on the hind legs, as the
                        Squirrel,  Bear,  and  Kangaroo,
                        use the front  limbs for holding
                        the food and  bringing  it to the
                        mouth, but never one limb alone.
                        The clawed animals, like the Cat
                        and Lion, make  use of their feet
                        in securing prey, all four limbs
                        being furnished  with  curved re-
                        tractile  claws;  but the food is
                        conveyed  into the mouth by the
                        movement of the head and jaws.
                        Man  and  the Monkeys employ
                        the hand in bringing food to the
                        mouth, and the  lips and tongue
 Fig. ir.-Arm of the Thumbless  in takmg {t mt°  the Cavity-  The
      Monkey (Ateles).       thumb on the  human  hand  is
longer and more perfect  than that of the Apes and Mon-
keys ; but the foot of the latter is also prehensile.
  2. The  Mouths of Animals.—In the  Parasites, as the
Tape-worm, which absorb nourishment through  the skin,
and Insects, as the May-fly and Bot-fly, which do all their
eating in the larval state, the mouth is either wanting or
rudimentary."  The Amoeba, also,  has  no mouth proper,
but wraps itself around its food.
  In the Animalcules it  is simply a round or oval open-
ing to the body-cavity, generally bordered with cilia, and
situated on the side of the body, or at one extremity.
  An elliptical or quadrangular orifice, surrounded with
 
HOW ANIMALS EAT.
55
 tentacles, and leading directly to the stomach, is the ordi-
 nary mouth of the  Polyps and  Jelly-fishes.   In those
 which  are  fixed,  as  the Actinia, Coral, and  Hydra, the
 mouth  looks  upward:  in those which freely move about,
 as the Jelly-fish, it is generally underneath, the position of
 the  animal being reversed.   In some, the margin, or lip,
 can be  protruded like a proboscis; and in  all it is exceed-
 ingly dilatable.
   The  mouth of the  Star-fish and Sea-urchin is a simple
 round aperture, followed by  a  very short  throat.  In the
 Star-fish, it is inclosed by a ring of hard tubercles.  In the
 Sea-urchin, it is armed with  five  sharp teeth,  resembling
 little conical wedges, set in as many jaws, and  surrounded
 by a muscular membrane and minute tentacles.
   Among the headless Mollusks which do not move about,
 the oral apparatus is  very simple, being inferior to that of
 the radiated animals.  Thus,  the immovable Ascidian has
 a mouth without tentacles or  lips, and in a strange place,
 for it is in the  interior of the body, at the bottom of the
 respiratory  sac; the  aperture at the top of the creature
 being really for the entrance of water for the double pur-
 pose of respiration and nutrition, and any alimentary par-
 ticles which enter with the water must find their way to
 the true mouth  below.   In  the  Oyster and Bivalves  gen-
 erally, the  mouth is an unarmed slit—a mere inlet to the
 stomach, situated in a kind of hood, formed by the union
 of the gills at their origin, and between two pairs of deli-
 cate lips.  These lips  make a furrow, along which pass the
 particles of food drawn in by  the cilia.
  Of the higher Mollusks, the little Clio (one of the Ptero-
pods) has a triangular mouth, with two  jaws armed with
sharp horny teeth, and a tongue covered with spiny hook-
lets all  directed backward.  Some Univalves have a sim-
ple fleshy tube.  Others, as the Whelk, have an extensible
proboscis, which unfolds itself, like the finger of a glove, 
56
COMPARATIVE ZOOLOGY.
and carries within it a rasp-like tongue, which can  bore
into the hardest shells.  Such as feed on vegetable matter,
                           as the Snail, have no probos-
                           cis, but on  the roof of the
                           mouth a curved  horny plate
                           fitted to cut leaves, etc., which
 fig. i8._jaw of the common snau   are pressed  against it by the
       (Helix albolabris).         y^ ftnd Qn the  floQr Qf the
mouth a  small tongue covered with delicate striae.   As
fast as the tongue is worn off by use, it grows out from
the root.
  The mouth of  the Cuttle-fish strikingly resembles that
of the Vertebrates, and is  the most elevated type below
the Fishes.  A broad circular lip nearly conceals a pair
of strong horny mandibles, not unlike the beak of a par-
rot, but reversed, the upper mandible being the shorter of
the two, and the jaws, which are cartilaginous, are imbed-
ded in a mass of muscles, and move vertically.   Between
them is a fleshy tongue covered with papillae and spines.
  The parasitic Worms, living within or  on the outside
of other animals, generally have a sucker  at one end or
underneath, serving simply for  attachment, and another
which is perforated.  The  latter is a true suctorial mouth,
being the sole  inlet  of food.  It is often surrounded with
hooklets or teeth, which serve both to scarify the victim
and secure a  firm hold.   In  the Leech, the mouth is a
triangular opening with thick lips, the upper one prolong-
ed, and microscopic teeth.   In many Worms it is a fleshy
tube, which can  be drawn in or extended, like the eye-
stalks of  the Snail,  and contains a minute  dental appara-
tus inside.
  Millepedes and Centipedes  have two lateral jaws and a
four-lobed lip.
  In Lobsters and Crabs, the mouth is situated underneath
the head, and consists of  a soft  upper lip, then a pair of
 
HOW ANIMALS EAT.
57
Pig. 19.—Mouth of a Locust dissected: 1, labrum, or upper lip; 2, mandibles; 3,
   jaws; 4. labium, or lower lip; 5, tongue.  The appendages to the maxillae and
   lower lip are palpi.
upper jaws provided with a short feeler, below which is a
thin bifid tongue; then follow two  pairs of membranous
under jaws, which are lobed  and hairy; and next, three
pairs of feet changed into jaws (Fig.  244). The Horse-shoe
Crab  has no jaws, the thighs answering the purpose.  The
Barnacle has a prominent mouth, with three pairs of rudi-
mentary jaws.
  With few exceptions, the mouths of Insects in the  larval,
or caterpillar, state are fitted only for biting, the two jaws
being horny shears.  But in the  winged, or perfect, state, 
58              COMPARATIVE ZOOLOGY.
Fig. 20.—Head of a Wild Bee (in-
 thophora retusa), front view:  a,
 compound eyes ; 6, clypeus ;  c,
 three simple eyes; d, antennas; e,
 labrum ; /, mandibles; i, maxillae;
 h, maxillary palpi; I, palpiper; j,
 labial palpi; m, paraglossse;  k,
 ligula.
of taking food.   In the
between the biting  and
per  jaws  " supply
the  place  of  trow-
els, spades, pickaxes,
saws,  scissors,   and
knives,"   while  the
maxillae are develop-
ed into a sheath to
inclose the long, slen-
der,   hairy  tongue
which  laps  up  the
sweets of flowers.  In
the suctorial Butter-
fly, the lips, mandi-
Insects may be divided into the
masticating (as the Beetle) and
the suctorial  (as the  Butterfly).
In  the former group,  the oral
apparatus  consists of two pairs
of  horny  jaws (mandibles and
maxillce),  which  work  horizon-
tally between an upper (labrum)
and an under (labium) lip.  The
maxillae and under lip carry sen-
sitive jointed threads, or feelers
(palpi).  The front edge of the
labium is  commonly  known as
the tongue (ligula).27   In such
a mouth, the mandibles are  the
most  important parts;  but  in
passing to the suctorial Insects,
we find that  the mandibles  are
secondary to the maxillae and la-
bium, which are the only means
Bee tribe, we  have a transition
 the sucking  Insects — the  up-
Fig. 21—Proboscis of a Butterfly. 
HOW ANIMALS EAT.
59
Fig. 22.—Mouth of the
 Horse - fly  (Tabanus
 lineola): a, antennae;
 m, mandibles;  mx,
 maxillae; mp, maxil-
 lary palpi; lb, labrum;
 I, labium, or tongue.
 bles, and  palpi  are  reduced  to  rudi-
 ments,  while the  maxillae  are  the only
 useful oral organs.  These are excessive-
 ly  lengthened into a  proboscis,  their
 edges locking by means of minute teeth,
 so  as to  form a central canal, through
 which the liquid food is pumped up into
 the mouth.  Seen under the microscope,
 the proboscis is made up of innumerable
 rings interlaced  with  spiral  muscular
 fibres.   The proboscis  of  the  Fly is a
                                modified lower lip;  that of
                                the  Bugs,  fitted both for
                                piercing  and  suction,  is
                                formed  by the  union of
                                four bristles, which are the
                                mandibles   and   maxillae
                                strangely altered.
                                  As most of the Arachnids
                                live by suction, the jaws are
                                seldom  used  for mastica-
                                tion.  In the Scorpion, the
                                apparent representatives of
                                the mandibles of an Insect
                                are transformed into a pair
                                of  small forceps, and  the
                                palpi,  so small in  Insects,
                                are  developed into  formi-
                                dable  claws: both of these
                                organs are  prehensile.   In
                                Spiders, the so-called man-
FiG.23.-Under Surface of Male Spider: a,  dibleS> which m0Ve  m0re
 c, poison-fang; 6, teeth on interior margin  Qr lggg vertically, end  in a
 of  mandible, e; f, labium; g, thorax; h,  ^              ,  . ,.,
 limbs ; i, abdomen ; I, spinnerets ;  m,
 maxillary palpus;  d, dilated terminal
 joiut.
fang; and the club-like pal-
pi,  often resembling legs, 
60
COMPARATIVE ZOOLOGY.
have nothing to do with ingestion or locomotion.   Both
Scorpions and Spiders have a soft upper lip, and a groove
within the mouth, which serves as a canal while sucking
their prey.   The tongue is external, and situated between
a pair of diminutive maxillae.
  The mouth of Vertebrates is a cavity with a fixed roof
(the hard palate) and a movable floor (the tongue and
lower jaw), having a  transverse opening in front,28 and a
narrow outlet behind, leading to the gullet.  Save in Birds
and some others, the cavity is closed in front with lips, and
the margins of the jaws are set with teeth.
  In Fishes, as in nearly all aquatic animals, the mouth is
the common entry  to  both the digestive and respiratory
organs; it is, therefore, large, and complicated by a mech-
anism for regulating the transit of the food to the stomach
and the aerated  water to the gills.  The slits  leading to
the gills  are provided  with  rows of processes  which, like
a sieve, prevent  the  entrance of food, and with valves to
keep the water, after it has entered the gills, from return-
ing to the mouth.   So that the mouths of Fishes may be
said to be armed at both ends with teeth-bearing jaws.  A
few Fishes, as the Sturgeon, are toothless; but, as a class,
they have an extraordinary dental apparatus—not only the
upper and  lower jaws, but  even the palate, tongue, and
throat, being sometimes studded with teeth.  Every part
of the mouth is  evidently designed for prehension.  Lips
are usually present; but the tongue is often absent, or very
small, and as often aids respiration as ingestion.
  Reptiles  have a wide mouth, even the insect-feeding
Toads and  the  Serpents can stretch theirs enormously.
Trae  fleshy  lips  are wanting; hence the savage aspect of
the grinning Crocodile.  With some exceptions, as Toads
and Turtles, the jaws  are armed with teeth.  Turtles are
provided with horny beaks.   The tongue  is rarely absent,
but is generally too thick  and short to be of much use. 
HOW ANIMALS EAT.
61
Fio. 24__Month of the Crocodile: d, tongue; e, glands; /, inferior, and g, superior,
        valve, separating the cavity of the mouth from the throat, h.
In the Toad, Frog, and Chameleon it is singularly exten-
sile : rooted in front and free behind, it is shot from the
mouth with such rapidity  that  the Insect is  seized and
swallowed more quickly than the eye can follow.  Snakes
have a slender forked tongue, consisting of a pair of mus-
cular cylinders, which is solely an instrument of touch.
  Birds are without  lips or teeth, the jaws being covered
with horn forming a beak.   This varies greatly in shape,
being extremely wide  in the Whip-poor-will, remarkably
long in the Pelican, stout in the Eagle,  and slender in the
Hummer.  It is hardest in those that tear or bruise their
food, and softest in  water-birds.  The tongue is also cov-
ered with a horny sheath, and generally spinous, its chief
function  being to secure the food when in the mouth.  It
is proportionally the largest and most  fleshy in  the Par-
rots.
  The  main characteristics of the mammalian mouth are
fleshy lips and  mobile cheeks/"  In the  duck-billed Mon-
otremes lips are  wanting, and in  the Porpoises  they are
barely  represented.   But in the herbivorous  quadrupeds 
62
COMPARATIVE ZOOLOGY.
they are the chief organs of prehension ; in the carnivo-
rous tribes they  are  thin  and  retractile;  while in the
Whale the upper lip  falls  down like  a curtain, overlap-
ping the lower jaw several feet.  As a rule, the mouth is
terminal; but in the Elephant, Tapir, Hog, and Shrew,
the upper lip blends with the nose to form a proboscis, or
snout.  The mouth is comparatively small in the Elephant
                        and in gnawing animals like the
                        Squirrel, wide in the  Carnivores,
                        short  in  the  Sloth, and long in
                        the Ant-eater.   Teeth are usual-
                        ly  present, but vary in form and
                        number with  the habits of the
                        animal.  The  Ant-eater  is tooth-
                        less, and the  Greenland Whale
                        has a sieve made of horny plates.
                        The tongue conforms in size and
                        shape with the lower  jaw, and is
                        a muscular, sensitive organ, which
                        serves many purposes, assisting in
                        the prehension, mastication, and
                        swallowing of food,  besides  be-
                        ing an organ  of taste, touch, and
                        speech.  Its  surface  is covered
  £?r'emOTeTTsecHonnofrthe with minute  prominences, called
  lower jaw.                papilldB, which are arranged  in
lines with mathematical precision.  In the Cats, these are
developed into recurved spines, which the animal uses in
cleaning  bones and combing  its fur.   Similar papillae
occur on the roof and sides of the mouth of the Ox and
other Ruminants.  The tongue is remarkably long in the
Ant-eater and Giraffe,  and almost immovable in the Gnaw-
ers, Elephants, and Whales.
   3. The Teeth of Animals. —Nearly  all animals have
certain hard parts within the mouth for the prehension or
Pig. 25.—Human Tongue and ad-
 jacent parts: a, lingual papillae;
 b, papillae forming V-shaped
 lines; d, fungiform papillae; e,
 filiform papillae;  g, epiglottis;
 m, uvula, or conical process,
 hanging from the soft palate,
    o, hard palate; r, palatine 
HOW ANIMALS EAT.
63
trituration of  solid food.   If  wanting, the legs are often
armed with spines, or pincers, to serve the same purpose,
as in  the  Horse-shoe  Crab; or the stomach is lined with
"gastric teeth," as in some marine Snails; or the deficiency
is supplied by a muscular gizzard, as in Birds, Ant-eaters,
Insects, and Cuttle-fishes.  Even the Lobster and Crab, in
addition to their complicated oral organs, have the stom-
ach furnished with a powerful set of teeth.
  The Sea-urchin is the first of  animals, and the only one
below Articulates and Mollusks, which exhibits any thing
like a dental  ap-
paratus.    Five
calcareous  teeth
having the shape
of   three - sided
prisms, each set in
a triangular pyr-
amid, or  " jaw,"
are moved  upon
each  other by a
Complex arrange- Fia' 26,—Ecninus bisected, showing masticating apparatus.
ment of levers and muscles.   Instead of moving up and
down, as in Vertebrates, or from right to left, as in Articu-
lates, they converge toward the  centre, and the food passes
between ten grinding surfaces.
   The  minute Rotifers (a group of  minute Articulates)
have  a curious pair of horny jaws.  That which answers
to the  lower  jaw is  fixed,  and  called the " anvil."  The
upper jaw consists of two pieces called " hammers," which
are sharply notched,  and beat upon the "anvil" between
them.
   The  horny-toothed mandibles of Insects, already men-
tioned, are mainly prehensile, but also serve to divide the
food  in a  measure.
   The three little white ridges  in the mouth of the Leech
 
64
COMPARATIVE ZOOLOGY.
are the convex edges of horny semicircles, each bordered
by a  row of nearly a hundred hard,  sharp teeth.   When
                                         the mouth, or suck-
                                         er, is  applied  to
                                         the skin, a  sawing
                                         movement is given
                                         to the horny ridges,
                                         so that the  " bite "
Fig. 27.—Teeth and Masticatory Apparatus of Ga-stero-   ,.  ,   j     -i  .
 pods: A, portion of odontophore, or " tongue," of Vel- OI tHe .LeeCH IS re-
 wind, enlarged; .B, portion of odontopbore of Whelk _]]_. „ eqw f<nr
 (Buccinum undatum), magnified —the entire tongue alJJ  °" °aw-CUL.
 has 100 rows of teeth; C, bead and odontophore of Lim-    The dentition of
 pet (Patella vulgata) • D, portion of same, greatly mag-
 nified, to show the transverse rows of siliceous teeth,  the  Univalve Mol-
lusks, or the Snails, is general-
ly lingual, *'. e., it consists of
microscopic teeth, usually si-
liceous  and  amber - colored,
planted in rows on the tongue.
The teeth are, in fact, the ser-
rated edges of  minute plates.
The  number  of  these plates
varies  greatly;  the   garden
Slug has 160 rows, with  180
teeth in each  row.
   All  Birds,  and  some  other
Vertebrates, as  Ant-eaters,30
Turtles, Tortoises, Toads,  and
Sturgeons,  have  no  teeth.
Their  absence  is generally
associated with a horny beak,
a wide gullet, and a muscular
stomach  (gizzard).
■   In a few Yertebrates, horny  PlG 28._Section of the Upper Jaw of a
plates take the  place of teeth,   Wnale (BaUmuptera), showing baleen-
r                l              '   plates: a, superior maxillary bone; 6,
as the Duck Mole (Ornitho-   ligamentous gum attaching the horny
                 ,   Trr.   .         body of the baleen-plate, c; d, fringe
rhynchlis)   and   Whalebone   of bristles; e, smaller plates.
 
HOW ANIMALS EAT.
65
 Whale.   In the former, the plates consist of closely set
 vertical hollow tubes; in the latter, the baleen, or whale-
 bone, plates, triangular in shape, and fringed on the inner
 side, hang in rows from the gums of the upper jaw.  In
 some Whales there are about 300 plates  composing the
 outer row in each jaw.81
   True teeth, consisting mainly of a hard, calcareous  sub-
 stance called dentine, are found only in back-boned ani-
 mals.  They are distinct from the skeleton, and differ from
 bone in containing more mineral
 matter, and in not showing, under
 the  microscope, any  minute cav-
 ities, called  lacunae.   A  typical
 tooth,  as  found in Man, consists
 of a central  mass of dentine, cap-
 ped with enamel and surrounded
 with cement.    The  first tissue is
 always present,  while the others
 may be absent.   It is a mixture
 of animal and mineral matter dis-
                                  Fig. 29.—Section of Human Mo-
 posed  in  the form of  extremely  iar,  enlarged: k, crown;  «,
 r.    .  ,      i   n       .   .      neck; /, fang; e, enamel; d,
 nne tubes and cells, so  minute as  dentine; c,  cement; P, PuiP-
 to prevent  the admission  of the  cavity"
 red particles of blood.  One modification of it is ivory,
 seen in the  tusks  of  Elephants.   Enamel  is the hardest
 tissue of the body, and  contains  not  more than two per
 cent, of animal matter.   It consists of six-sided  fibres set
 side by side, at right angles to the surfaces of the dentine.
 Cement closely resembles bone, and is present only in the
 teeth of the higher animals.
  Teeth are usually confined to the jaws; but the number,
size, form, structure, position, and mode of attachment vary
with the food and  habits of the animal.  As a rule,  ani-
mals developing large numbers of teeth in the back part
of the mouth are inferior to those having fewer teeth, and
                         5
 
66
COMPARATIVE ZOOLOGY.
those nearer the lips.   The teeth of  Mammals only have
fangs.
  The  teeth of  Fishes present the greatest variety.  In
number, they range from zero to hundreds.  The Hag-fish
(Myxine) has a single tooth on the roof of the mouth, and
two serrated plates on the tongue; while the mouth of the
Pike is crowded with teeth.   In the very  lowest of the
class, we find teeth, short and blunt, in the shape of cubes,
or prisms, arranged like mosaic work.   Such pavement-
teeth (seen in some Rays) are fitted for grinding sea-weed
                          and  crushing shell-fish.   But
                          the cone is the  most common
                          form: sometimes  so slender
                          and   close  as  to   resemble
                          plush, as in the Perch; or  of
                          large size, and flattened like
                          a  spear-head  with serrated
                        f edges, as in  the  Shark; but
Pig. 30.—Jaws and Pavement-teeth of    o   '                 >
       a Ray (Myiwbaua).        more often like  the canines
of Mammals,  curved  inward  to  fit  them for grappling.
In the  Shark, the teeth are confined to the fore part of the
mouth; in the Carp, they are all situated on the bones  of
the throat; in the  Parrot-fish,  they occupy both back and
front;  but in most Fishes, the  teeth are developed also  on
the roof, or palate, and,  in fact, on nearly  every bone  in
the mouth.  They seldom  appear (as in the Salmon)  on
the upper maxillary.  As to mode of  attachment, the teeth
are generally anchylosed (fastened by bony matter)  to the
bones which support them, or simply bound by ligaments,
as in the Shark.  In a few Fishes, the teeth consist  of
flexible cartilage; but  almost invariably  they  are com-
posed of some kind of dentine, enamel and cement being
absent.
   Of Reptiles, Toads, Turtles, and Tortoises are toothless;
Frogs  have teeth in the upper  jaw  only;  Snakes have a 
HOW ANIMALS EAT.
67
more complete set, but Saurians possess the most perfect
dentition.   The  number is  not fixed  even in the same
species:  in  the Alligator it varies from 72 to 88.   The
teeth are limited to the jawbones in  the higher forms
(Saurians);  but in others, as the Serpents, they  are planted
also  in the roof of the mouth.  With few exceptions, they
are conical  and curved  (Fig. 35).   In  the Serpents  they
are longest  and sharpest; and the venomous species have
two  or more fangs in the upper jaw.  These fangs contain
a canal,  through  which  the
poison is forced by muscles
which compress  the  gland.
The  bones  to which they
are  attached  are movable,
and  the fangs ordinarily lie
flat  Upon the gums, but  are  Fig. 31.—Poison Apparatus of the Rattle-
,      v ,  . .        .  •.    .    snake: g, gland, with duct, leading to
Drought intO a Vertical pOSl-   the fang, /;  m, elevator muscles of the
tinn  in  +l.a    f s\£  t- '1 '     jaw, which, in contracting, compress the
lion in  tne  act OI SUlKlIlg.   giand; 8, salivary glands on the edge of
As a rule, the teeth of Rep-   theJaws? n<aostliL
tiles are simply soldered  to the bone which supports them,
or lodged in a groove; but those of Crocodiles are set in
sockets.  Reptilian teeth are made of dentine and a thin
layer of cement, to which  is  added in most  Saurians a
coat of enamel on the crown.
  In the majority of Mammals, the  teeth are limited in
number and definite in their forms.  The number ranges
from 1 in the Narwhal (but  the longest tooth in the king-
dom) to 220 in the Dolphin.  The average is 32, occurring
in Ruminants, Apes, and Man;  but 44 (as  in the Hog and
Mole) is called the typical or  normal number, and this
number  is exceeded only in the lowest groups.  When
more than 44, the teeth  are of the Reptilian type, small,
pointed, and of nearly equal size, as in the Porpoise.   In
the  higher  Mammals, the teeth  are comparatively  few,
and  differ so much in size, shape, and  use, that they can 
68
y COMPARATIVE ZOOLOGY.
be classed into  incisors, canines,  premolars, and molars.
For such a dental  series  exhibits a double purpose, pre-
hension and  mastication.   The chisel - shaped front teeth
are fitted for cutting the food, and hence called incisors.
These vary in number: the Lion has six in each jaw;  the
Squirrel has  two in each jaw, but remarkably developed;
the Ox has none in the upper jaw, and the Elephant none
in the lower; while the Sloth has none  at all.38   The ca-
Fig. 32—Skull of the Babirusa, or Malayan Hog, showing growth and curvature of
                       the canines.
nines,  so  called because  so prominent  in the Dog,  are
conical, and, except in Man, longer than the other teeth.
They are  designed for seizing and tearing; and they are
the  most formidable  weapons  of  the  wild  carnivores.
There  are never more than four.   They are wanting in
all  Rodents, and in nearly all herbivorous  quadrupeds.
The molars, or grinders, vary greatly in shape, but close- 
HOW ANIMALS EAT.
69
ly correspond with the structure and habits of the animal,
so that  a single tooth is sufficent to  indicate the mode of
life and to identify the species.33   In the Ruminants, Ro-
dents, Horses, and Elephants, the  summits of the molars
are flat, like mill-stones, with transverse or  curving ridges
of enamel.   In the Cats and Dogs,  they are narrow and
sharp, passing by  each  other like the blad«s of scissors,
and therefore cutting,  rather  than  grinding, the  food.
The more purely carnivorous  the species,  and the  more
it feeds upon living prey, the fewer the molars.   In ani-
mals living  on mixed  diet, as  the  Hog  and Man, the
crowns  have  blunt tubercles.   Premolars, or bicuspids,
are those which were preceded by  milk-teeth;  the true,
or back, molars had no predecessors.
   The dentition of Mammals is expressed  by a formula,
which  is  a combination  of  initial letters  and figures in
Pig. 33.—Teeth of the right lower jaw of adult male Chimpanzee (Troglodytes niger),
      natural size.  The molar series does not form a curve, as in Man.

fractional form, to show the number  and kind  of  teeth
on each  side of both  jaws.  Thus, the formula for Man
•  .  •  2-2     1-1.  ^2 2. ^  3-3    on

  The teeth of Mammals are always restricted to the mar-
gins of the jaws, and form a single row in each.  But they
never form  an unbroken series in any living species,  ex- 
70
COMPARATIVE ZOOLOGY.
cept Man.34  The teeth implanted in the premaxillary bone,
and in the corresponding part of the lower jaw, whatever
their number, are incisors.  The first tooth behind the pre-
maxillary, if sharp and projecting, is a canine.
  Each tooth has its particular bony socket.36  The molars
are still further strengthened by having two or more  di-
verging fangs, or roots, a feature peculiar to  this class.
The incisors and canines have but one  fang;  and those
that are perpetually growing,  as the  incisors of  Rodents
and  Elephants, have none at all.   The  teeth of flesh-eat-
ing Mammals usually consist of hard dentine, surrounded
with cement and capped with enamel.   In  the  herbivo-
rous tribes, they are very complex, the enamel and cement
being inflected into the dentine, forming folds, as in the
molar of the Ox, or plates, as in the compound tooth of the
Elephant.   This  arrangement  of  the  three tissues, which
Fig. 34.—Upper Molar Tooth of Indian Elephant (Elephas Indicus), showing trans-
   verse arrangement of dentine, d, with festooned border of enamel plates, e; c,
   cement; one-third natural size.

differ in hardness, secures a surface with prominent ridges,
well adapted for grinding.   The cutting teeth of the Ro-
dents consist of  dentine, with a plate of enamel on the
anterior surface, and the unequal wear preserves a  chisel-
like edge.  Enamel is sometimes wanting, as in the molars
of the Sloth and the tusks of the Elephant.
   In  Fishes  and Reptiles, there  is  an almost unlimited
succession of teeth;  but Mammalian teeth are cast and
renewed but once in life. 
HOW ANIMALS EAT.
71
  Yertebrates use their teeth for the prehension of food,
as weapons of  offense or defense, as aids in locomotion,
and  as instruments for uprooting or cutting down  trees.
But  in the higher class, they are principally adapted for
dividing or grinding the food.38  While in nearly all other
Vertebrates the food is bolted entire, Mammals masticate
it before swallowing.  Animals that masticate most thor-
oughly, digest  most  rapidly.   Mastication,  however, is
more essential  in  the  digestion  of vegetable than of ani-
mal  food;  and hence  we find the dental apparatus most
efficient in the herbivorous quadrupeds.  The food is most
perfectly reduced by the Rodents.
  Teeth, as we shall see, are appendages of the skin, not
of the skeleton, and, like other superficial organs, are lia-
ble to be modified in accordance with the  habits of  the
creature.  They are, therefore, of great zoological value;
for, such is the harmony between them and their uses, the
naturalist can predict the food and general structure of an
animal from a sight of the teeth alone.  For the same rea-
son, they form important guides in the classification of ani-
mals ; while their durability renders them available to the
paleontologist in the determination  of the nature and af-
finities of extinct species, of which they are often the sole
remains.   Even the structure is  so  peculiar that a frag-
ment will sometimes suffice.
  4. Deglutition, or  How Animals Swallow.—In  the
lowest forms of life, the mouth is but an aperture opening
immediately into the body-cavity, and the food is drawn
in by ciliary currents.   But in the majority  of animals, a
muscular tube,  called the gullet, or oesophagus, intervenes
between  the mouth and  stomach, the circular  fibres of
which contract, in  a wave-like manner, from above  down-
ward, propelling  the morsel  into the stomach.37  In  the
higher Mollusks, Articulates, and Yertebrates, deglutition
is generally assisted by the tongue, which presses the food 
72              COMPARATIVE ZOOLOGY.

backward, and  by a glairy juice, called  saliva,  which fa-
cilitates its passage through the gullet.38  Vertebrates have
a cavity behind the mouth, called the throat, or pharynx,
which may be considered as a funnel to the oesophagus.39
In air-breathers, it has openings  leading to  the  windpipe,
nose, and ears.  In Man,  as  in  Mammals generally,  the
process of deglutition  is in  this wise: the food, masticated
by the teeth and lubricated by the saliva, is  forced by the
tongue and cheeks into the pharynx; the soft palate keep-
ing it out of the nasal aperture, and the valve-like epiglot-
tis falling down to form a  bridge over the opening to the
windpipe.  . The moment the pharynx receives  the food,
it grasps  it tightly, and, the  muscular  fibres contracting
above  it and left lax  below it, it is rapidly thrust into the
oesophagus.  Here, a similar  movement (the peristaltic)
strips the food  into the stomach.40  The rapidity  of these
contractions transmitted  along the oesophagus may be ob-
served in the neck of  a Horse while drinking.
  Deglutition in the Serpents is painfully slow,  and some-
what peculiar.  For how is  an animal, without limbs or
molars, to swallow its prey, which  is often much larger
than its own body?   The Boa-constrictor, e.g., seizes the
Fig. 35.—Skull of Boa-constrictor: 1, frontal; 2, prefrontal; 4, postfrontal; 5, basl-
   occipital; 6, sphenoid: 7, parietal; 12, mastoid;  13, alisphenoid; 17, premaxil-
   lary; 18, maxillary; 20, nasal; 24, transverse; 25, internal pterygoid; 34, dental,
   lower jaw; 35, 36, angular; a, tympanic; 8, prenasal; v, petrosal. 
THE ALIMENTARY CANAL.
73
head  of  its victim with its sharp recurving  teeth, and
crushes the body with its overlapping coils.  Then, slow-
ly uncoiling, and covering the carcass with a slimy mucus,
it thrusts the head into its mouth by main  force, the mouth
stretching marvelously,  the  skull being  loosely  put to-
gether.  One jaw is then unfixed, and the  teeth  withdrawn
by being pushed forward, when they are again  fastened
farther back upon the animal.  The other jaw is then pro-
truded and refastened ; and thus, by successive movements,
the prey is slowly and spirally drawn  into the wide gullet.
                   CHAPTER IX.
                THE ALIMENTARY CANAL.
  The Alimentary Canal is the great route by which nu-
tritive matter reaches the interior of the body.  It is the
most universal organ in the animal kingdom, and the rest
are secondary or subservient to it.  In  the higher animals,
it consists of a mouth, pharynx, gullet, stomach, and in-
testine.
  It is a general  law, that food can  be  introduced  into
the living system only in a fluid state.   While plants send
forth  their roots to seek  nourishment  from without, ani-
mals,  which may be likened to plants turned outside in,
have their roots (called  absorbents) directed inward along
the walls of a central tube or  cavity.   This cavity is for
the reception and preparation of the food, so that animals
may be said to carry their soil about with them.  The ne-
cessity for such a cavity arises not only from the fact that
the food, which is usually solid, must be dissolved, so as to
make  its way through the delicate walls of the cavity into
the system, but also from the occurrence of  intervals be- 
74
COMPARATIVE ZOOLOGY.
tween the periods of eating, and the consequent need of
a reservoir.  For animals, unlike plants, are thrown upon
their own wits to procure food.
  The  alimentary canal is  a continuation of the skin,
which is reflected inward,  as we  turn the finger of  a
glove.  We find every grade of this  reflection, from  the
mere depression in the side of the body  of the Amoeba, to
the sac of the Sea-anemone and the long intestinal tube of
the Ox.  So that food in the stomach is  still outside of the
true body.  In fact, there are certain Worms, living inside
of other animals, which have neither mouth nor stomach,
but  imbibe  nourishment through their skin.41  Such  a
method of taking food  is a link between the plant's out-
side mode of nutrition and the internal mode of the ani-
mal : fundamentally, there is no difference.
  The feeblest sign of a digestive cavity is that extempo-
rized by the jelly-like Amoeba.   Wherever a minute sea-
weed or animalcule  happens to come in contact with its
                                     body,  the  spot  re-
                         ■f           tracts, forming a de-
                       '  $   /i       pression, which sinks
                                     deeper  and  deeper,
                                     till the edges meet,
                                     and the  prey is in-
                                     gulfed.    After  the
                                     soluble   parts   are
                                     dissolved, the  indi-
                                     gestible  residue  is
                                     brought  to  the sur-
                                     face  by  a  reverse
Fig. 36.—Dissected Actinia: a, the thick opaque skin prOCeSS
 consisting of ectoderm and endoderm, lined with
 muscular fibres; c, the tubular tentacles communi-
 cating with the interspaces, k, between the mem-
 braneous vertical folds, g; g', orifices in the walls
 allowing passage of respiratory  water from one SOl'la and the Hydra,
 compartment to another; d, mouth leading to gas-              .
 trie cavity, e.                           where a definite oral
  A step higher than
this is seen in Inf u- 
THE ALIMENTARY CANAL.
75
orifice, or mouth, leads to a permanent body-cavity, and
serves both for  the inlet of food and the  outlet of mat-
ters not wanted.  These animals may be likened to a tube
or bag with one opening.  There is no great difference be-
tween the membrane which lines this "stomach" and that
which clothes the body;  for the Hydra has been turned
inside out, and digested  as well.  The Polyps have also
but one external opening; but from this  hangs down  a
short tube, open at both ends, reaching about half-way
to the bottom of the  body-cavity.   Such an arrangement
would be represented by a bottle  with  its neck  turned
inward.  In  this suspended sac, which is somewhat con-
stricted  at the extremities, digestion takes  place; but the
product passes freely into all the  surrounding  chambers,
along with the  water for respiration.  The Medusae, or
Jelly-fishes,  preserve  the same type of a digestive appa-
ratus ; but the sac is  cut off from the general cavity, and
numerous canals radiate  from it to a circular canal near
the margin of the disk.  In the Star-fishes, the sac sends
off two branches, or canals, to each ray.   But these radi-
ating canals serve a double purpose, for they not only carry
nutritive matter, but bring back the excretions.
   Thus far we have seen but one opening to the digestive
cavity, rejected  portions  returning  by the same road by
which they enter.   But  a true alimentary canal  should
have an anal aperture distinct from  the oral.   The sim-
plest form of such a canal is exhibited by the Sponge, in
its system of absorbent pores  for the entrance of liquid,
and of several main channels for its  discharge.  The ap-
paratus, however, is not marked off from the general cav-
ity of the body, and, as in the preceding cases, digestion is
not distinct from circulation.43
  The Sea-urchin presents us with an important advance
— one cavity with  two orifices; and the complicated ap-
paratus of higher animals is but the development of this 
76
COMPARATIVE ZOOLOGY.
                                   type.   This  first  rudi-
                                   ment of an alimentary
                                   canal begins in a mouth
                                   well provided with teeth
                                   and  muscles,  and  ex-
                                   tends   spirally  to  its
                                   outlet,  which generally
                                   opens on the upper, or
                                   opposite, surface. More-
                                   over, while in many of
                                   the Worms the canal is
                                   a  simple tube running
                                   through the axis of the
                                   cylindrical body  from
                                   oral orifice to  anal ap-
                                   erture, the  canal of the
                                   Sea-urchin shows a dis-
                                   tinction of parts,  fore-
shadowing the  pharynx, gullet, stomach, and  intestines of
Man  himself.   Both  mouth and vent  have  muscles for
constriction  and  expansion ;  and,  as the vent  is on  the
summit of the  shell, and the latter is covered with spines,
the ejected particles are seized by delicate forks (pedicel-
lariai), and passed on from one to the other down the side
of the body, till they are dropped off into the  water.43
   The next  higher modification  we find in the  Articulate
subkingdom.  In the Worms, the digestive tract is either
a  straight,  unvarying tube,  or  divided  up  into pouches
(sacculated), as in the Leech, with clusters of little glands,
called follicles, along the  side, which are the rudiments of
a liver.   In Myriapods and Larvae, the same general plan
is  continued, the canal passing in a straight line from one
extremity to the  other, but showing  a division into gullet,
stomach, and intestine.44   Crustaceans, like the Lobster,
have  a short gullet leading  to a large  cavity,  situated in
Fig. 37.—Diagrammatic Section of a Sea-urchin
 (Echinus):  a, mouth ; 6, oesophagus; c, stom-
 ach ; d, intestine; /, madreporiform tubercle ;
 g, sand-canal;  ft, ambulacral ring; k, Polian
 vesicles, which  are probably reservoirs of flu-
 id ; m, ambulacral tube;  o, anus; p, ambula-
 cra, with their contractile vesicles; r, nervous
 ring around the gullet; ■*, two nervous trunks,
 the right terminating, at anal pole, in a small
 ganglion ; t, blood-vascular rings connected by
 v, the contractile heart; w, two arterial trunks
 radiating from the anal ring; *, an ovary open-
 ing at the anal pole in a genital plate, y;  z,
 spines, with their tubercles. 
THE ALIMENTARY CANAL.
77
the  head of  the
animal, which  is
a  gizzard, rather
than stomach,  as
it has  thick  mus-
cular  walls  arm-
ed with teeth.   A
well-marked con-
striction separates
this  organ   from
the intestine. The
liver is highly de-
veloped ;  instead
of numerous fol-
licles,  there  is  a
large symmetrical
organ,  divided in-
to two lobes, pour-
ing  its  secretion
into  the  upper
part of the  intes-
tine, which is the
true stomach.
   Among Insects,
there is great va-
riation in the form
and  length of the
canal.   The  fol-
lowing  parts  can
generally  be  dis-
tinguished :  gullet, crop, gizzard, stomach, and large and
small intestines, with many glandular  appendages.   The
crop, gizzard, and large intestine are  sometimes absent,
especially in  the carnivorous species.   In Bees, the crop
is  called the " honey-bag."   The gizzard is  found in  In-
FiG. 38.—Anatomy of a Caterpillar: g, h, oesophagus ; h,
  ?', stomach ;  k, hepatic vessels; I, m, intestine; q, r,
  salivary glands ; p, salivary duct; a, b, c, longitudinal
  tracheal trunks; d, e, air-tubes distributed to the vis-
  cera ; /. fat-mass ;  v, x, y, silk-secretors ;  z, their ex-
  cretory ducts, terminating in t, the spinneret, or fu-
  mlus. 
78
COMPARATIVE ZOOLOGY.
sects having mandibles, and is frequently lined with rows
of horny  teeth,  which are  specially developed  in Grass-
Piq.39—Alimentary Canal of a Beetle:   Fig. 40. — Alimentary Canal of the  Bee
  a, pharynx; 6, gullet, leading to crop,    (Apis melliflca): a, gullet; b, crop; c, d,
  c, gizzard, d, and stomach, e; f, deli-    stomach; e, small intestine; /, large in-
  cate biliary tubes; g, intestine; ft,    testine ; g, anal orifice ; ft,' biliary ves-
  other secreting organs.              sels; i, auxiliary glands.
hoppers, Crickets, and  Locusts.   The intestines  are  re-
markable for  their  convolutions.    Insects have no  true
liver;  but  its functions are performed  by  little tubes
(caeca) around the stomach.46
   The alimentary canal  of  Spiders is short and straight,
the pharynx and  gullet being very minute.  The stomach
is characterized by sending  out tubular prolongations, and
the intestine ends in a large bladder-like expansion.  Scor-
pio. 41.—Anatomy of a Sphinx Moth: n, nervous cord: n', brain sending off nerves to
   the legs, V, I", V", and for the wings at n"; ft, dorsal vessel, or heart; c, crop; «,
   stomach; i, intestines; o, reproductive organs; </, oviduct; 8-20, segments.
 
THE ALIMENTARY CANAL.
79
pions have no stomachal cavity—a straight intestine passes
directly through the body.
  In  bivalve  Mollusks, like the Clam, the mouth (which
is a mere aperture) opens at once  into the stomach, which
lies imbedded in  a large liver, and the intestine, describ-
ing a few turns,  passes directly through the heart.46  In
the univalve Mollusks, like the Snail, the  gullet is long,
and frequently expands into a crop; the stomach is often
double, the anterior being  a gizzard provided with teeth
for mastication ; the intestine passes through the liver, and
ends in the fore part of the body, usually on the right side.
  The highest Mollusks, as the Cuttle-fish and Nautilus,
exhibit  a  marked advance.   A   mouth  with  powerful
mandibles leads to a  long  gullet,  which ends in a strong
muscular gizzard  resembling  that of a  fowl.47   Below
this is a cavity, which is either a stomach or duodenum;
it  receives  the  bile  from  a large liver.   The intestine
is a tube of uniform  size, which, after one or two slight
curves, bends up,
and opens into the
"funnel "near the
mouth.
  Fishes  have  a
simple, short, and
wide   alimentary
canal. The stom-
ach  is  separated
from  the intestine
by a narrow " py-
loric" orifice,  or
valve, but  is not
so clearly  distin-
guished  from the
gullet,   so  that
regurgitation    is
Fig. 42. — Alimentary Canal of the Oyster: a, stomach
 laid open; d, liver; 6, c, d, /, convolutions of the intes-
 tine ; g, anal aperture; n, o, auricle and ventricle; I, m,
 adductor muscle; ft, k, lobes of mouth divided to show
 the venous canals at the base of the gills. 
80                  COMPARATIVE ZOOLOGY.
Fig. 43.—Anatomy of a Gasteropod (Snail): a, mouth ; b, foot; c, anus ; d, lung; e,
   stomach, covered above by the salivary glands; /, intestine; fir, liver; ft, heart;
   i, aorta; j, gastric artery; I, hepatic artery; k, artery of the foot; m, abdominal
   cavity, supplying the place of a venous sinus ; n, irregular canal communicating
   with the abdominal cavity, and carrying the blood to the lung; o, vessel carry-
   ing blood from the lung to the heart.
easy.48   Indeed, it is  common for Fishes to  disgorge  the
indigestible parts  of  their  food,  and some,  as  the  Carp,
g   hikealmn      f       o
Fig. 44.—Anatomy of a Lamellibranch (Mactra): a, shell;  &, mantle ; c, tentacles, or
   lips; d, mouth; e, nerves; /, muscles; g, anterior and, n, posterior ganglion; ft,
   liver; i, heart; k, stomach; I, intestine passing through the heart; m, kidney; o,
   anal end  of the  intestine; p, exhaleut and, q, inhalent respiratory tubes, or si-
   phons ; r, gills; «, foot. 
THE ALIMENTARY CANAL.
81
send the food back to the pharynx to be masticated.  The
stomach is  usually bent, like  a siphon; but  the intestine
is  nearly straight, and  without  any marked  distinction
into small and large.   Its appendages are a  large liver
and a  rudimentary pancreas.
   In the amphibious Reptiles, as the Frogs,  the digestive
apparatus is very similar to that of  Fishes;  but the two
kinds of intestines can be more
readily distinguished. The high-
er  Reptiles generally  have  a
long wide gullet, which passes
insensibly into the stomach, and
a short intestine (about twice the
length of  the  body)  very dis-
tinctly  divided into  small and
large by a constriction.49   The
vegetable-feeding Tortoises have
a  comparatively long intestinal
tube;  and the  Serpents  have  a
slender stomach, but little wider
than the rest of the alimentary  FlG- 46.-Anatomy of a cephaiopod
                              ^   (diagram): a, tentacles; b, masti-
Canal.                             catory apparatus; c, eye; d, sali-
   mi         1    r -v  r\     j-i    vary gland;  e, nervous ganglia;/,
   lhe Stomach Of the UrOCOdlle   oesophagus; g, internal shell, or
is  more complex than any hith-   ^XL*] Tt™£ S
erto mentioned.  It resembles   )pk-bag: « ovary; « oviduct; P,
                                  liver; r, gill contained m the bran-
that of the Cuttle-fish, but offers   chial chamber;  8, branchial heart;
                         .         t, systemic heart;  v, mantle.
a still  more striking analogy  to
the gizzard of  a Bird, having very  thick  walls, and  the
muscular fibres radiating precisely in the same manner.
So that, in  this respect, the Crocodile may be  considered
as the connecting link between Reptiles and Birds.60  It is
in Crocodiles also that the duodenum, a small pouch, with
which  the intestine begins, is first distinctly defined.  Into
this pouch, the liver and pancreas, or sweet-bread, pour
their secretions.  Furthermore, in the lower  animals, the
                            6
 
82
COMPARATIVE ZOOLOGY.
intestines lie more or  less loose in  the abdomen;  but in
the Crocodile, and likewise Birds  and Mammals, they are
supported by a membrane called mesentery.
  In Birds, the length of the alimentary canal varies with
Fig. 46.—Anatomy of the Carp: br, branchiae, or gill-openings; c, heart; /, liver; tn,
   swimming bladder;  ci, intestinal canal; o, ovarium; «, urethra. The side-view
   shows the disposition of the muscles in vertical flakes. 
THE ALIMENTARY CANAL.
83
their diet, being greatest in those living on grain and fruit.
The gullet corresponds in length with the neck, which is
longest in the long-legged tribes, and in width with the
food.  In those that swallow large fish entire, the gullet is
dilatable, as  in Snakes.   In nearly all Birds, the food is
delayed in some cavity before digestion: thus, the Pelican
has a bag under the lower jaw, and the Cormorant has a
capacious gullet, where  they store up fishes; while those
that gorge them-
selves   at inter-
vals, as the Vult-
ure, or  feed on
seeds and grains,
as  the  Turkey,
have   a  pouch,
called  the  crop,
developed   near
the lower end of
the gullet.61  The
Ostrich,  Goose,
Swan,  most  of
the Waders, and
the  fruit  or in-
sect eating Birds,
which find their
food in tolerable
abundance,  and
take it in small quantities, have  no such reservoir
eons have two crops.
   In all  Birds, the food  passes from the  gullet into the
proventriculus, or stomach proper, where it is mixed with
a  "gastric juice" secreted  from glands on  the surface.
Thence  it goes into the gizzard, an oval sac  of  highly
muscular texture,  and lined with a tough horny  skin.62
The gizzard  is most highly developed, and of a deep-red
Fig. 47.—Stomach of the Crocodile: a, muscular fibres
 radiating from a central tendon, b; d, commencement
 of duodenum; c, oesophagus; /, intestine.
Pig- 
84
COMPARATIVE ZOOLOGY.
color, in the Scratchers and flat-billed Swimmers (as Fowls
and Swans);  but comparatively thin  and feeble  in  Birds
of Prey (as the Eagle).   The gizzard is followed by the
                                     intestines, which are
                                     longer  than  those of
                                     Reptiles: the small in-
                                     testine  begins with a
                                     loop (the duodenum),
                                     and is  folded several
                                     times upon itself; the
                                     large intestine is short
                                     and  straight,  termi-
                                     nating  in the sole out-
                                     let of  the body, the
                                     cloaca.   A  liver and
                                     pancreas  are always
                                     attached to  the  upper
                                     part of the  small in-
                                     testine.
                                        The  alimentary ca-
                                     nal  in Mammals  is
                                     clearly separated into
                                     four distinct cavities:
                                     the pharynx, or throat;
                                     the oesophagus, or gul-
                                     let; the stomach; and
                                     the intestines.
                                        The   pharynx   is
                                     more complicated than
Fig. 48.—Digestive Apparatus of the Fowl:  1, in Birds.  It is  a fun-
  tongue; 2, pharynx; 3, 5, oesophagus;  4, crop;    ,  ,     ■,  ,      ,
  6,proventriculns; 7, gizzard; 8,9,10, duodenum; nel-SnapeOl  Dag,  nav-
  11,12, small intestine; 13, two caeca (analogue of  •     cPVpti   nnPniri0-<;
  the colon of mammals); 14, their insertion into IJ,&   btJVen   opeiiuigft
  the intestinal tube; 15, rectum; 16, cloaca; 17, leading" into  it:  tWO
  anus; 18, mesentery; 19,20, left and right lobes       °
  of liver; 21, gall-bladder;  22, insertion of pan- from the nostrils, and
  creatic and biliary ducts; 23,pancreas; 24, lung;       -       ,
  25, ovary; 26, oviduct.                    two  from  the   ears ;
 
THE ALIMENTARY CANAL.
85
one  from the windpipe,
guarded  by the  epiglot-
tis; one from the mouth,
with a fleshy curtain call-
ed the  soft palate ; and
one from the oesophagus.
It is the  natural passage
for  food  between  the
mouth  and  the oesopha-
gus, and  of air between
the  nostrils  and  wind-
pipe.   Like  the mouth,
it is lined  with a soft
mucous membrane.
  The  oesophagus  is  a
long and narrow  tube,
formed of  two  muscu-
lar layers:  in  the out-
side one, the  fibres run
lengthwise;  in the oth-
er,   they  are   circular.
It is also covered  more
or less  with a  sheath of
striated fibres, and  lined
with mucous membrane.
While in all Fishes, Rep-
tiles, and Birds the ven-
tral  chamber  is  one, in
Mammals it is divided,
by   a  partition  called
the diaphragm, into two
Fig. 49.—Digestive Apparatus of Man : 1, tongue; 2, pharynx; 3, oesophagus; 4, soft
  palate; 5, larynx; 6, palate; 7, epiglottis; 8, thyroid cartilage; 9, beginning of
  spinal marrow; 10,11,12, vertebrae, with spinous processes; 13, cardiac orifice of
  stomach ; 14, left end of stomach; 18, pyloric valve; 19, 20, 21, duodenum ; 22,
  gall-bladder; 27, duct from pancreas; 28, 29, jejunum of intestine; 30, ileum;
  34, coecum; 36, 37, 38, colon, or large intestine; 40, rectum.
 
86
COMPARATIVE ZOOLOGY.
cavities—the thorax, containing the heart, lungs, etc.; and
the abdomen, containing the stomach, intestines, etc.   The
Fig. 50.—Ideal Section of a Mammalian Vertebrate: A, pectoral, or fore limb; B,
   pelvic, or hind limb; o, mouth;  &, cerebrum; c, cerebellum ; d, nose;  e, eye; /,
   ear; 0, oesophagus; ft, stomach;  i, intestine;j, diaphragm, or midriff; k, rectum,
   or termination of intestine; I, anus; m, liver; n, spleen ; 0, kidney; p, sympa-
   thetic system of nerves; q, pancreas; r, urinary bladder; 8, spinal cord; w, ure-
   ter ; v, vertebral column; w, heart; x, lung; y, trachea, or  windpipe; z, epi-
   glottis.

oesophagus passes through a slit in the diaphragm, and al-
most immediately expands  into the stomach.
   In the majority of Mammals, the stomach is a muscular
bag  of an irregular oval  shape, lying obliquely across the
abdomen.  In the Flesh-eaters, whose food is  easy of solu-
tion, the stomach  is usually simple, and lies nearly in the
                               course  of the alimentary ca-
                               nal ;  but in  proportion as
                               the food departs more wide-
                               ly  in  its  composition from
                               the body itself,  and is there-
                               fore more difficult to  digest,
                               we find the stomach increas-
                               ing in size and complexity,
                               and turned  aside from the
                               general course  of the canal,
                               so  as  to  retain the  food a
                               longer time.63   The inlet, or
opening, into the oesophagus is called cardiac; the outlet,
Fig. 51.—Section of Horse's Stomach: A,
  left sac; JS, right sac; C, duodenum. 
THE ALIMENTARY CANAL.
87
or opening, leading into the intestines
is called pyloric.   In the Carnivores,
Apes, and most odd-toed quadrupeds,
the stomach resembles that of  Man.
That of the toothless  Ant-eater has
the lower part turned into  a kind of
gizzard for crushing its food.   The fig. 52—stomach of the p0r-
Elephant's is subdivided by  numerous P°ise: c'cardiac''*pyloric'
folds.  In the Horse, it is constricted in the middle;  and
in the Rodents, Porpoises, and Kangaroos, the constriction
is carried so far as to make two or three sections.  But
animals that  chew the cud (Ruminants)  have  the most
                                    complex stomach.  It
                                    is  divided  into four
                                    peculiar   chambers:
                                    First,   the   paunch
                                    (rumen), the largest
                                    of  all, receives  the
                                    half-masticated food
                                    when first swallowed.
                                    The inner  surface  is
                                    covered with papillae,
except in the Camel, which has large cells for storing up
water.   From this, the food passes into the honey-comb
stomach (reticulum), so named from its structure.  Liq-
uids swallowed usually go directly to  this cavity, without
passing through the paunch, and hence it is sometimes
Fig. 53.—Stomach of the Lion: c, cardiac orifice, or
      entrance of oesophagus; p, pyloric.
Fig. 54.—Complex Stomach of a Ruminant: a, gullet; b, rumen, or paunch;  c, reticu-
 lum ; d\ psalterium, or manyplies; e, abomasus; /, pylorus lending to duodenum. 
 88              COMPARATIVE ZOOLOGY.

 called  the water-bag.  Here the food is made into little
 balls, and returned to the mouth to undergo a thorough
 mastication.  When finally swallowed,  it is  directed, by
 a groove from  the oesophagus, to the third, and smallest,
 cavity, the many plies  (psalterium), named from its nu-
 merous folds, which  form  a strainer  to keep back any
 undivided food; and  thence it passes into the true stom-
 ach (abomasus), from which, in  the calf, the rennet  is
 procured for  curdling milk  in the manufacture of cheese.
                          This  fourth cavity is  like the
                          human stomach in  form  and
                          function,  and is the only part
                          which  secretes  gastric  juice.
                          The rumen and reticulum are
                          rather dilatations of the oesoph-
                          agus than parts of  the stomach
                          itself;  wliile the  latter is di-
                          vided by  constriction into two
                          chambers,  the psalterium  and
                          abomasus,  as  in many other
                          animals.
                            In structure, the stomach re-
                          sembles the  oesophagus.  The
                          smooth  outside  coat (perito-
                          neum)  is  a  reflection  of the
                          membrane,  which  lines  the
                          whole abdomen.   The middle,
                          or muscular,  coat  consists  of
                          three  layers of  fibres, running
lengthwise around and obliquely.   The successive con-
traction and relaxing of  these fibres produce the  worm-
like motion of the stomach, called peristaltic.   The inner-
most, or mucous, membrane, is soft, velvety, of a reddish-
gray color in  Man, and filled with multitudes of glands,
which secrete the  gastric juice.   The  human stomach,
Fig. 55.—Vertical Section of the
 Coats of the Stomach:  1, surface
 of mucous membrane, and months
 of gastric follicles; 2, gastric tubu-
 li, or follicles; 3, dense connective
 tissue ; 4, submucous tissue ;  5,
 trausverse muscular fibre; 6, lon-
 gitudinal muscular fibre; 7, fibrous,
 or serous, coat. 
THE ALIMENTARY CANAL.
89
when distended, will  hold about five pints;  that of  the
Kangaroo is as long as its body.
  The intestinal canal in Mammals begins at the pyloric
end  of  the stomach,  where there is a kind  of  valve or
circular  muscle.  Like the stomach, it varies greatly, ac-
cording to the nature  of the food.   It is generally longest
in the Vegetable-feeders, and shortest in the Flesh-feeders.
The greater length in the former is due to the fact that
vegetable food requires  a longer
time  for  digestion,  and  that  a
greater  bulk  of such food is re-
quired to obtain a given quantity
of nutriment.   The intestines meas-
ure 150 feet  in a full-grown  Ox,
while they are but three times the
length of  the body in  the Lion,
and  six  times in Man.    Save in
some lower forms, as  the Whales,
there are two main divisions, the
"small" and "large"  intestines, at
the junction of which is a valve.
The former is the longer of the
two,  and in  it digestion  is com-
pleted,  and  from  it absorption
takes place.  The large intestine is Fig, 56.-section of the wall of
            ..  . .    .     _    ,     the Human Intestine (ileum),
a temporary lodging-place for the   xso: o,viiii; 6andd,glands;
   i          r ,i   r  j     ,'i  ., •    e and e, mucous membrane; /,
useless part of the food, until  it is   circular  muscles; 9t A,i0ngi-
expelled from  the body.  The be-   tudinal mU8cle8-
ginning  of the small intestine is called  the  duodenum,
into  which  the ducts  from the liver and  pancreas open.
The  intestinal canal has the same  structure as the stom-
ach, and by a peristaltic motion its  contents are propelled
downward.  The  inside surface of the small intestine is
covered with a host of thread-like processes (villi), resem-
bling the pile of velvet.
 
90
COMPARATIVE ZOOLOGY.
  In taking this general survey of the succession of forms
which the digestive apparatus presents among the princi-
pal groups of animals, we can not fail to trace a gradual
specialization.   First, a simple excavation in the body,
one orifice serving as  inlet for food and outlet for indi-
gestible matter; next, a sac, or short tube, with walls of
its own suspended in the body-cavity;  then, a canal pass-
ing through the body, and, therefore, having both mouth
and vent; next, an apparatus for mastication, and a swell-
ing of the central part of the canal into a stomach, having
the special endowment of secreting gastric juice;  then, a
distinction between the small and large intestine, the for-
mer thickly  set with villi, and receiving the secretions of
large glands.   We also notice that food, the means of
obtaining  it, the instruments for mastication, and the size
and complexity of  the alimentary canal,  are closely re-
lated.
                    CHAPTER X.
                 HOW ANIMALS  DIGEST.

  The object of the digestive process is the reduction
of food into such a state that it can be absorbed into the
system.   For this purpose, if solid,  it is dissolved; for
fluidity  is  a  primary condition, but  not the only one.
Many  soluble substances  have to undergo  a chemical
change  before they can form parts  of the living body.
If albumen or sugar be injected into the veins, it will not
be assimilated, but be cast out unaltered.
  To produce these two essential  changes,  solution  and
transmutation, two agencies are used — one  mechanical,
the other chemical.   The  former is  not always needed,
for many animals find their food already dissolved, as the 
HOW ANIMALS DIGEST.
91
Butterfly ;  but solid  substances,  to  facilitate their solu-
tion, are ground or torn into pieces  by teeth, as in Man;
by jaws, as in the Lobster; or  by  a gizzard, as in the
Turkey.
  The chemical preparation of food  is indispensable.64  It
is accomplished by one or more solvent fluids secreted in
the alimentary canal.   The most important, and one al-
ways  present, is the gastric juice, the secretion of which
is restricted to the stomach, when that cavity exists.  In
the higher animals, numerous glands pour additional flu-
ids  into the digestive tube, as saliva into the upper part
or mouth,  and bile and pancreatic  juice into the upper
part of the  intestine.   In fact,  the mucous membrane,
which lines the alimentary canal throughout, abounds with
secreting pores.
  The Digestive Process is substantially the same in all
animals, but  it is carried further in the more highly de-
veloped forms.  In the Infusoria, the food is acted upon
by some secretion from the walls of the body-cavity, the
exact nature of which is unknown.   In the  Star-fish and
Sea-urchin, we find  two  solvents — a gastric juice, and
another resembling bile; but the two appear to mingle in
the stomach.   Mollusks  and Articulates show a clear dis-
tinction between  the stomach and intestine, and  the con-
tents  of the liver are poured into the latter.  There are,
therefore, two stages  in the digestive act: first,  the food
is dissolved by the gastric juice  in  the stomach, forming
chyme; secondly, the chyme, upon entering the intestine,
is changed into chyle by the action of the bile, and is then
ready to be absorbed into the system.
  In Yertebrates, a third solvent is  added, the pancreatic
juice, which  aids the bile in completing digestion.  But
Mammals have a still  more perfect and elaborate  proc-
ess; for in them  the saliva of the mouth acts chemically
upon  the food; while the  saliva  in  all other animals has 
92
COMPARATIVE ZOOLOGY.
no other office, so far as we  know, than to moisten the
food for swallowing.
  Taking Man as an example, let us note the main facts
in the process.  During mastication, by which the relative
surface  is increased, the food  is  mixed with saliva, which
converts the starch into sugar.66  Passed into the stomach,
the now sweetish, pulpy mass is subjected to the action of
the gastric juice, a peculiar acid, which has no effect on
starch or oil,  but readily dissolves the albumen,  fibrine,
gelatine, and like constituents of the food.66  While this
solution is going on, the  muscular walls  of the stomach
successively contract and relax, rolling the food  about,
and mixing it thoroughly with  the gastric  juice,  and, at
the same time, moving the whole mass toward the pyloric
                         orifice.67   Some of the chyme
                         thus produced  is at once ab-
                         sorbed into the blood-vessels of
                         the stomach;  but  the  greater
                         part is pressed forward through
                         the pylorus into the duodenum.
                         As  soon as the  chyme enters
                         this cavity, it separates  into a
                         white, creamy substance, called
 Fig. 57.-chyie corpuscles, x 500.  chyle,   and  a  residuary mass,
which  is  gradually converted into faeces,  and expelled
from the system.68  Exactly how this change from chyme
to chyle is produced is not known; but it is the most im-
portant  part of the digestive process.
  Chyme differs from food  in  having starchy particles
changed into  sugar, and much  of the  albuminous  por-
tion chemically altered by the gastric juice; but the con-
version  of the starch  is not  complete, and certain albu-
minous  parts, and all of the oily  particles, remain un-
touched.  In the duodenum, the whole mass is acted upon
by secretions  from the  liver and pancreas.   While the
 
THE ABSORBENT SYSTEM.
93
gastric secretion  was acid, these are alkaline;  and their
office seems to be — the subdivision of the  fatty matter
into minute particles (emulsion),  till  they  are  diffused
through the liquid, like atoms of butter in milk, thus fit-
ting it to be absorbed into the blood; and the completion
of the work begun by the saliva and gastric juice.  In this
they are assisted  by  the "intestinal juice" secreted by the
mucous lining of the small intestine, the action of which
is merely  supplementary.  The chyle  is  slowly driven
through  the small intestine  by the creeping,  peristaltic
motion of  its  walls,  the nutritious portion being taken  up
by the absorbents, as described in the next chapter, while
the undigested part remaining is discharged from the large
intestine.
                   CHAPTER XL
                 THE ABSORBENT SYSTEM.

  The nutritive matter (chyle), prepared by the digestive
process, is still  outside of  the organism.  How shall it
enter the  living tissue?
  In animals, like the Infusoria and Polyps, whose digest-
ive  department is not separated from the body-cavity, the
food, as soon as dissolved, mingles freely with the tissues
and organs it has to nourish.   In the higher Invertebrates
having an alimentary canal, the chyle passes, by simple
transudation, through the walls of the canal directly into
the  soft tissues, as in Insects, or is absorbed from the canal
by  veins in contact with  it, as in Sea-urchins, Mollusks,
Worms, and Crustaceans, and then distributed through the
body.
  In Yertebrates only do we find a special absorbent sys-
tem.  Three sets of vessels are concerned in the general 
94
COMPARATIVE ZOOLOGY.
process by which fresh material is taken up and added to
the blood : Yeins, Lacteals, and Lymphatics.  Only the
two former draw material from the alimentary canal.
   It is a general law that the food is absorbed as fast as
it is dissolved, and, therefore, there is a constant loss in
the  passage down the canal.  In the mouth and oesopha-
gus, the absorption is slight; but much of that which has
yielded to the gastric juice, with  most of  the  water, is
greedily absorbed by the veins of the stomach, and made
to join the current of blood which is rushing to  the liver.
Absorption by the veins also takes place from the skin
and lungs.  Medicinal or poisonous gases  and liquids are
readily introduced into the  system by these channels.
   We have seen that the oily part of the food passes un-
changed from the stomach into the small intestine, where,
acted upon by the pancreatic juice, it is  cut up  into  ex-
tremely minute particles.   These, and the remaining nu-
                            tritive substances not taken
                            up by the blood-vessels, enter
                            the system in a roundabout
                            way.  A  multitude  of  mi-
                            croscopic tubes form a net-
                            work in the walls of the  in-
                            testine,  and  even  run into
                            the little velvety villi  with
            ?  ^j^r        which the intestine  is lined.69
Fig. 58.—Lacteal System of Mammal: a, rp,    .,,.    .   .
 descending aorta, or principal artery;  J-De Villi, projecting into the
 b, thoracic  duct;  c, origin  of lacteal J•__<.,_j  c^^a  vi       1.1 j.
 vessels, g, in the walls of?the intestine, <"gested  food,  like rootlets
 d; e mesentery, or membrane attach- into  the  sojl   abSOrb  the
 ing the intestine to walls of the body;               '
 /, lacteal, or mesenteric, glands.      chyle, which  is immediately
passed into the net-work of tubes, called  lacteals, from
the milky character  of  the chyle which they convey, in
Mammals.  These lacteals unite into  larger trunks, which
lie in the mesentery (or membrane which suspends the in-
testine to the back wall of  the abdomen),  and these  pour
 
THE ABSORBENT SYSTEM.
95
 their contents into one large vessel, the thoracic duct, ly-
 ing along the backbone, and joining the great jugular vein
 in the neck.
   While the lacteals spring from the intestine only, the
 lymphatics come from  all parts of the body,60 more es-
 pecially the skin.  The two
 are closely allied  in struct-
 ure  and office,  and  both
 empty   into  the  thoracic
 duct;  so that they  may
 be  regarded as two sets of
 roots issuing from one com-
 mon trunk.  They differ in
 the nature of the fluid they
 contain, the lymphatics car-
 rying  transparent lymph,
 abounding   with  minute
 colorless disks,  or  cells;
 while  chyle has, in addi-
 tion,  a crowd of oil-glob-
 ules, which render it white
 and opaque.61  Both coag-
 ulate upon exposure to the
 air.  The chief function of
 the lymphatics seems to be
 to gather up matters which
 have  served a purpose in
 the system, but which may
 again be used in the blood,
 and to return to  the circu-
lation that superfluous part of the blood poured out upon
the tissues, over and above what was needed.
  Like  the  roots of Plants, the absorbent vessels do not
commence with open mouths; but the fluid which  enters
them  must traverse  the membrane which  covers their
Fig. 59.—Principal Lymphatics of the Hu-
 man Body: a, union of left jugular and
 subclavian veins; b, thoracic duct; c, re-
 ceptacnlum chyli. The oval bodies are
 glands. 
96
COMPARATIVE ZOOLOGY.
minute extremities.  The pores of this membrane (which,
though invisible, are demonstrable) are so many short cap-
                        illary tubes; so that  the passage
                        of the chyle is a physical neces-
                        sity, and is not dependent on a
                        vital  or some other mysterious
Fig. 60.—Diagram of a  Lymph  principle.   But  as capillary  at-
 Gland, showing change from ep-      .               .       .
 itheiiai to nucleated cells.      traction can not produce motion
beyond  the  capillary tubes, another force is  called into
play, namely, diffusion.  The chyle, having passed through
the membrane, meets a fluid (lymph), with which it is mis-
cible, and diffuses itself into that fluid, leaving room for a
fresh supply to enter the membrane.62  In this way a slow
but strong current is kept up, which is probably aided by
a rhythmical contraction of successive portions of the lac-
teals themselves.  In Fishes and Reptiles, the absorbents
are provided with pulsating sacs, called lymphatic hearts.
In Birds  and Mammals, they are furnished with valves,
which prevent the backward flow of the contents.
                  CHAPTER XII.
                THE BLOOD OF ANIMALS.
  The Blood, is that peculiar fluid derived from digested
food which carries to the living tissues the materials nec-
essary to their growth and repair.   The great bulk of the
body is occupied with apparatus for the preparation and
circulation of this vital fluid.
  The blood of the lower animals (Invertebrates) differs
so widely from  that of Man  and other Yertebrates, that
the former were long supposed to  be  without blood.  In
them the blood  is commonly colorless; but it has a bluish
 
THE BLOOD OF ANIMALS.
97
cast  in Crustaceans; reddish, yellowish, or  greenish, in
Worms;  and reddish, greenish, or brownish,  in  Jelly-
fishes.   The  red liquid which appears when  the head of
a Fly  is crushed is not blood, but  comes from  the eyes.
In Fishes, Reptiles, Birds, and Mammals, the  blood  is red,
excepting the white-blooded fish, Amphioxus*3
  As a rule, the more simple the fabric of the body, the
more simple the nutritive fluid.  In Sponges and Infu-
soria, it is sea-water carrying organic particles; in the low
Polyps and Jelly-fishes, it is  merely chyme; in all other
Invertebrates, having a complete alimentary canal, it  is
chyle; in the backboned animals, it is a highly complex
and  distinct fluid.
  In all animals,  however,  from  Sponge  to  Man, the
blood, apparently a clear, homogeneous fluid, really con-
sists of minute grains, or globules, of organic matter float-
ing in water.  If the blood of a Frog be poured on a filter
of  blotting-paper,  a
transparent fluid (call-
ed plasma)  will pass
through,  leaving red
particles,  resembling
sand, on the upper sur-
face.  Under the mi-
croscope, these  parti-
cles  prove to be cells,
or flattened disks (call-
ed corpuscles) contain-
ing  a nucleus;  some
                j   i   Fig. 61.—Red Blood-corpuscles  of Man:  a, shows
are  COlorleSS, and Oth-   circular contour; b, a biconcave section; c, a
ersred.  The red disks   g™Pinchains-
have a tendency to  run together;  the colorless ones re-
main single.  Meanwhile, the plasma  separates into two
parts by coagulating; that is, minute fibres form, consist-
ing  of fibrine,  leaving a pale yellowish fluid, called se-
                           7
 
98
COMPARATIVE ZOOLOGY.
rum."*  Had the  blood not been filtered,  the corpuscles
and fibrine would have mingled, forming a jelly-like mass,
known as clot.   Further, the serum will  coagulate if heat-
ed, dividing into  hardened  albumen and  a watery fluid,
called serosity, which  contains  the  soluble  salts of  the
blood.
   These several parts may be expressed thus:
            , f.     .   (colored 1
            ( Corpuscles ^f-
Blood
=clot.

Plasma
                    (fibrine^jalbumen.
                    (serum  (serosity = water and salts.
If now we examine the nutritive fluid  of the simplest
                               animals,  we  will  find
only a watery fluid con
taining  granules.    In
Radiates  and  the  low-
est Articulates and Mol-
lusks, there is a similar
fluid, with the  addition
of a few white corpus-
cles.   But  there  is no
fibrine, and, therefore, it
                     In
does not coagulate.
Fig. 62.—Nucleated Blood-cells of a Frog, x 250.  the  higher  Articulates
and Mollusks, the circulating fluid resembles the chyle as
we find it in the thoracic duct of Yertebrates, containing
colorless nucleated  cells, and  coagulating.66   In Yerte-
brates, there are,  in  addition to  the
plasma and white corpuscles of  In-
vertebrates, red corpuscles, to which
their blood owes its peculiar hue.   In
Fishes, Reptiles, and Birds, i. e.,  all
the backboned animals born  from
eggs, these red  corpuscles are nucle- T
 00 '      #       r                  Fig. 63.—Elliptical Corpuscle
ated; but in those of Mammals,  no   of  «« Frog, showing a
   ,    ,    ,      ,.       , .«          white prominence at the
nucleus has been discovered.           centre.
 
THE BLOOD OF ANIMALS.
99
  All blood-corpuscles are microscopic.   The  white are
more uniform in size than the red; and generally smaller
(except in Mammals), being about -g-gVo- of an inch in di-
ameter.   The red  corpuscles are largest in Reptiles (those
of the amphibious Proteus being the  extreme, or -3-^5- of
an inch), next in Fishes, then Birds and Mammals.  The
smallest known are those of the Musk-deer. In Mammals,
the size agrees with the size of the  animal only within a
natural order; but in Birds  the correspondence holds good
throughout the class, the largest being found in the Os-
trich, and the smallest in the Humming-bird.   In Man,
they measure 3-3V0- of an  inch, so that it  would take
40,000 to cover the head of  a pin.
  As to shape, the colorless  corpuscles are ordinarily glob-
ular, or sac-like, in  all animals; but they are constantly
changing.  The form of the red disks is more permanent,
although they are soft and elastic,  so that they squeeze
through very narrow passages.   They are oval, circular,
Fig. 64.—Comparative Size and Shape of the red Corpuscles of various Animals. 
100
COMPARATIVE ZOOLOGY.
or angular, in Fishes; oval in  Reptiles, Birds, and the
Camel tribe; and circular in the rest of Mammals.   They
are double-convex when  oval, and double-concave  when
circular.
  Blood is always heavier than water; but is thinner in
cold-blooded than in  warm-blooded animals, in herbivores
than in carnivores.   The blood of birds, which is the hot-
test known, being 10° higher than Man's, is richest in red
corpuscles.  In  Man, they constitute about one-half the
mass of blood.   The  white globules are far less numerous
than the red; they are more abundant in venous than ar-
terial blood, in  the sickly and ill-fed than in the healthy
and vigorous, in the  lower Yertebrates than in Birds and
Mammals.
  There is less blood in cold-blooded than in warm-blood-
ed animals; and the larger the animal, the greater  is the
Fig. 65.—Capillary Circulation in the Web of a Frog's Foot, X 100: a, b, small veins;
   d, capillaries in which the oval corpuscles are seen to follow one another in sin-
   gle series; e, pigment-cells in the skin. 
THE BLOOD OF ANIMALS.
101
proportion of blood to the body.  Man has about a gallon
and a half, equal to one-tenth of his weight.  The heart of
the Greenland Whale is a yard in diameter.
  The main Office of the Blood is  to  supply nourish-
ment to, and take away waste matters from, all parts of
the body.  It is at  once purveyor and scavenger.  In its
circulation, it passes, while in the arterial half of the cap-
illaries, within an infinitesimal distance of the various tis-
sues.   The plasma, carrying the nutritive  matter  needed,
exudes through the walls of the capillary tubes;  the tissue
assimilates or makes like to itself whatever is suitable for
its growth and  repair; and the lymphatics  (the escape-
pipes) take up  any surplus, and return it to the blood.
At the same time, the venous part of the capillary net-
work absorbs the waste products of the tissues, expelling
the gases by the lungs, and the solid matters by the skin
and kidneys.  The special function of the several constit-
uents of the blood  is not clearly known.  The colorless
corpuscles in Yertebrates are supposed to be the source of
the red disks.   The latter are probably  the carriers of
gases, absorbing oxygen in the lungs, which  they give to
the tissues, and receive carbonic acid in exchange.
  Like  the  solid tissues, the blood, which is  in  reality a
liquid tissue, is  subject to waste and  renewal, to growth
and  decay.   Its source is the product of digestion,  not
only  the lacteal chyle, which is the only fluid  in the body
that  has  never  formed  part of the  blood, but also  the
chyme  absorbed by  the  blood-vessels  of the  stomach.
Chyle differs from blood chiefly in containing  less albu-
men  and fibrine, and no red disks.  The transmutation is
accomplished by the small glands (mesenteric) attached to
the lacteals, the  lymph, and the lungs.  In the low organ-
isms, the nutritive  fluid is  prepared  by contact with  the
tissues, without passing through special organs. 
102
COMPARATIVE ZOOLOGY.
CHAPTER XIII.
            THE CIRCULATION OF THE BLOOD.
  The Blood is kept in continual motion in order to
nourish and purify the body and itself.  For as life means
work, and work brings waste, there is constant need of
fresh material to make good the loss in every part of the
system, and of the removal of matter which is  no longer
fit for use.
  In the very lowest animals, where every  part of the
                              structure is equally capa-
                              ble of absorbing  the di-
                              gested food and is in con-
                              tact  with it, there  is no
                              occasion  for any  circula-
                              tion,  although  in  them
                              even it is  not allowed to
                              stagnate.    But in propor-
                              tion  as the power of ab-
                              sorption is  confined to cer-
                              tain  parts,  the more need
                              and the greater  complexi-
                              ty of an apparatus for con-
                              veying the  nutritious fluid
                              to the various tissues.
                                In  nearly all animals,
                              the nutritive fluid is con-
                              veyed to the various parts
                              of  the body by  a system
                              of  tubes, called  blood-ves-
Fig. 66.—Venous Valves.  They usually oc-
     cur iu pairs, as represented.       Sets.   The  higher forms
 
THE CIRCULATION OF THE BLOOD.        103
ib^t
"Wffl
       nun
MM
have two sets — arteries  and veins,  in  which the blood
moves  in opposite directions, the former carrying blood
from a  central reservoir or heart, the latter taking it to
the heart.   The walls of these tubes are made of three
coats, or layers, of tissue, the ar-
teries being elastic, like rubber,
and many of the veins being fur-
nished with valves.67   In Yerte-
brates,  the  great artery coming
out of the  heart is called  aorta,
and  the  grand venous trunk, en-
tering  the  heart on the  opposite
side, is  called vena cava.   Both
sets  divide  and subdivide  until
their branches are finer than hairs;
and  joining these finest arteries
and  finest veins are intermediate
microscopic tubes, called capilla-
ries (in Man about -g-sVo- of an inch
in diameter).68  In these only, so
thin and delicate are their walls,
does the blood  come in contact
with the tissues or the air.
   In all animals having  a special
organ  for respiration (gills  or lungs), there are two sets of
capillaries,  since there are two circulations—the systemic,
from the heart around the  system to the heart again, and
the pulmonary, from the  heart  through the respiratory
organ  back to the heart.  This double  course may be il-
lustrated by the figure 8.
   There is  no true system of blood-vessels below the  Star-
fish.  The  canals of the  Sponge are  really on the outside
of the animal.  In the Infusoria and Polyps, the nutritive
matter mixed with water rises and falls in the body-cavity.
The simplest  provision for the distribution of the products
Fig. 67. — Relation of artery, a,
 vein, b, and capillaries, c, as
 seen in the muscles of a Dog. 
104
COMPARATIVE ZOOLOGY.
of digestion  is shown by the Jelly-fish,  whose stomach
sends off radiating tubes.
  The first Approach to a Circulatory System is made
by the Star-fish and  Sea-urchin.  A vein runs along the
whole length of the alimentary tube, to absorb the chyle,
and forms a circle around each end of the  tube.  These
circular vessels send off branches to various parts of the
body; but as they are not connected by a net-work of cap-
illaries, there can be no circuit (Fig. 37).
  The next higher type is exhibited  by the Articulates.
H we examine the back of any thin-skinned Caterpillar,
                 a long  pulsating tube is seen running
                 beneath the skin from one  end  of the
                 body to the other.   This  dorsal vessel,
                 or heart, as it is called, is  open at  both
                 ends, and  divided by valves into com-
                 partments, permitting the blood to go
                 forward, but not backward.  Each com-
                 partment communicates by a pair  of
                 slits, guarded by valves, with the body-
                 cavity, so that fluids may enter, but can
                 not escape.   " Circulation " is very sim-
                 ple.  We have seen  that the chyle ex-
                 udes through the walls of the aliment-
                 ary canal directly into the cavity of the
                 abdomen,  where it  mingles with  the
                 blood already there.   This mixed fluid
                 is drawn  into the dorsal tube through
                 the valvular  openings  as  it expands;
Fio. 68.—Part of the Dor-
 sal Vessel, or Heart, of
 a Cockchafer bisected:
 a, b, muscular walls;
 d, valves between the and Upon  its Contraction, all the  Side-
 compartments ; c, valve   •■          ,   ,     n
 defending one of the valves  are  closed, and
 ttZZSES forced toward the head.
IS
                                           the fluid
                                            Passing out
                 at the front opening,  it is again dif-
fused among and between the tissues of the body.  The
blood,  therefore, does  not  describe a circle in  definite
of the abdomen. 
THE CIRCULATION OF THE BLOOD.        105
channels  so as to return  constantly  to  its point of  de-
parture.
  Certain Worms (as the Earth-worm) have  a ventral
tube, conveying the blood from head  to tail, with numer-
ous cross  canals joining the ventral and dorsal tubes.  In
the  Lobster and  Crab, Spider  and Scorpion, the dorsal
Pig. 69.—Circulation in a Lobster: o, heart; 6, artery for the eyes; c, artery for an-
   tennae ; d, hepatic artery; e, superior abdominal artery; /, sternal artery; g, ve-
   nous sinuses transmitting blood from the body to the branchise, h, whence it
   returns to the heart by the branchio-cardiac vessels, *'.
tube sends off a system of arteries (not found in Insects);
but the blood, as it  leaves these  tubes, escapes into the
general cavity, as in other Articulates.   The Lobster and
Crab, however, show a great advance in the concentration
of the  propelling power  into a  short muscular sac—the
first rudiment of a true heart.
  A third development of the circulatory system  is fur-
nished  by the Mollusks.   Comparatively sluggish,  they
need  a powerful  force-pump in the form  of a compact
heart.   In the Oyster and  Snail,  we find such an organ
having two cavities — an  auricle and a ventricle, one for
receiving, and the other for distributing, the blood.  The
auricle injects the blood into  the ventricle, which propels
it by  countless arteries to the various organs.  Thence it
passes, not immediately to the veins, as in higher animals,
but into the spaces around the alimentary canal.  A part
of this is carried  by  vessels to the  gills  or lung, and  then
returned with  the unpurified portion to the auricle.  The 
106
COMPARATIVE ZOOLOGY.
—k
whole of the blood, therefore, does not make  a complete
circuit.
  A still higher form is seen in the Cuttle-fish, the high-
                         est of  Invertebrates.   Instead of
                         a double heart, we find a triple
                         one, a ventricle  to  throw  the
                         blood  over the system, and  two
                         auricles to force it into the gills.
                         Hitherto, there has been a  pro-
                         pelling cavity on  the  " arterial"
                         side only; but now  the veins
                         which collect the blood from the
                         system to send it back  to the
                         heart  by the  way of  the  gills,
                         are furnished  with two branchi-
                         al  hearts, which  accelerate the
                         circulation through those organs.
                         Many of the  arteries and veins
                         are joined by capillaries, but not
                         all;  so that in no invertebrate
                         animal is the  blood returned  to
                         the heart by a continuous closed
                         system of blood-vessels.
                            As  a rule, in all animals  hav-
                         ing any  circulation  at all, the
                         current always takes  one direc-
                         tion.  This is generally necessi-
                         tated  by valves.   But a  curious
                         exception is  presented  by  the
                         Ascidians (a group of low Mol-
                         lusks), whose  tubular  heart is
                         valveless, and the contractions
                         occur alternately at one end and
                         then the other; so that the blood
oscillates to  and fro, and  a given vessel is at  one  time a
Fig. 70.—Circulating Apparatus in
 the Fish: a, branchial artery; 6,
 arterial bulb; c, ventricle; d,
 auricle; e, venous sinus; /, port-
 al vein; g, intestine; h, vena
 cava; i, branchial  vessels; k,
 dorsal artery, or aorta; I, kid-
 neys ; m, dorsal artery. 
THE CIRCULATION OF THE BLOOD.        107
vein and  at  another an artery.   In this respect it resem-
bles the foetal heart of higher animals.
  In Yertebrates only is the circulating  current  strictly
confined to the blood-vessels; in no case does it escape
into the general cavity of the body.  In  other respects,
there is no great advance in the apparatus of  the lowest
Yertebrates over that of the highest Mollusks.   The heart
of Fishes, as in the Oyster, is double, but its position is re-
versed.   Instead of driving arterial blood over the body,
it receives the returning,  or venous, blood, and sends it to
the gills.  Re-collected from
the gills  by  minute  tubes,
called capillaries, the blood
is passed into a large artery,
or  aorta,  along  the back,
which distributes  it  by a
complex  system  of  capil-
laries  among the  tissues.
These capillaries unite with
the ends of the veins which
pass the blood into the auri-
cle of the heart.69
  In  Reptiles generally  (as
Frogs, Snakes, Lizards, and
Turtles), the  heart  has  three cavities—two auricles  and
one ventricle.   The  venous blood  from  the body is re-
ceived into the right auricle, and the purified blood from
the lungs into the  left.   Both throw their contents  into
the ventricle, which pumps the mixed blood in two direc-
tions—partly to the lungs, and partly around the  system.
Circulation is, therefore, incomplete, since the  whole  cur-
rent does not pass through the lungs, and three kinds of
blood are found in the body—arterial, venous, and mixed.
  The ventricle of these Reptiles is  partially divided by a
partition.  In the Crocodile, the division is complete, so
Fig. 71.—Diagram of a single Heart: d,
 auricle; e, ventricle; c, veins leading to
 auricle; a, aorta, or main artery. 
108
COMPARATIVE ZOOLOGY.
that there are really four cavities—two auricles, and  two
                             ventricles.  But  both  ventri-
                             cles send  off  aortas  which
                             cross one another, and  at that
                             point a small aperture brings
                             the  two  in  communication.
                             The venous and  arterial cur-
                             rents  are,  therefore,  mixed,
                             but  not  within the heart, as
                             in the other  Reptiles,  nor so
                             extensively.  In the structure
Fig. 72—Heart of the Dugong, a typical of the heart, as  Well  as  giz-
 four-chambered heart, the parts being
 more separated than in higher ani- Zard, Crocodiles approach the
 mals: E, right ventricle; L, left ven- -p.  -,
 tricle; D, right auricle;  F, pulmonary -DiruS.
 artery; K, left auricle; A, aorta.       The Highest Form Of the
Circulating System is possessed  by the warm-blooded
Yertebrates, Birds, and Mammals.  Not  a  drop  of blood
can make the circuit of the body without passing through
the lungs, the circulation  to and from those organs being
as perfect  as  the distribution      /   g    t\    g
of arterial blood.  The heart
consists of  four  cavities — a
right  auricle  and  ventricle,
and a left auricle and ventri-
cle.   In  other  words, it  is a
hollow  muscle  divided inter-
nally by  a vertical  partition
into  two distinct  chambers,
each of  which is  again  di-
vided by a Valve  into an ail- Fl£ *3.-Theoretical Section of the
        J                          Human Heart: a, right ventricle :
ricle and  a ventricle.   The
work of the right auricle and
ventricle  is  to   receive   the
blood  from  the  veins,  and
send  it  to the lungs;  while  the other two receive  the
b, inferior vena cava; c, tricuspid
valve; d, right auricle ; e, pulmona-
ry veins; /, superior vena cava; g,
pulmonary arteries; h, aorta; k, left
auricle ; I, mitral valve; m, left ven-
tricle ; n, septum. 
THE CIRCULATION OF THE BLOOD.
109
monary artery; e, pul-
monary veins, bringing
blood from the gills, d,
and uniting in the aorta,
/; g, vena cava.
blood  from the lungs, and propel it
over  the  body.    The  left ventricle
has more  to do than any other cavi-
ty.   The two auricles contract at the
same instant;  so,  also, do the  ventri-
cles.   The sounds  which they respect-
ively make may be  imitated  by  the
words lubb, tup.   The course  of the
current  in Birds and Mammals is as
follows:  the   venous  blood  brought
from the system is discharged by two
or three large trunks70 into the right
auricle,  which immediately forces it Fig. 74.—piaTof circuia-
   ,      1   ti • ,   .1    •  1 ,    ,  • i     tion in Fishes: a, auri-
past a valve  into  the right ventricle.   de. 6iVentricie;  c, Pui-
The ventricle then contracts, and the
blood  rushes  through the  pulmonary
artery past its semilunar valves  into
the lungs, where it is changed from blue to crimson, re-
turning  by the pulmonary vein to the left auricle.   This
                                    sends  it  past the  mi-
                                 ^  tral valves into the left
                                    ventricle, which drives
                                    it  by  the  semilunar
                                    valves  into  the aorta,
                                    and thence, by its  ram-
                                    ifying arteries and cap-
                                    illaries,  into  all  parts
                                    of the body except the
                                    lungs.  From the cap-
                                    illaries, the blood, now
                                    changed from crimson
Piob.75 76  -A, Plani of Circulation in Amphibia t  ^    •  gathered by
  and Reptiles; B, Plan of Circulation in Birds        '   r>          J
  and Mammals: a, right auricle receiving venous the veins  and Conveyed
  blood from the system; b, left auricle receiving         '
  arterial blood from the lungs; c, c>, ventricles; back  to the heart.
  d, e, f, systemic artery, vein, and capillaries; g,                      ,
  h, k, pulmonary artery, vein, and capillaries.       The Rate  OI   tn6
 
110
COMPARATIVE ZOOLOGY.
Blood-current generally  increases with the activity of
the animal, being most rapid in Birds."  In Insects, how-
ever, it is comparatively slow; but this is because the air
is taken to the blood—the whole body being bathed in air,
so that the blood has no need to hasten to a special organ.
Nevertheless, the pulsations in a Bee at rest are nearly
doubled when it is lively.  The motion in the arteries is
two or three times faster than in the veins, but diminishes
as the distance from the heart increases.   In the aorta of
the Horse, the blood moves 12^ inches per second; in that
of Man, 10^; in the capillaries of Man, 2 inches per min-
ute ; in those of a Frog, 1.
  The Cause of the Blood-current may be  cilia, or the
contractions  of  the body,  or  pulsating  tubes or hearts.
In the higher animals, the impulse of the heart is not the
sole means:  it is aided by the contractions of the arteries
themselves, the movements  of the chest in respiration, and
the attraction of the  tissues for the arterial blood in the
capillaries.   In the Chick, the blood moves before the
heart begins to beat; and if  the heart of an animal be
suddenly  taken  out, the motion in  the capillaries  will
continue as before.   It has been estimated that the force
which the human heart  expends in twenty-four hours is
equivalent to lifting 124 tons one foot.
                  CHAPTER XIY.
             HOW ANIMALS  BREATHE.

  Arterial Blood, in passing through the system, both
loses and gains  certain substances.   Its loss is made good
by fresh products of digestion; and the solid or fluid waste
matters which it has taken up are removed by excretion. 
HOW ANIMALS BREATHE.            Ill
But  it becomes tainted, also, by carbonic acid gas, and
loses the free oxygen which it  possessed.  It is this dif-
ference in the gaseous  contents which makes the great
difference between the crimson blood of the arteries and
the dark-purple blood of the veins in the higher animals.
  The First Object of Respiration is to convert venous
into  arterial blood.  It is done  by bringing it to the sur-
face, so that the carbonic acid may be exhaled and oxygen
absorbed.  The apparatus for this purpose is analogous to
the one used for circulation. In the lowest animals, the
two  are combined.   But in the  highest, each is essentially
a pump, distributing a fluid (in one case air, in the other
blood) through a series  of tubes to  a  system of cells or
capillaries.   They are also closely related to each other:
the more perfect the circulation, the more careful the pro-
vision made for respiration.
  Respiration is performed  either in  air or  in water.
So  that  all animals  may  be classed as air-breathers or
water-breathers.  The latter are,  of course, aquatic,  and
seek the air which is dissolved in the water.  Land-snails,
Myriapods, Spiders,  Insects, Reptiles, Birds, and Mam-
mals breathe air directly; the  rest,  with  few exceptions,
receive it through the medium of water.  In the former
case, the organ is internal; in the latter, it is more or less
on  the  outside.  But however  varied the organs—tubes,
gills, or lungs—they are all constructed on the same prin-
ciple.
  (1) Sponges, Infusoria, and Polyps have no separate respir-
atory apparatus,  but absorb air, as well as food, from the
currents of water passing through them.
  In the Star-fish,  Sea-urchin, and the like, we find the
first distinct respiratory  organs, although none are exclu-
sively devoted to respiration.   There are two sets of ca-
nals—one carrying the nutrient fluid, and the other, radi-
ating from a ring around the mouth, distributing aerated 
112
COMPARATIVE ZOOLOGY.
               water.    This  may be  called  the "water-
               pipe system."  Besides this, there are nu-
               merous gill-like fringes,  which probably
               aid  in  respiration.
                 Fresh-water "Worms, like the Leech and
               Earth-worm, breathe  by  the skin.    The
               body is always covered by a  viscid  fluid,
               which  has the property of absorbing air.
               The air is, therefore, brought into immedi-
               ate  contact with  the soft skin, underneath
               which  lies a  dense net-work of blood-ves-
               sels.
                 All  the rest of  water-breathing animals
               have gills.   The simplest  form  is seen  in
               Marine Worms:  delicate  veins  projecting
               through the  skin make a series of arbores-
               cent tufts along  the side  of the body;  as
               these float in the water, the blood is puri-
Fig.77.—Lob-worm  fied.  Bivalve Mollusks have four flat gills,
 riZT* dtSTat  consisting of delicate membranes filled with
 chiate,  showing  blood - vessels
 the tufts of capil-
 laries, or external  and   covered
 gills.  The large   . ,    .,.     T
 head is without  Wltn Cilia.   In
 eyes or jaws.     the Oyster, these
ribbon-like folds are exposed
to the water  when  the  shell
opens;  but  in the Clam, the
mantle incloses them, forming
a tube, called  siphon, through
which the water is driven  by
the cilia.73  The aquatic Gas-
teropods   (Univalves)    have
either  tufts, like  the "Worms,
or comb-like  ciliated gills in
a  cavity  behind the  head, to
Fig. 78. — Diagrammatic Section of a
 Lamellibranch (Anodon): a, lobes of
 mantle; 6, gills, showing transverse
 partitions; c, ventricle of heart; d,
 auricles; e, pericardium; /, g, gland-
 ular sacs; A, venous sinus; k, foot;
 A, branchial, or pallial, chamber;  B,
 epibranchial chamber. 
HOW ANIMALS BREATHE.
113
which the water is admitted by a siphon.   The Cuttle-fish
has similar flat gills covered by the mantle; but the water
is drawn in by muscular contractions instead of by cilia.
The end  of the siphon through which  it  is ejected is
called the funnel.  The leaf-shaped gills of Lobsters and
Crabs are also placed in tubular cavities, and a  current is
kept up by a little valve worked by the jaws.
  The perfection of apparatus for aquatic respiration is
seen in Fishes.  The gills are comb-like fringes  supported
on four or five bony or cartilaginous arches, and consist of
myriads of microscopic capillaries, the object being to ex-
pose the venous blood in a state of minute subdivision to
streams of water.  The gills are always covered; and the
water taken in by the  mouth passes between the gills, and
escapes by a single opening on each side, in most  Fishes,
but  by  five slits in  the Sharks.   The act of "breathing
water" resembles swallowing, only the water enters the
gills instead of the gullet.
  (2) Air-breathers have tracheae, or  lungs.   The  former
consist of two principal tubes, which pass from one end
of the body to the other, opening on the  surface by aper-
tures,  called  spiracles,
resembling a  row of
button-holes along the
sides of the  abdomen,
and ramifying through
the  smallest and most
delicate  organs, so  that
the  air may follow the
blood  wherever  it cir-
culates.   To keep the
pipes  ever  open,  and
at the same time leave
,,     n   ...   ,,           Fig. 79.—Spiracle of an Insect, X 75.
them flexible, they are
provided with an elastic  spiral  thread,  like the  rubber
 
114
COMPARATIVE ZOOLOGY.
tube of a drop-light.   Respiration is  performed by the
              movements  of the  abdomen,  as  may  be
              seen in the Bee  when at rest.  This "air-
              pipe system," as  it may  be termed, is best
              developed in Insects.
                The "nerves" of an  Insect's wing con-
              sist of a tube within a tube:  the inner one
              is a trachea carrying air,  and  the outer one,
            ,  sheathing it, is a blood-vessel.   So perfect
Fig. 80. —Tracheal  .        &   .                       r
 Tube of an insect,  is  the aeration  of the  whole  body,  from
 highly magnified,  .         „     t    i -i   i  •         .
 showing elastic  brain to feet, the  blood  is  oxygenized at
 spiral thread.    ,1          .   1        i     , 1         i
              the moment when, and  on the spot where,
it is carbonized;  only one kind of  fluid is,  therefore, cir-
culating — arterial.
It is difficult to drown an  Insect, as
the water can not enter the pores;  but if a drop of oil be
applied to the abdomen, it falls dead  at  once, being suf-
                h
Pig. 81.—Ideal Section of a Bee:  a, alimentary canal; h, dorsal vessel; t, trachea;
                       n, nervous cord.
focated.   The  largest  spiracle  is  usually  found  on the
thorax, as under the wing of a Moth:  such may be stran-
gled by pinching the thorax.
   In  Millipedes and Centipedes, the  spiracles  open  into 
                  HOW ANIMALS BREATHE.              115

little  sacs   connected  to-
gether  by  tubes;  in  Spi-
ders   and  Scorpions,  the
spiracles, usually  four  in
number, are the mouths of
sacs without the tubes, and
the  interior of  the  sac is
gathered into folds;  land
Snails have one spiracle, or
aperture, on the left side of
the neck, leading to a large
cavity,  or sac, lined  with
fine blood-vessels.   These
sacs  represent the  primi-
tive idea of a lung,  which
is but an infolding of the
skin,  divided  up  into cells,  and  covered with  capillary
veins.74
   Like  the  alimentary canal, the lungs of an animal are
really an inflected portion of the outer surface; so that
Fig. 83. — Part of a transverse section of a Pig's Bronchial Twig, X 240: a, outer
   fibrous layer; 6, muscular layer; c, inuer fibrous layer;  d, epithelial layer; /,
   one of the neighboring alveoli.
Fig. S2.—Section through a bronchial tnbe,
  Lung of a Bird, magnified: o, the cavity ;
  6, its lining membran; supporting blood-
  vessels ; c, perforations at the orifices of
  the lobular passages, d;  e, interlobular
  spaces, containing the terminal branches
  of the pulmonary vessels supplying the
  capillary plexus, /, to the meshes of which
  the air gets access by the lobular passages. 
116
COMPARATIVE ZOOLOGY.
breathing by the skin and breathing by lungs are one in
principle.  Indeed, in many animals, especially Frogs, res-
piration is carried on by both lungs and skin.
  All Yertebrates have two kinds of respiratory organs in
the course of their life.   From Fish to Man, all have gills
             in  the  embryo state.76  Fishes, and a few
             Amphibians,  keep them through life;  but
             in  the  rest they  disappear.  All, too, have
             lungs; but fully developed only in Reptiles,
             Birds, and Mammals.  The lung of the Fish
             (the  air-bladder) remains rudimentary.   A
             few  adult Amphibians, as Proteus  and Si-
             ren, retain both gills  and lungs, thus forming
             a link  between  Fishes and  Reptiles.  But
             Frogs and Salamanders begin life  as water-
             breathers, and when mature have lungs only.
               The lungs of Yertebrates are elastic mem-
             branous sacs, divided more or less into cells
             to  increase the surface.  Upon  the  walls of
             the cells are  spread  the capillary blood-ves-
             sels.  The smaller the cells,  the greater the
             extent of  surface upon which the blood  is
             exposed to the  influence of the  air, and,
             therefore,  the more  active  the respiration
             and  the purer  the  blood.    The  lungs are
             relatively  largest in Reptiles, and  smallest
             in  Mammals.  But in  the cold-blooded Am-
             phibians and Reptiles, the air-cells are  few
             and large; in the warm-blooded  Birds  and
             Mammals, they  are exceedingly numerous
             and minute.78 In Birds and Mammals, the
             blood in the  capillaries is exposed to the air
on all sides;  in  the Reptiles, on one only.   Respiration is
most perfect  in  Birds;  they require, relatively to  their
weights, more air than Mammals or Reptiles, and most
Fig.  84.— Lungs
 of a Reptile: a,
 trachea; 6, its
 bifurcation; c,
 pulmonary ar-
 tery ;  d,  pul-
 monary  vein ;
 the sac, B, is
 rudimentary. 
HOW ANIMALS BREATHE.
117
quickly die for  lack of  it.   In Birds,  respiration is  not
confined to the lungs;  but, as in  Insects, extends through
a great part of  the body.  Air-sacs  connected with  the
lungs  exist in the abdomen  and under  the skin  of  the
neck,  wings, and  legs.   Even the bones are  hollow  for
this purpose;  so  that if  the windpipe be  tied,  and an
opening be made in the wing-bone, the Bird  will continue
to respire.   The right lung is usually the larger; in some
Snakes, the left is wanting entirely.    In Mammals,  the
lungs  are freely  suspended in the thorax; in other Yerte-
brates, they are  fastened to the back.
   The lungs communicate with the  atmosphere  by means
of the trachea, or windpipe, formed  of a series of cartilag-
inous  rings, which keep it constantly open.   It begins in
Pig. 85.—Lungs of a Frog: a, hyoid
 apparatus; b, cartilaginous ring at
 root of the lungs;  c, pulmonary
 vessels; d, pulmonary sacs having
 this peculiarity common to all cold-
 blooded air-breathers, that the tra-
 chea does  not divide into bron-
 chial branches, but terminates ab-
 ruptly  by orifices which  open at
 once into the general cavity.  A
 cartilaginous net-work divides the
 space into little sacs, on the walls
 of which the capillaries are spread.
Fig. 86.—Distribution of Air-tubes in Mam-
 malian Lungs: a, larynx ; b, trachea;  c, d,
 left and right bronchial tubes ; e, /, g, the
 ramifications. In Man the subdivision con-
 tinues until the ultimate tubes are one-twen-
 ty-fifth of an inch in diameter.  Each lobule
 represents in miniature the structure of the
 entire lung of a Frog.
the back part of the  mouth, opening into the pharynx by
a slit, called the glottis, which, in Mammals, is  protected 
118
COMPARATIVE ZOOLOGY.
by  the  valve-like epiglottis.
                               The trachea passes along
                                     the neck in front of
                                     the oesophagus,  and
                                     divides  into  two
                                     branches, or bronchi,
                                     one for  each lung.
                                     In Birds and Mam-
                                     mals,  the bronchial
                                     tubes, after  entering
                                     the lungs, subdivide
                                     again  into   minute
                                     ramifications.
        fig. 87.—skeleton of a Frog.           Yertebrates   are
the only animals that breathe through the  mouth or nos-
trils.   Fishes inspire only.   Frogs, having no ribs, and
Turtles, whose ribs are sol-
dered together into a shield,
are compelled to swallow
the air.  Snakes, Lizards,
and Crocodiles   draw it
into the lungs by the play
of the ribs.77  Birds, unlike
other  animals, do not in-
hale the  air  by  an  active
effort;  for that is done by
the springing-back of the
breast-bone  and  ribs to
their natural  position.  To
expel  the air, the  breast-
bone  is  drawn  down to-
ward   the   backbone by
muscles, which compresses
the lungs.
   Mammals  alone have a
perfect thorax, i.e., a closed
Fig. 88.—Human Thorax:  o, vertebral col-
 umn ; 6, 6', ribs, the lower ones false;  c,
 clavicle; e, intercostal muscles, removed
 on the left side to show the diaphragm, d;
 f, pillars of the diaphragm attached to the
 lumbar vertebrae; g, muscles for elevating
 the ribs; h, sternum. 
HOW ANIMALS BREATHE.
119
cavity for the heart and lungs, with movable walls (breast-
bone and ribs) and a diaphragm, or muscular partition,
separating it from the abdomen.78  Inspiration (or filling
the lungs) and expiration (or emptying the lungs) are both
accomplished by muscular exertion;  the former, by rais-
ing the ribs and lowering the diaphragm, which enlarge
the capacity of the chest, and the air rushes in to prevent
a vacuum; the latter, by the  ascent of the diaphragm and
the descent of the ribs.
  As a rule, the more active and more muscular an ani-
mal, the greater the  demand for  oxygen.  Thus, warm-
blooded animals live  fast, and their  rapidly decaying tis-
sues call for rapid respiration;  while in the cold-blooded
creatures the waste is comparatively slow.   Respiration is
most active in Birds, and least in water-breathing animals.
The sluggish  Toad respires  more slowly than the busy
Bee, the Mollusk more slowly than the Fish.  But respi-
rations, like beats  of  the  heart, are fewer in large Mam-
mals than in small ones.  An average Man inhales about
700 cubit feet of air per day.
  Another result of respiration, besides the purification of
the blood, is the production of heat.   The chemical com-
bination of the oxygen in the air with the carbon in  the
tissues is a true combustion;  and, therefore, the more act-
ive the animal and its breathing, the higher its tempera-
ture.   Birds and Mammals have  a temperature of about
108° and  100° respectively, and are called warm-blooded.
Fishes and Reptiles have a lower and more variable tem-
perature,  ranging  from 35°  to 80°,  and  are called cold-
blooded.  The Bee is  from 3° to 10°, and the Earth-worm
and Snail from 1£° to 2°, higher than the  air.  The mean
temperature of the Carp and Toad is 51°; of Man, 98°;
Dog, 99°; Cat, 101°;  Squirrel, 105°;  Swallow, 1110. 
120
COMPARATIVE ZOOLOGY.
CHAPTER XV.
                SECRETION  AND  EXCRETION.
  In the circulation of the blood, not only are the nutrient
materials deposited within the body in the form of tissue,
but  certain special fluids  are separated and conveyed  to
the external or internal surfaces of the body.   These flu-
ids are of  two kinds:  some, like saliva, gastric juice, bile,
milk, etc., are for useful purposes; others, like  sweat and
urine, are expelled from the system as useless or injurious.
                              The separation of the  for-
                              mer is  called secretion; the
                              removal  of  the latter is ex-
                              cretion.  The two functions
                              differ also in  this, that se-
                              cretion is intermittent, as, e.
                              g., the  gastric juice is form-
                              ed  when  wanted,  while  ex-
                              cretion is constant day and
                              night.  Both processes, how-
                              ever, are substantially alike.
                                 In the lowest forms, there
                              are  no special organs, but
                              secretion and excretion take
Fig. 89-Three plans of secreting Mem- place from  the genera) gur_
  branes.  The heavy line represents the r               o
  areolar-vascular layer;  the next line is face.   Even  in the  higher
  the basement, or limiting membrane;    .                       .
  and the dotted line the epithelial layer: animals, there are Secreting
  a shows increase of surface  by simple       •,            m,
  plaited  or fringed projections;  b, five membranes.    liie mem-
  modes of increase by recesses, forming i1rjnips linino-  tbp nnup nnd
  simple glands, or follicles; c,  two forms Dranes ilnmg  me nose anQ
  of compound glands.              alimentary canal and inclos-
ing the lungs, heart, and joints, secrete lubricating fluids.
 
SECRETION AND EXCRETION.
121
  The infolding of such a membrane into little sacs or
short tubes (follicles), each having its own outlet, is the
type of all secreting and  excreting organs.   The  lower
tribes have nothing higher, and the apparatus for prepar-
ing the gastric fluid attains no further development even
in Man.  When a cluster of these follicles, or sacs, dis-
charge  their contents by  one common duct, we have a
gland.   But whether membrane, follicle, or gland, the
organ is covered with a net-work of blood-vessels, and
lined with epithelial cells, which are the real agents in the
process.
  The chief Secreting Organs are the salivary glands,
gastric follicles, pancreas, and  liver, all  situated  along
the digestive tract.
  1. The salivary  glands,  which  open into the mouth,
secrete  saliva.   They exist in nearly all animals, except
Fishes,  Crocodiles,  and "Whales, and are most largely de-
veloped in such as live on vegetable
food.   The saliva  serves  to lubricate
or  dissolve  the food for  swallowing,
and, in Mammals that  masticate, aids
also in digestion.79
  2. The gastric follicles are minute
tubes in the walls  of the  stomach  se-
creting gastric juice. They are found
in all Yertebrates, and  in the higher
Mollusks and Articulates.  In the low-
er  forms, a simple  membrane  lined
with cells serves  the same  purpose.
Under  the  microscope,  the soft  mu-
cous  membrane  of the  human stom-
ach  presents  a  honey-comb  appear-
ance, caused by numerous depressions
or cells.  At the bottom of these de-
pressions  are clusters of spots, which are  the  orifices of
Fig. 90.—Follicles from the
 Stomach of a Dog, x
 150; near the mouth, a,
 there is a lining of co-
 lumnar epithelium. 
122
COMPARATIVE ZOOLOGY.
the tubular follicles.   The  follicles are less than -g-oVo- of
an inch in  diameter, and number millions.
  3. The pancreas, or "sweetbread," so important in  the
process of  digestion, when present, exists only in the Yer-
                               tebrates and  the  higher
                               Mollusks.   In its struct-
                               ure  and  its secretion it
                               closely resembles the  sal-
                               ivary glands.  In the Cut-
                               tle-fish, it is represented
                               by a sac; in Fishes, by
                               a  group of follicles.   It
                               is proportionally  largest
                               in  Birds whose salivary
                               glands are deficient.  The
Fig. 91.—Pancreas of Man, o; g, gall-bladder; °        ...
  «, cystic duct; c, duct from the liver; p, py- pancreatic j uice  enters
  loric valve; e, i, duodenum.            , i    -j   j
                               the duodenum.
  4. A liver in some form is found in all animals having
a distinct digestive cavity.  In Mollusks and Yertebrates,
it is the largest gland in the body.   The higher the ani-
mal, the more compact the organ.  Thus, in Polyps it is
represented by yellowish cells lining the stomach;  in In-
sects, by delicate  tubes along the intestine; in  Mollusks,
by a cluster of sacs, or follicles, forming a loose compound
gland.  In Yertebrates, the liver is well defined, and com-
posed  of a multitude of lobules (which give it  a granular
appearance)  arranged on the capillary veins, like grapes
on  a  stem, and containing nucleated  secreting cells.   It
is of variable shape, but  usually two, three, or  five lobed,
and is centrally situated — in  Mammals, just  below  the
diaphragm.  In most Yertebrates, there is an appendage
to the liver, called the gall-bladder, which is simply a res-
ervoir for the bile when not wanted.
  The liver is both a secretory  and excretory organ.   For
while  the  bile performs  an essential, though mysterious, 
SECRETION AND EXCRETION.           123
part in the  digestive  process, it is decomposing matter
filtered from the blood, and, if not cast out of the system,
Fig. 92.—Liver of the Dog, P, F; D, duodenum and intestines; P, pancreas;  r,
   spleen; e, stomach; /.rectum;  R, right kidney; B, gall-bladder; ch, cystic
   duct;  F, lobe of liver dissected  to  show distribution of portal vein, VP, and
   hepatic vein, vh; d, diaphragm; VC, veua cava; C, heart.

produces jaundice.   It is  reabsorbed  by the lacteals, but
is finally discharged by  oxidation through the lungs.  In
animals of  slow  respiration, as  Crustaceans, Mollusks,
Fishes, and Reptiles,  fat accumulates in  the  liver.   " Cod-
liver oil" is  an example.
   The  great Excreting Organs  are the  lungs,  the kid-
neys,  and the skin;  and  the substances  which  they  re-
move from the system—carbonic acid, water, and urea—
are the products  of decomposition,  or organic  matter on
its way back to the mineral kingdom.60  Different as these 
124
COMPARATIVE ZOOLOGY.
organs  appear, they are constructed upon  the same prin-
ciple :  each consisting of a very thin sheet of tissue sepa-
rating the blood to be purified from the atmosphere, and
straining out, as  it were, the noxious matters.  All, more-
over, excrete the same  substances, but in very  different
proportions: the lungs  exhale carbonic acid and water,
with a  trace of  urea; the kidneys expel water, urea, and
a little  carbonic  acid; while the skin partakes of the nat-
ure of both, for it is not only respiratory, especially among
the lower animals, but it performs the work of the kidneys
when they are diseased.
  1. The lungs (and likewise gills) are mainly excretory
organs.  The oxygen they impart sweeps  with the blood
through every part of  the body, and  unites  with,  i. e.,
burns up, the effete matters,  which, set free by muscular
and  nervous exertion,  would  poison the  system, if  not
removed.  The carbonic-acid gas thus generated is car-
ried  by the veins  to the lungs, and there exhaled in breath-
                     ing.   This process is more immedi-
                     ately necessary to life than any oth-
                     er : the arrest of respiration is fatal.
                        2. "While the lungs (and skin also,
                     to a slight degree)  are  sources  of
                     gain as well as loss to the blood, the
                     kidneys are purely excretory organs.
                     Their sole function  is to eliminate
                     the solid products of  decay which
                     can  not pass out by the lungs.   In
                     Mammals, they  are discharged  in
                     solution; but  from other animals
Fig. 93.-Section of Human Wh° drmk  little the  excretion is
 Kidney,  showing the  tubu- more or less solid.   In Insects, the
 lar portion, 3, grouped into .                              '
 cones; 7, the ureter, or out- kidne}7s are groups of tubes; in the
 let of the secretion.       i • i    -»r n  i    i
                     higher Mollusks, they are represent-
ed by spongy masses of  follicles;  in Yertebrates, they are
 
THE SKIN AND SKELETON.
125
well-developed glands, two in number, and consisting of
closely packed tubes.
  3. The  skin of the  soft-skinned  animals, particularly
of Amphibians and Mammals, are  covered with  minute
pores, which are the ends of as many delicate tubes that
lie coiled up into a knot within the true skin.   These are
the sweat-glands, which excrete watery vapor, and with it
certain salts and gases.   The importance of this excretion,
known as perspiration, is shown by the fact that if the
skin be varnished over, the  animal will die.   On  the ac-
cession of Leo X. to the papal chair,  a child was  gilded
all over, at Florence, to represent the Golden Age, and it
died in a few hours.
  Besides these  secretions and excretions,  there are oth-
ers, confined to particular animals, and designed for spe-
cial purposes: such are the oily matters secreted from the
skin of  quadrupeds for lubricating the hair and keeping
the skin flexible; the tears of Reptiles, Birds,  and Mam-
mals ; the milk of Mammals; the ink of the Cuttle-fish;
the poison in the stings of  Jelly-fishes and Insects; and
the silk of Spiders and Caterpillars.
                  CHAPTER XYI.
                THE SKIN AND SKELETON.

  The Skin, or Integument, is that layer of tissue which
covers the outer surface of the body.  The term Skeleton
is applied to the hard parts of the body, whether external
or internal, which serve as a frame-work or protection to
the softer organs, and afford  points of attachment to mus-
cles.  If external, as the crust of the Lobster, it is called
Exoskeleton;  if internal, as the bones of Man, it is called 
126
COMPARATIVE ZOOLOGY.
Endoskeleton.   The former is but a modification of the
skin.
  1. The Skin.—The skin is a very complex tissue, since
it serves not only for a covering, but also for an organ of
excretion, absorption, and touch.  In the lowest forms, as
Amoebae and Infusoria, it is an extremely delicate film, or
membrane, but little more consistent than the body which
it envelopes.  But throughout the animal kingdom, from
the slimy coat of the Polyp to the thick hide of the Rhi-
noceros, the skin shows a similar structure—an inner  and
an outer layer; the former called dermis ;  the latter,  epi-
dermis.91
  Except in the low and immature forms,  as "Worms  and
Caterpillars, the skin of Articulates is hardened into a
crust.  The loose skin, called the m,antle, which envelopes
the body of the Mollusk corresponds to the true skin of
higher animals. The border  of the mantle is surround-
ed with a delicate  fringe,  and, moreover, contains minute
glands, which  secrete the  shell and the coloring matter
by which it is adorned.  The Tunicates have  a leathery
epidermis, remarkable for containing, instead of lime, a
substance resembling vegetable cellulose.
  In  Mammals, whose  skin is most fully developed, the
dermis is a sheet of tough  elastic tissue, consisting of in-
terlacing fibres, and containing blood-vessels, lymphatics,
sweat-glands, and nerves.  It i^ the part  converted  into
leather when hides are tanned, and attains the extreme
thickness  of three  inches in the Rhinoceros.  The  upper
surface is covered with a vast number  of  minute projec-
tions, called papilla), each the  termination of  a nerve;
these are the  essential agents  in the  sense of touch.63
They are best seen on the tongue of an Ox or Cat,  and
on the human fingers, where they are arranged in rows.
  Covering this sensitive layer,  and accurately molded to
all its furrows and ridges, lies the bloodless and nerveless 
THE SKIN AND SKELETON.
127
epidermis.  It is that part of  the skin which is raised  in a
blister.  It is thickest where there is most pressure or hard
usage: on the back of the Camel it attains  unusual thick-
ness.   The lower portion of the epidermis (called rete mu-
Fig. 94.—Section of Skin, from Horse's Nostril: E, epidermis; D, derma; 1, horny
   layer of epidermis ;  2, rete mucosum ;  3, papillary layer of derma; 4, excretory
   duct of a sudoriparous, or sweat, gland; 5, glomerule, or convoluted tube of the
   same; 6, hair follicle; 7, sebaceous gland; 8, internal sheath of the hair follicle;
   9, bulb of the hair; 10, mass of adipose tissue.
cosum)  is  comparatively soft, and consists of nucleated
cells  containing  pigment-granules, on  which the color of
the animal depends.   Toward the surface, the cells be-
come flattened, and finally, on the outside, are changed to
horny scales (Fig. 2, c).
   These scales, in the higher animals, are constantly wear-
ing off in  the  form of  scurf, and  as constantly being re-
newed from below.  In Lizards and Serpents, the old epi-
dermis is cast  entire, being stripped off from the head to
the tail; in the Toad, it comes off in  two  pieces;  in the
Frog, in shreds;  in Fishes and some Mollusks, in the form
of slime.   However modified the  epidermis, or whatever
its appendages, the like process of removal goes on.  Mam-
mals  shed their  hair;  Birds, their feathers;  and  Crabs, 
128             COMPARATIVE ZOOLOGY.

their shells.   "When the loss is periodical, it  is  termed
moulting.
   2.  The Skeletons. — (l) The  Exoskeleton is  developed
by the  hardening of the skin, and, with very few excep-
tions, is the  only kind  of skeleton possessed by inverte-
brate animals.  The usual forms are coral, shells, crusts,
scales,  plates, hairs,  and feathers.  It is horny or calca-
reous;  while  the endoskeleton  is  generally a  deposit of
earthy  material within  the body,  and is nearly confined
to the Yertebrates.
   The  microscopic  particles of living jelly, called Poly-
cistines and Foraminifera, cover themselves  with sili-
ceous and  calcareous shells of  the most  beautiful  pat-
terns.    The Sponge  has  an internal skeleton of horny
fibres, which  is  the sponge of commerce.  Coral is  the
Fig. 95.—1, Vertical Section, and, 2, Transverse Section, of a Sclerodermic Corallite:
   a, mouth; b, tentacles; c, stomach; d, intermesenteric chamber; e, mesentery;
   /, septum ; g, endoderm; h, epitheca;  k, theca, or outer wall; m, columella; n,
   short partitions; p, tabula, or transverse partitions, a characteristic of extinct
   corals; r, sclerobase; s, coenenchyma, or common substance connecting a neigh-
   boring corallite ; t, ectoderm; x, pali, or imperfect partitions.
solid frame-work of certain Polyps. There are two kinds:
one  represented by the common white coral, which is  a
calcareous secretion within  the body of the  Polyp, in the
form  of  a cylinder, with partitions  radiating  toward  a
centre (sclerodermic); the  other, represented by the solid
red coral of jewelry, is  a central axis deposited by a group
of Polyps on the outside (sclerobasic). 
                THE SKIN AND SKELETON.             129

  The skeleton of the Star-fish is a leathery skin studded
with  calcareous particles.    The Sea-urchin  is  covered
with an inflexible
shell  of elaborate
and beautiful con-
struction.    The
shell  is  really  a
calcified skin, be-
ing a net-work of
fibrous  tissue and
earthy matter.  It
varies  in  shape
from a sphere to
a disk;  and  con-
      _ .    .   ,  Fig. 96.—Shell of Sea-urchin (Cidaris) without its spines.
sists of hundreds
of angular pieces  accurately  fitted  together, like mosaic-
work.   These form  ten  zones, like the  ribs of  a melon,
five broad ones alternating with five narrower ones.  The
former  (called interambulacra)  are covered with tuber-
Fig. 97.—Structure of Sea-urchins' Spines: 1, a, spine of Cidaris cut longitudinally ;
   t, 8, ball and socket joint; p, pedicellarise; 2, 3, transverse sections of spines of
   Cidaris and Echinus.
cles  bearing  movable spines.   The narrow zones  (called
ambulacra, as they are likened to walks through a forest)
                             9
 
130
COMPARATIVE ZOOLOGY.
are pierced with small holes, through  which  the animal
sends out fleshy tentacles.
  The skin of the Crab and Lobster is hardened by cal-
careous deposit into a " crust," or shell ;83 but, instead of
forming one piece, it is divided into a series of segments,
which move on  each other.   The number of these  seg-
ments, or rings, is usually  twenty-one—to the head,  tho-
rax, and  abdomen,  seven each.   In  the adult, however,
the rings of the head and  thorax are  often soldered to-
gether into one shield, called cephalo-thorax;  and in the
Horseshoe Crab all the divisions of the  skeleton are quite
obliterated.  The shell of Crustaceans is periodically  cast
off, for they continue to  grow even after they have reach-
ed  their mature form.   This molting is a very remarka-
                                      ble operation. How
                                      the   Lobster   can
                                      draw its legs from
                                      their cases without
                                      unjointing or split-
                                      ting them has long-
                                      been  a   puzzle.84
                                      But  the   cast - off
                                      skeleton  is  a per-
                                      fect  copy  of  the
                                      animal, retaining in
                                      their places the  del-
                                      icate coverings of
                                      the  eyes  and  an-
                                      tennae, and even the
                                      lining membrane of
                                      the  stomach with
                                      its teeth!
                                         The horny crust
                                      of  Insects  differs
                                      from that of Crus-
Fig. 98.—Diagram of an Insect: A, head bearing the
 eyes and antennae; B, prothorax, or pronotum, car-
 rying the first pair of legs;  C, mesothorax, carrying
 the second pair of legs and first pair of wings; D,
 carrying the  third pair of legs and second pair of
 wings; E, abdomen, with ovipositor, F; 1, coxa, or
 hip ; 2, trochanter;  3, femur, or thigh ; 4, tibia, or
 shank; 5, tarsus, or foot; 6, claw. 
THE SKIN AND SKELETON.
131
taceans in consisting mainly of chitine and in containing
no  lime.   The head, thorax, and abdomen are distinct,
and usually consist of thirteen visible segments—one for
the head, three for the thorax (called prothorax, mesotho-
rax, and  metathorax), and nine for the abdomen.  The
antennae,  or  feelers, legs, and  wings, as well  as  hairs,
spines, and scales, are appendages of the skeleton.  As
Insects grow only during  the larval, or caterpillar, state,
molting is confined to that period.
  The shells of Mollusks are well-known examples of exo-
skeletons.  The mantle, or loose skin, of these animals se-
cretes calcareous earth in successive layers, converting the
epidermis into  a " shell."85  So various and characteristic
is the  microscopic  structure of  shells, that a fragment is
sometimes sufficient to determine the group to which it
belongs.   A large class of shells  is represented  by that of
the Oyster, which is composed of three parts: the external
brown epidermis, of horny texture ;  then  the prismatic
portion, consisting of minute columns set perpendicularly
to the surface; and the internal nacreous layer,  or " moth-
er-of-pearl," made  up of exceedingly thin plates.   The
pearly lustre of the last is due to light falling upon the
outcropping edges of wavy laminae.86  In many cases, the
prismatic and  nacreous  layers  are  traversed by minute
tubes.  Another typical shell-structure is seen in the com-
mon Cone, a section of which shows three layers, besides
the epidermis,  consisting of minute  plates set at different
angles.   The Nautilus is composed of two distinct layers:
the outer one having the  fracture of broken china; the
inner one, nacreous.
  Most living shells are  made of one piece, as the Snail;
these are  called "univalves."   Others, as the Clam, con-
sist of two parts, and are called "bivalves."   In  either
case, a valve may be regarded as  a hollow cone, growing
in a spiral form.  The ribs, ridges,  or spines on the out- 
132
COMPARATIVE ZOOLOGY.
side of a shell mark the successive periods of growth, and,
therefore,  correspond with the age  of the animal.   The
following  figures show  the principal parts of the ordinary
bivalves and univalves.   The valves of a bivalve are gen-
erally  equal, and the umbones, or beaks, a little in front
of the centre.   The valves are bound together by a liga-
ment near the umbones, and often,
also, by means of a " hinge " form-         ^^
ed by  the  " teeth" of one valve in-
terlocking into cavities in the oth-
er.   The aperture of a univalve is
Fig. 99.—Left Valve of a Bivalve Mollusk (Cytherea
 chione): h, hinge ligament; u, umbo;  I, lunule;
 c, cardinal, and t, t', lateral, teeth; a, a', impres-
 sions of the anterior and posterior adductor mus-
 cles ; p, pallial impression; e, sinus occupied by
 the retractor of the siphons.
Fig. 100.—Section of a  Spiral
 Univalve (Triton corrugatus):
 a, apex; b, spire; c, suture;
 d, posterior canal; e, outer
 lip of the aperture; /, anteri-
 or canal.
frequently closed  by a  horny or calcareous plate,  called
"operculum,"  which  the animal carries on its back, and
which is a part of the exoskeleton.
  Imbedded in the back of the Cuttle-fish is a very light
spongy " bone," which, as already observed, is a  secretion
from the  skin, and, therefore, belongs to the exoskeleton.
It has no resemblance  to true  bone, but is formed, like
shells, of a number  of  calcareous plates.   Nevertheless,
the Cuttle-fish does  exhibit the first traces of  an  endo- 
THE SKIN AND SKELETON.
133
skeleton:  these  are plates of cartilage, one of which sur-
rounds the brain, and hence may be called a skull.   To
this cartilage, not  to  the "cuttle-bone," the  muscles are
attached.
   In Yertebrates,  the exoskeleton  is subordinate to the
endoskeleton, and is feebly  developed in comparison.  It
is  represented   by  a
great variety of append-
ages to the skin, which
are  mainly  organs  for
protection, not for sup-
port.  Some  are horny
developments of the ep-
idermis, such  as hairs,
feathers,   nails,   claws,
hoofs, horns,  and  the
scales  of Reptiles; oth-
ers arise from the hardening of the dermis by calcareous
matter, as the scales  of Fishes,  the plates  of Crocodiles
and Turtles, and the shield of the Armadillo.
   The scales of Fishes (and likewise the spines of their
vertical fins) lie  imbedded in the overlapping  folds of the
skin, and are covered  with a thin epidermis.   The scales
of the bony Fishes (Perch, Salmon, etc.)  consist of  two
Fig. 101.—Skeletal Architecture in the Armadil-
 lo, showing the relation of the carapax to the
 vertebral column.
Fig. 102.—Diagrammatic Section of the Skin of a Fish (Carp): a, derm, showing lam-
   inated structure with vertical fibres, b; c, gristly layer; e, laminated layer, with
   calcareous granules; d, superficial portion developing into scales; /, scale-pit.

layers, slightly calcareous, and  marked by concentric and
radiating lines.  Those of the Shark have the structure of
teeth, while the scutes, or plates, of the Crocodiles, Turtles,
and Armadillos are of true bone. 
134
COMPARATIVE ZOOLOGY.
  The   scales  of
Snakes and Lizards
are horny epiderm-
ic plates covering
the     overlapping
folds of the  true
skin.  In some Tur-
tles these plates are
of great size, and
are   called   " tor-
toise - shell;"  they
cover  the   bony
Pig. 103.—Vertical Section of the Forefoot of the Horse  dermal plates.  The
scales  on
of  Birds,
the  tail
the legs
 and  on
 of  the
 (middle digit):  1, 2, 4, proximal, middle, and distal,
 or ungual, phalanges; 3, sesamoid, or nut-bone ; 5,
 6, 7, tendons; 9, elastic tissue; 8, 10, internal and
 external floor of the hoof; 11, 12, internal and ex-
 ternal walls.
Beaver and  Rat, have  the same
structure.     Nails  are  flattened
horny plates developed from the
upper surface of the fingers  and
toes.    Claws are   sharp  conical
nails, being  developed  from the
sides as  well as upper surface;
and  hoofs are  blunt  cylindrical
claws.   Hollow  horns,  as of the
Ox,  may be  likened  to  claws
sheathing a  bony case.  The horn
of the Rhinoceros  is a solid mass
of  epidermal  fibres.    "Whale-
bone,"  the rattles  of the  Rattle-
snake, and  the  beaks  of  Turtles  fig. i04.-sec^TonL Root and
                                     part of the Shaft of a Human
                                     Hair; it is covered with epi-
                                     dermic scales, the inner layer,
                                     c, forming the outer covering
                                     of the shaft, being imbricated;
                                     the  root  consists  of angular
                                     cells loaded with pigment.
and Birds, are likewise epidermal.
  Hairs, the characteristic clothing
of Mammals, are elongated horny
cones,  composed of "pith"  and 
THE SKIN AND SKELETON.            135
 "crust."   The latter is an outer layer of  minute  over-
 lapping scales, which  are directed
 toward the point, so that rubbing
 a human hair or fibre of wool be-
 tween the thumb and finger pushes
 the  root-end  away.   The  root is
 bulbous, and  is contained in a  mi-
 nute depression, or sac, formed by
 an infolding of the skin.  Hairs are
 usually set obliquely into the skin.
 Porcupine's quills and  Hedgehog's
 spines make  an easy transition to
 feathers,  which differ  from  hairs
 only in splitting up into numerous
 laminae.  They are the most com-
 plicated of all the modifications of
 the  epidermis.   They consist of a
 "quill" (answering  to the bulb of
 a  hair), and a "shaft," supporting
 the  " vane," which is  made  up of
 " barbs," " barbules," and interlock-
 ing  "processes."   The  quill alone
 is hollow,  and has an  orifice at
 each end.   The teeth  of  Mollusks
 and  Articulates are also epidermal
 structures;  but the teeth of Yerte-
 brates are  developed from the der-
 mis.    In all cases, teeth belong to
 the exoskeleton.   A human tooth and an oyster-shell rep-
resent each other, structure for structure.
   (2) The Endoskeleton, as  we have seen, has its first rep-
resentative in the Cuttle-fish.  "With this exception, it is
 peculiar to Yertebrates.   In the  Cuttle-fish, and some
Fishes, as  the  Sturgeon and Shark,  it consists of carti-
lage; but in all others (when adult) it is bone or osseous
Fig. 105.—Parts of a Feather:
 a, quill, or barrel; b, shaft; c,
 vane, or beard ;  d,  accessory
 plume, or down; e, /, lower
 and  upper umbilicus, or ori-
 fice, leading to the interior of
 the quill. 
136
COMPARATIVE ZOOLOGY.
tissue.   Yet there is  a diversity in the composition of
bony skeletons: that of fresh-water Fishes  contains the
least earthy matter, and that of Birds the most.  Hence
the density  and ivory-whiteness of  the bones of the lat-
ter.  Unlike the shells of Mollusks  and the crust of the
Lobster, which grow by the addition of layers  to  their
borders, bones are  moist, living parts, penetrated by blood-
vessels and nerves, and covered with a tough membrane,
called periosteum, for  the attachment of muscles.
  The surface of bones is compact;  but the  interior may
be solid or spongy (as the bones of Fishes, Turtles, Sloths,
and "Whales), or hollow  (as  the long bones of Birds and
the active quadrupeds).   There are  also cavities (called
"sinuses") between the inner and outer walls of the skull,
as remarkably  shown  by the Elephant.   The cavities in
the long bones of quadrupeds  are  filled  with marrow;
those in the long bones of Birds and  in skulls contain
air.
  The number of bones not only differs in  different ani-
mals, but varies with the age of  an individual.  In very
early life there are no  bones at all; and ossification, or the
conversion of cartilage into bone, is not completed until
maturity.   This process begins at a multitude of points,
and  theoretically there are as many bones in a skeleton as
centres of ossification.  But the actual number is usually
much  less — a result of  the tendency of these centres to
coalesce.  Thus, the thigh-bone in youth is composed of
five  distinct portions,  which gradually unite.  So  in the
lower  Yertebrates many parts remain  distinct  which in
the  higher  are joined  into one.   The  occiput or back-
bone of Man's skull is the union of three or four  bones,
which are seen separate in the skull  of the Fish.
  A complete  skeleton, made up of all the  pieces  which
might enter into  its composition, does not exist.  Every
animal has  some  deficiency.  All have a skull and back- 
THE SKIN AND SKELETON.
137
bone; but  in  the  development of the various parts, and
especially  of  the  appendages, there  is  endless  variety.
Fishes come nearest to the archetype skeleton;  but while
they possess a complete set of skull-bones, they have no
representatives of fingers and toes.  The Snake has plenty
of ribs and tail, but no breast-bone; the Frog has a breast-
bone, but neither tail nor ribs.  As the skeleton of a Fish
is too complicated for the  primary student, we will select
for illustration the skeleton of a Lion—the type of quad- 
138
COMPARATIVE ZOOLOGY.
rupeds.  It should be remembered, however, that all Yer-
tebrates are formed on one plan.
  The vertebrate skeleton consists  of a series of rings,
called vertebra},  arranged  along the  back of the body,
with  certain appendages, as limbs, ribs, etc.   The verte-
brae are always  present, but  the appendages  are  incon-
stant.  In  the  lowest Fishes, the spinal column is  a con-
tinuous cylinder, with scarcely a trace of division, and in
Birds, also, it is much consolidated;  but usually the verte-
brae are separable.   They range in number from 10 in  the
Frog (not counting the head) to 305 in the Boa-constrictor.
  A typical  vertebra consists of a number of bony pieces
so arranged as  to form two arches, or hoops, connected by
Fig. 107.—Vertebra—A, cervical; B, dorsal: 2, centrum ; 4, transverse process, con-
   taining foramen, a, for artery; 5, articular process; 3, spinous process, or neural
   spine; 1, neural canal; 6, facets for head of rib, the tubercle of the rib fitting
   in a facet on the process, 4; b, laminae or neurapophyses.

a central bone, or centrum.6''  The upper  hoop is called
the neural arch, because it encircles the spinal marrow;
the lower hoop is called the haimal arch,  because  it in-
closes the heart and the great central blood-vessels.   An
actual vertebra, however, is subject to  so many modifica-
tions, that it deviates more or less from this ideal type.
Selecting one from the middle of  the back for an exam-
ple, we see that the centrum sends  off from its dorsal side
two branches, or processes, called neurapophyses.   These
meet  to form the neural arch, under which is the neural
canal, and  above  which is  a process  called the neural 
THE SKIN AND SKELETON.
139
 spine.   On the anterior and posterior edges of the arch
 are smooth surfaces, or zygapophyses, which in the natural
 state are covered with cartilage, and come in contact with
 the corresponding surfaces of the preceding and succeed-
 ing vertebrae.  The bases of the arch are notched in front
 and behind, so that  when two vertebrae are put together a
 round opening (intervertebral foramen)  appears between
 the pair, giving passage to the nerves issuing from the spi-
 nal cord. From the  sides of the arch, blunt transverse proc-
 esses project outward and  backward, called diapophyses.
 Such are the main  elements  in a representative vertebra.
 The haemal arch is  not formed by  any part of the  verte-
 bra, but by the ribs and breast-bone.  Theoretically, how-
 ever, the ribs are considered  as elongated processes from
 the centrum (pleurapophyses),  and in a few  cases  a hce-
 mal spine is developed from the breast-bone  correspond-
 ing to the neural spine.
   The  vertebrae  are united  together  by ligaments,  but
 chiefly  by a very tough, dense, and elastic substance be-
 tween the centra.   The neural arches form  a continuous
 canal which contains and protects the spinal cord; hence
 the vertebral column is called  the neuroskeleton.   The
 column  is always more or less curved; but the beautiful
 sigmoid  curvature   is peculiar  to  Man.   The vertebrae
 gradually increase in size from the head toward the end
 of  the trunk, and then  diminish to the  end of the tail.
 The neural  arch and centrum are seldom wanting;  the
 first vertebra in the  neck has no centrum, and the last in
 the tail is all centrum.   The  vertebrae  of  the  extremities
 (head and tail) depart most widely from the typical form.
  The vertebral column in Fishes and Snakes  is divisible
into three regions — head, trunk,  and  tail.   But in  the
higher animals there are six  kinds of vertebrae:  cranial,
cervical,  dorsal, lumbar, sacral, and caudal.
  The cranial vertebral form the skull.  They are greatly 
140
COMPARATIVE ZOOLOGY.
Fig. 110. 
THE  SKIN  AND SKELETON.                 1±1
BONES  OF  THE  MAMMALIAN SKULL*


                       BRAIN-CASE.
NASAL.
       LAC
NOSE.


ETHMOID.
    FRONTAL.            PARIETAL.                 SUPRAOCCIPITAL
HRYMAL.                             SQUAMOSAL.

ORBITOSPHENOID.  EYE. ALISPHENOID. PERI- EAR. OTIC.  EXOCCIPITAL.

                     MALAR.          TYMPANIC.

  PRE8PHEN0ID.        BASISPHENOID.                BASIOCCIPITAL.
         VOMER.                                       HYOID ARCH.
PREMAXILLA.   MAXILLA.   PALATINE.   PTERYGOID.

             LOWER JAW, OR MANDIBLE.
                        THE  SKULL OF THE  DOG.

Fig. 108.—Under surface.   Fig.  109.—Upper surface.   Fig. 110.—Longitudinal ver-
   tical section ;  one-half natural size: SO, supraoccipital; ExO, exoccipital; BO,
   basioccipital;  IP, interparietal; Pa, parietal; Fr, frontal;  Sq, squamosal; Ma,
   malar;  L, lachrymal; Mx, maxilla; PMx, premaxilla;  Xa,  nasal; MT, maxillo-
   turbinal; ET, ethmoturbinal; ME, ossified portion of the mesethmoid;  CE, cri-
   briform, or sieve-like, plate of the ethmoturbinal;  V0, vomer; PS, presphenoid ;
   OS, orbitosphenoid; AS, alispheuoid;  BS, basisphenoid; PI, palatine;  Pt,
   pterygoid;  Per, periotic ; Ty, tympanic bulla; on, anterior  narial aperture; ap,
   or apf,  anterior palatine foramen; ppf, posterior palatine foramen ; io, infra-
   orbital foramen ; pof, postorbital process of frontal bone ; op, optic foramen ; sf,
   sphenoidal fissure; fr,  foramen rotundum, and  anterior opening of alisphenoid
   canal;  as, posterior opening  of alisphenoid canal; fo, foramen ovale ; Jim, fora-
   men lacerum medium ;  of, glenoid fossa; gp, postglenoid process;  pgf, post-
   glenoid foramen ;  earn, external auditory  meatus;  sm, stylomastoid foramen;
   ftp, foramen lacerum posterins ; cf, condylar foramen ; pp, paroccipital process;
   oe, occipital condyle ; fm, foramen magnum ; a,  angular process ; s, symphysis of
   the mandible where it unites with the left ramus ; id, inferior dental canal; cd,
   condyle; cp, coronoid process; the  * indicates the part  of the cranium to which
   the condyle is articulated when the mandible  is  in place;  the upper border  in
   which the teeth are implanted is called alveolar; sh, eh, ch, bh, th, hyoidean ap-
   paratus, or os linguae, supporting  the  tongue.   In the skulls of old animals,
   there are three ridges:  occipital, behind;  sagittal median, on the upper snrface;
   and stiper orbital, across the frontal, in the  region of the eyebrows.  The last is
   highly developed in the Gorilla.
   • In this diagram, modified from Huxley's, the italicized bones are single; the
rest are double.  Those in the line of the Ethmoid form the Cranio-facial Axis:
these, with the other sphenoids and occipitals, are developed in cartilage; the rest
are membrane bones.  In the Human skull, the three occipitals coalesce into one. 
142
COMPARATIVE ZOOLOGY.
modified, as the neural arches are expanded to inclose the
brain.  The number of distinct bones composing the skull
is greatest in Fishes, and least in Birds:  this arises from
the fact that the bones remain separate in the former case,
while those  of the chick  become united  together (anchy-
losed) in the  full-grown Bird.   A skull consists  of  the
brain-case and the face.   The principal parts of  the skull,
as shown in  the Dog's, are:  1. The occipital bone behind,
containing a large  hole, or foramen magnum, on each
side of which are rounded  prominences, called condyles,
by  which the  skull  articulates with  the first cervical ver-
tebra.   2. The parietal.   3. The frontal.   These three
form the main walls  of the brain.   4. The sphenoid, on
the floor of the skull in  front of the occipital, and con-
sisting  of three pieces.   5. The temporal, in which is sit-
uated the ear.   In Man this is one bone;  but in most ani-
mals there are three—the periotic, tympanic, and squa-
mosal.    6.  The  malar,  or  "cheek-bone,"  which sends
Fig. 111.—Skull of the Horse:  1, premaxillary bone; 2, upper incisors;  3, upper
   canines;  4, superior maxillary; 5, infraorbital foramen ; 6, superior maxillary
   spine; 7, nasal bones ;  8, lachrymal; 9, orbital cavity; 10, lachrymal fossa; 11,
   malar; 12, upper molars; 13, frontal; 15, zygomatic arch;  16, parietal; IT, oc-
   cipital protuberance; 18, occipital crest; 19, occipital condyles; 20, styloid proc-
   esses ; 21, petrous bone; 22, basilar process;  23, condyle of inferior maxillary;
   24, parietal crest; 25, inferior maxillary; 26, lower molars; 27, anterior maxillary
   foramen; 28, lower canines; 29, lower incisors. 
THE SKIN AND SKELETON.
143
back a process to meet one from the squamosal, forming
the zygomatic arch.  7. The nasal, or roof of the nose.
8. The maxilla; that part of the upper jaw in which the
canines, premolars, and molars are lodged.  9. The pre-
maxilla, in which  the  upper incisors are situated.  10.
The palatine, which, with the maxillary bones, forms the
roof of the mouth.  There are two  appendages  to the
skull:  the mandible, or  lower jaw, whose condyles, or
rounded extremities,  fit into a cavity (the glenoid) in the
temporal bone; and the hyoid, situated at the root of the
tongue.
   The cervical vertebral, or bones of the neck, are peculiar
in having an orifice on each side of the centrum for the
passage of an  artery.  The  first, called  atlas,  because  it
supports the head,  has no centrum, and  turns on the sec-
ond, called axis, around a blunt process, called the odon-
toid.   The centra  are  usually wider than  deep, and the
neural spines very short, except in the last one.  The num-
ber of cervical vertebrae ranges from 1 in the Frog to 25
in the Swan.
   The dorsal vertebral  are such as bear  ribs, which, unit-
ing with the breast-bone, or sternum, form a bony arch
over the heart and lungs, called the thorax.  The sternum
may be wanting, as in Fishes and Snakes, or greatly de-
veloped,  as  in  Birds.   When present, the first  vertebra
whose ribs are  connected  with it is the first dorsal.  The
neural spines of the dorsal series are generally long, point-
ing backward.
   The lumbar vertebral are the massive vertebrae lying in
the loins between the dorsals and the hip-bones.
   The sacral vertebral lie between the hip-bones, and are
generally consolidated into  one  complex  bone, called sa-
crum.
   The  caudal  vertebral  are placed behind the sacrum,
and form the tail.    They diminish in size, losing proc- 
144
COMPARATIVE ZOOLOGY.
esses and neural arch, till finally nothing is left but the
centrum.  They number from 3 or 4 in Man to 270 in
the Shark.
  Besides the  lower jaw, hyoid,  and  ribs, Yertebrates
have other appendages to the spinal column — two pairs
of limbs.™   The fore limb is divided into the pectoral
arch (or shoulder girdle), the arm, and  the hand.   The
arch is fastened to  the  ribs and  vertebrae by  powerful
muscles, and consists of three bones, the scapula, or shoul-
der-blade, the coracoid,  and the  clavicle, or collar-bone.
The scapula and coracoid are generally united, the  latter
forming a  process of the former;  and the clavicles are
frequently  wanting,  as in the hoofed  animals.  The hu-
merus,  radius, and  ulna are the  bones  of the  arm, the
first articulating by  ball-and-socket joint with the scapu-
la, and  by  a  hinge-joint  with the radius and ulna.   The
humerus and radius are always present; but the ulna may
be absent.  The bones of the hand are divided into those
of the carpus, or wrist; the metacarpus, or palm; and the
phalanges, or fingers.  The fingers, or " digits," range in
number from 1 to 5.
  The hind limb is  composed of the pelvic arch (or hip-
bones),  the leg, and  the foot.   These parts  correspond
closely  with  the skeleton  of the fore limb.    Like the
shoulder, the  pelvic arch, or os innominatum, consists of
three bones—ilium, ischium, and pubis.  The  three are
distinct in Reptiles and in the young of higher animals;
but in adult Birds and Mammals they become united to-
gether,  and are also (except in Whales) solidly attached to
the sacrum.  The two pelvic arches and the sacrum thus
soldered into one make the pelvis.  The  leg-bones consist
of the femur,  or thigh; the tibia, or shin-bone; and the
fibula, or splint-bone.  The rounded head of the femur
fits into a  cavity (acetabulum) in the pelvic arch, while
the  lower  end articulates  with the tibia,  and sometimes 
THE SKIN AND SKELETON.
145
(as in  Birds)  with the fibula  also.   An extra bone, the
patella, or knee-pan, is hung by a muscle in front of the
joint between the femur and tibia of the higher animals.
The foot  is made up of  the tarsus, or ankle; the meta-
tarsus, or lower instep;  and the phalanges, or toes.  The
toes number from 1  in the Horse to 5 in Man.
   Certain parts of the skeleton, as  of  the skull,  are firm-
ly joined  together by zigzag edges  or  by overlapping;  in
either  case the  joint  is called a suture.   But  the great
majority of the  bones are intended to move one upon
another.    The  vertebrae  are locked  together   by their
processes, and also by a tough fibrous substance between
the centra, so that a slight motion  only is allowed.  The
limbs furnish the best examples of movable articulations,
as the ball-and-socket joint at the shoulder, and the hinge-
joint at the elbow.  The bones are  held together by liga-
ments, and, to prevent friction, the extremities are cover-
ed with cartilage, which is constantly  lubricated with  an
unctuous fluid called synovia.
CHEMICAL COMPOSITION OF BONES.
Phosphate of Lime, with trace of
  Fluate of Lime....................
Carbonate of Lime...................
Phosphate of Magnesia.............
Sulphate, Carbonate, and Chlorate
  of Soda..............................
Glutine and  Chondrine.............
Oil.......................................
Cod.	Tortoise.	Hawk.	Man.
57.29	52.66	64.39	59.63
4.90	12.53	7.03	7.33
2.40	0.82	0.94	1.32
1.10	0.90	0.92	0.69
32.31	31.75	25.73	29.70
2.00	1.34	0.99	1.33
100.00	100.00	100.00	100.00
10 
OS
112 -Skeleton of the Perch (Percafiuviatilia)- 1, frontal; 4, postfrontal; 7, parietal; 8, supraoccipital;  9, exoccipital;  11, alisphenoid; 12, mastoid;
  ?rnasal  17  nremaSrvTmS lary -19,  prenasal; 20, suborbital plates;  21, supratemporal (peculiar to Fishes); 23, mastotemporal; 24, trans-

  vie bone%P,^                       orflap*, closing the gill-openings; 30, preoperculum; 31 ^^ <P<^ SSESSS
  33, interopercuium; 34, dental-that part of the lower jaw containing the teeth;  35 supra-angular;  36 angu ar; 4J, nrohyj, lying^between the

  branches of the os hvoides- 46  suprascapular; 47, scapula;  48, coracoid; 50, digital rays of pectoral fin;  51, ulna; 52, radius, 5d, carpus, t>f, o»,

  S^/^i?SfS«iS;^lSS.m«;t of the caudal fin, 71; 72, ribs, 73, styliform processes; 74, 79, interspinous bones; 75,  dorsal fins;

  80, pelvic bone; 81, ventral fin; 83, 6, hsemal spines; 85, a, parapophyses; 86,  anal fin.
O
>

•—i



N
O
o
t-l
o 
THE SKIN AND SKELETON.
147
to   ^^
 
148
COMPARATIVE ZOOLOGY.
Fig. 115.—Skeleton of the Tortoise (plastron removed):  a, cervical vertebrse; c, dor-
   sal vertebrae; d, ribs; e, sternal ribs (the marginal bones of the carapax); I, scap-
   ula ; fc, clavicle; 6, coracoid bone; /, pelvis; i, femur: g, tibia: h, fibula.
Fio. 116.—Skeleton of a Vulture:  1, cranium—the parts of which are separable only
   in the chick; 2, cervical vertebra?;  3, dorsal; 4, coccygeal, or caudal; the lumbar
   and sacral are consolidated together, and to the ribs, 5;  6, sternum, or breast-
   bone, extraordinarily developed; 7, furculum, clavicle, or " wish-bone;" 8, cora-
   coid; 9, scapula; 10, humerus;  11, ulna, with rudimentary radius; 12, metacar-
   pals; 13, phalanges of the great digit of the wing; 19, thumb; 14, pelvis; 15, fe-
   mur ; 16, tibia and fibula, or crus; 17, metatarsus (tarsus is wanting in Birds); 18,
   internal digit, or  toe, formed  of three phalanges; the middle toe has four pha-
   langes ; the outer, five;  and the back toe, or thumb, two. 
THE  SKIN AND  SKELETON.                149
Fig. 117.—Skeleton of the Horse (Equus caballus): 22, premaxillary; 12, foramen in
   the maxillary;  15, nasal; 9, orbit;  19, coronoid process of lower jaw; 17, surface
   of implantation for the masseter muscle; there are seven cervical vertebrse, nine-
   teen dorsal, D-D; five lumbar, a-e; five sacral,/-2; and seventeen caudal, p-r;
   51, scapula, or shoulder-blade; i, spine, or crest; h, coracoid process (acromion
   wanting);  1, first pair of ribs (clavicle wanting, as in all Ungulates); e, sternum;
   a, shaft of humerus; 6, deltoid ridge;  g, head fitting in the glenoid cavity of the
   scapula—near it is a great tuberosity for the attachment of a powerful muscle; k,
   condyles; 54, radius, to which is firmly anchylosed a rudimentary ulna, 55, called
   olecranon; 56, the seven bones of the carpus, or wrist; 57, large metacarpal, or
   "cannon-bone," with two " splint-bones;" 58, fetlock-joint; 59, phalanges of the
   developed  digit, corresponding to  the third finger  in  Man;  62, pelvis;  63, the
   great trochanter,  or  prominence on the femur, 65; 66, tibia;  67, rudimentary
   fibula; 68, hock, or heel, falsely called knee; 69, metatarsals. 
150
COMPARATIVE ZOOLOGY.
Fig. 118.—Skeleton of the Ox (Bos taurw).
ii

Fig. 119__Skeleton of an Elephant (Elephas Indicus). 
THE SKIN AND SKELETON.
151
Fig. 120.—Skeleton of the Chimpanzee (Troglodytes niger). 
152
COMPARATIVE ZOOLOGY.
                  CHAPTER XVII.
               HOW ANIMALS MOVE.
  1. Muscle.—The power of animal motion is vested in
protoplasm, cilia, and muscles.   The simplest forms  of
life, as  the structureless  Sponge  and Amoeba, move by
the contraction and extension of the protoplasm of which
they consist.   In nearly all animals we find with  the  mi-
croscope myriads of little hair-like cilia, which are inces-
santly vibrating.  They are seen on the outside of  Infu-
soria, serving  as paddles for locomotion; they fringe  the
gills of  the Oyster, creating currents for respiration; and
they line the passage to  our lungs to  expel the  mucus.
The cause of ciliary  motion is unknown; no muscle or
nerve has been traced to them.   Water seems to be a nec-
essary condition.
  But muscular tissue is the great motor agent, and exists
in all animals from the Coral to Man.  The power of con-
tractility, which in the Amoeba is  diffused throughout the
body, is here confined to  bundles  of highly elastic fibres,
called muscles. When a muscle contracts, it tends to bring
Fig. 121.—A Contracting Muscle.
its two  ends together, thus shortening itself, at the same
time increasing in thickness.  This  shrinking property is
excited  by external stimulants, such as  electricity, acids,
alkalies, sudden heat or cold, and even a sharp blow; but 
HOW ANIMALS MOVE.
153
the ordinary cause of contraction is an influence from the
brain conveyed  by a nerve.  The property,  however,  is
independent of  the  nervous system, for it does not cease
immediately  after  death.    The  amount  of  force with
which  a muscle contracts depends on the number of its
fibres; and the amount of shortening, on their length.
  As a rule, muscles are white in cold-blooded animals,
and red in the warm-blooded.   They are white in all the
Invertebrates, Fishes, Batrachians,  and Reptiles, except
Salmon, Sturgeon, and  Shark; and  red in  Birds and
Mammals, except in the breast  of the common fowl, and
the like.89
  It is also a rule, with  some exceptions, that the  volun-
tary muscles of Yertebrates, and  all the  muscles  of the
Lobster, Spider, and Insect tribes, are striated; while the
involuntary muscles of Vertebrates, and all  the muscles
of Radiates, Worms, and Mollusks, are smooth.  All mus-
cles attached  to internal bones, or  to a jointed external
skeleton, are striated.  The voluntary muscles are  gener-
ally solid ;  and the involuntary,  hollow.90
  This leads  to another classification  of muscles:  into
those which are attached to solid parts within  the body;
those which are attached to the skin or its modifications;
and those having no attachments, being complete in them-
selves.  The last are hollow or  circular muscles, inclosing
a cavity or space, which they reduce by contraction.  Ex-
amples of such are seen in the heart, blood-vessels, stom-
ach, iris of the eye, and  around the mouth.  In the lower
Invertebrates, the  muscular system is a net-work of longi-
tudinal, transverse, and oblique fibres intimately blended
with the skin, and not divisible into separate muscles.  As
in the walls of the human stomach, the  fibres are usually
in three distinct layers.  This arrangement is exhibited by
soft-bodied animals, like the Sea-anemone, the Snail, and
the Earth-worm.  Four thousand fibres have been counted 
154
COMPARATIVE ZOOLOGY.
in a Caterpillar.  There are also "skin-muscles"  in  the
higher animals,  as those by which the  Horse produces a
twitching of the skin  to shake  off Insects, and those by
which the hairs of the  head and the feathers of Birds  are
made to stand on end.   Invertebrates, whose skin is hard-
ened into a shell or crust, have muscles attached to the in-
side of such  a skeleton.  Thus, the Oyster has a mass of
parallel fibres connecting its two valves; while in the Lob-
ster and  Bee, fibres go from ring to ring, both longitudi-
nally  and spirally.  The muscles of all Invertebrates  are
straight parallel fibres, not in  bundles, but distinct, and
usually flat, thin, and soft.
  The great  majority  of the muscles of Vertebrates  are
attached to the bones, and such are voluntary.  The fibres,
which are coarsest in Fishes (most of all in the Rays), and
finest in  Birds, are bound into  bundles by a web-like  tis-
sue ; and the muscles thus made up are arranged in layers
around the skeleton.   Sometimes their extremities are at-
tached to the bones (or rather to the periosteum) directly;
but generally by means of white  inelastic  cords, called
tendons.   In Fishes, the chief  masses of muscle are dis-
posed along the sides of the body, apparently in longitu-
dinal  bands,  reaching  from head to tail, but really in a
series of vertical flakes, one for each vertebra.  In propor-
tion as limbs are developed, we  find the muscles concen-
trated about  the shoulders and hips, as in quadrupeds.
The bones of the limbs are used as levers in locomotion,
the fulcrum being the  end of a bone with which the mov-
ing one is articulated.   Thus, in raising the arm, the  hu-
merus is a lever working upon the scapula as a fulcrum.
The most important muscles are called extensors and flex-
ors.  The former are such as pass over the back of a joint
to extend the bone beyond it; while the flexors lie in front
of the joint to bring the same bone into an angle with its
fulcrum—as in bending the arm. 
HOW ANIMALS MOVE.
155
   2. Locomotion.—All animals have the power of vol-
untary motion, and all, at  one time or another, have the
means of moving themselves from place to place.  A few
are free  in  the embryo-life, and fixed when adult, as the
Sponge,  Coral, Crinoid, and Oyster.   There may be no
regular well-defined means of progression, as in  the Amoe-
ba, which extemporizes arms  to creep over the surface;
or  movement may be accomplished by  the contraction
of  the whole body, as in the Jelly-fish,  which, pulsating
about  fifteen times in a minute, propels  itself backward
through  the water.  So the Worms  and  Snakes swim by
the undulations of the body.
   But, as a rule, animals are provided with special organs
for locomotion.   These become reduced in number, and
progressively  perfected,  as we  advance in the scale of
rank.  Thus, the Animalcule is  covered with  thousands
of  hair-like cilia; the Star-fish has hundreds of soft, un-
jointed, tubular suckers; the  Centipede  has from 30 to
40 jointed hollow legs;  the Lobster, 10; the Spider, 8;
and the  Insect, 6;  the Quadruped has 4 solid limbs for
locomotion ;  and Man, only 2.
   (1) Locomotion in Water.—As only the lower  forms of
life are aquatic, and as the weight of the body is partly
sustained by the element, we must expect to find the or-
gans of  progression  simple and feeble.   The  Infusoria
swim with great rapidity  by  the  incessant vibrations of
the delicate filaments, or cilia, on their bodies.   The com-
mon Squid on our coast admits water into the interior of
the body, and then suddenly forces it out through a  fun-
nel, and thus moves backward, or forward, or around, ac-
cording as the funnel is turned—toward the head, or tail,
or to one side.   The Lobster has a fin at the  end of its
tail, and  propels itself backward by a quick down-stroke
of the abdomen.
  But  Fishes, whose bodies offer  the least resistance to 
156
COMPARATIVE ZOOLOGY.
progression through water, are the most perfect swimmers.
Thus, the Salmon can go twenty miles an hour, and  even
,t-porsnl
             Fig. 122.—The Fins of a Fish (Pike-perch).
ascend cataracts.  They have fins of two kinds:  those set
obliquely to the body, and in pairs;  and those which are
vertical, and single.   The former, called pectoral and ven-
tral fins, represent the fore and hind limbs of quadrupeds.
The vertical fins, which are  only expansions of  the skin,
vary in number; but in  most Fishes there are  at least
three: the caudal, or tail-fin; the dorsal,  or back-fin;
                    and the anal,  situated on  the abdo-
                    men,  near the  tail.  The  chief loco-
                    motive agent is the tail, which sculls
                    like a stern - oar;  the other fins  are
                    mainly  used to balance and raise the
                    body.   When  the two  lobes of  the
                    tail are equal, and the vertebral  col-
                    umn stops short at its base, as in the
                    Trout, it  is said to be homocercal.
                    If the vertebrae extend into the  up-
                    per lobe, making it  longer than the
                    lower one, as in the Shark, the tail is
                    called heterocercal.  The latter is the
                    more effective for varying the course;
                    the Shark, e. g., will accompany  and
                    gambol  around  a ship in  full  sail
                      The Whale swims by striking the
Fig. 123.—Diagram illustrat-
  ing the locomotion of a
  Fish.  The tail describes
  the arc of an ellipse; the
  resultant of the two im-
  pulses is the straight line
  in front.
across the Atlantic 
HOW ANIMALS MOVE.
157
water  up and down, instead of laterally, with a fin-like
horizontal tail.  Many air-breathing animals swim with
facility on the surface, as the Water-birds, having webbed
toes, and most of the Reptiles and Quadrupeds.
  (2) Locomotion in Air.—The power of flight requires a
special modification of  structure  and an  extraordinary
muscular  effort, for air is 800 times lighter  than water.
Nevertheless, the velocity attainable by certain Birds is
greater than that of any Fish or Quadruped; the Hawk
being able to go 150 miles an hour.   The bodies of  In-
sects and  Birds  are  made as light  as possible by the dis-
tribution of air-cavities.91
  The wings  of Insects are generally four in  number;
sometimes only  two, as in the Fly.   They are moved by
muscles lying inside the thorax.   They are simple expan-
sions of the  skin, or  crust, being composed of two delicate
films of the epidermis stretched upon a net-work of tubes.
There  are three main varieties:  thin and transparent, as
in the Dragon-fly;  opaque, and covered with minute col-
ored scales,  which are  in reality flattened hairs, as in  the
Butterfly; and hard and opaque,  as  the first pair (called
elytra) of the Beetle.
  The wings of Birds, on the other hand, are  modified
fore-limbs, consisting of three sets of feathers (called pri-
mary,  secondary, and tertiary), inserted on the hand, fore-
arm, and  humerus.  The muscles  which give the down-
ward stroke of the wing are fastened to the breast-bone;
and their power, in  proportion to the weight of the Bird,
is as 10,000  to 1. Yet the Insect is even superior in vigor
and velocity of flight.98   In ascending, the Bird slightly
rotates the  wing, striking downward and  a  little back-
ward ;  while the tail acts as a rudder.  A short,  rounded,
concave wing, as in the common Fowl, is not so well fit-
ted for high and prolonged flight as the long, broad, point-
ed,  and flat wing of the  Eagle.   The wing is folded by 
158
COMPARATIVE ZOOLOGY.
means  of an elastic skin connecting  the  shoulder and
wrist, which is stretched when the wing is expanded.
Fig. 124.—Flamingoes taking Wing.
Besides Insects and Birds, a few other animals have the
power of flight, as Bats, by means of long webbed fingers;
Flying Fishes, by large pectoral fins; and Flying Reptiles,
Flying Squirrels, and the like, by membranes between the
fore and hind legs.
  (3) Locomotion on Solids.—This requires less muscular
effort  than swimming  or flying.  The more unyielding
the basis of support, the greater the amount of force left
to move the  animal along.   The simplest method is the
suctorial, the  animal attaching itself to some fixed object,
and then, by  contraction, dragging the body onward.  But
the higher and  more common method is by the use  of
bones, or other hard parts, as levers.
  The  Star-fish creeps by the  working of hundreds  of
tubular suckers, which are extended by being filled with
fluid forced into them by little sacs.  The Clam moves
by fixing and contracting a muscular  appendage,  called
a " foot."  The Snail  has innumerable short muscles on
the under side of its body, which, by successive contrac-
tions resembling minute undulations, enable the animal
to glide  forward apparently  without effort.  The  Leech 
HOW ANIMALS MOVE.
159
has a sucker at each end: fixing itself by  the  one on its
tail, and  then stretching  the body,  by contracting the
muscular  fibres which run around  it,  the creature fastens
its mouth by suction, and draws forward the hinder parts
by  the contraction of longitudinal muscles.   The  Earth-
worm lengthens  and shortens itself in the same way  as
the Leech, but instead of suckers for holding its position,
it has numerous  minute spines pointing backward; while
the Caterpillar has short legs for the  same purpose.  The
legless Serpent  moves by means  of  the scutes, or large
scales, on the under side of the body, acted upon  by the
ribs.  In  a straight line, locomotion is slow; but by curv-
ing the body laterally or vertically,  it can glide or leap
with great rapidity.
  Most animals  have movable jointed limbs, acted upon
as levers by numerous muscles.  The Centipede has forty-
two legs,  each with five joints and a claw.   The Crab has
five pairs of six-jointed legs; but the front pair is modi-
fied into  pincers for  prehension.   With  the rest, which
end in a  sharp claw, the Crab  moves backward, forward,
or  sideways.   The Spider has eight legs, usually seven-
jointed,  and  terminating in two claws  toothed  like a
Fig. 125__Diagrammatic section of Star-fish: a, mouth; b, stomach; c, caecum, or in-
   testine ; d, dorsal surface;  e, ambulacral plates; /, ovarium; g, tubular feet; ft,
   Internal sacs for extending the feet.

comb, and a third which  acts like a  thumb.  In running,
it moves the first right leg, then the fourth left; next,  the
first left, and then the fourth right; then the third right 
160
COMPARATIVE ZOOLOGY.
and  second left together;  and, lastly, the third  left and
second  right together.   The front and  hind pairs are,
therefore, moved like those of a quadruped.  The Insect
                                   has six legs, each of
                                   five joints: the coxa,
                                   or hip; trochanter;
                                   femur;  tibia,  or
                                   shank; and tarsus.
                                   The  last is  subdi-
                                   vided  usually  into
                                   five   joints   and  a
                                   pair of claws.  Such
                                   as can walk  upside
                                   down,  as  the  Fly,
                                   have,  in  addition,
                                   two  or three suckers
                                   between the claws.93
                                   While the leg-bones
                                   of Vertebrates  are
                                   covered by the mus-
cles  which moved them, the limbs of  Insects are hollow,
and  the muscles inside.  The fore legs are directed for-
ward, and the  two hinder pairs backward.  In  motion,
the fore and hind feet on one side, and the middle one
on the other, are moved simultaneously, and then  the re-
maining three.
  The four-legged animals have essentially the same ap-
paratus and method  of motion.   The Crocodile  has  an
awkward gait, owing to the fact that the limbs are short,
and  placed far apart, so that the muscles act at a mechan-
ical  disadvantage.  The Tortoise is proverbially slow for
a similar reason.  Both swim better than they walk.  Liz-
ards are light and agile, but progression is aided by a wrig-
gling of the body.
  The locomotive organs of the mammalian quadrupeds
Fig. 126. — Feet of Insects: A, Bibio febrilis; B,
  House - fly (Musca domestica); C, Water - beetle
  (Dytiscus). 
                  HOW ANIMALS MOVE.               161

are much  more  highly organized.  The bones are more
compact; the vertebral column is arched, and yet elastic,
between the shoulder and hip, and the limbs are placed
vertically underneath the body.   The bones of the fore
limb  are nearly  in a  line; but  those of the  hind  limb,
which is mainly used  to project the body  forward,  are
more  or less inclined  to one  another,  the  angle  beino-
greatest in animals of great speed, as the Horse.   Some
walk  on hoofs, as the Ox (Ungulate); some on the toes,
as the Cat (Digitigrade); others  on the sole, touching  the
ground  with the  heel, as the Bear (Plantigrade).  In  the
Fig. 127. —Feet of Carnivores:  A, Plantigrade (Hear);  B, Pinnigrade (Seal); C,
                      Digitigrade (Lion).
Pinnigrade Seal, half of the fore limb is buried under the
skin, and the hind limbs are turned backward to form a
fin with the tail.   The normal  number of toes is five;  but
some  may be wanting,  so that we  have one-toed  animals
(as Horse), two-toed (as Ox),  three-toed (as Rhinoceros),
four-toed  (as Hippopotamus),  and five-toed (as  the Ele-
phant).  The Horse steps on what corresponds to  the nail
of the middle finger; and  its swiftness is mainly owing
to the solidity of the  extremities of the limbs.   Horses
of the greatest speed have the shoulder-joints  directed at
a considerable ans^le with the arm.
                          11 
162
COMPARATIVE ZOOLOGY.
  The order  in  which  the legs  of quadrupeds succeed
each other determines the  various modes of progression,
called  the walk, trot, gallop, and leap.   Many, as  the
Horse, have all these movements; while some only leap,
as the Frog and Kangaroo.   In leaping animals, the hind
limbs  are extraordinarily  developed.    In many  Mam-
mals, like the  Squirrel,  Cat,  and Dog, the fore legs  are
used for prehension as well as locomotion, and such have
a collar-bone.   Monkeys use all four, and also the tail, for
Fio. 128.—Feet of Hoofed Mammals:  A, Elephant; B, Hippopotamus; C, Khinoc-
   eros;  D, Ox; E, Horse, a, astragalus; cl, calcaneum, or heel; s, naviculare; b,
   cuboides; ce, ci, cm, cuneiform bones; the numbers indicate the digits in use.
locomotion and prehension, keeping a horizontal attitude;
while the Apes, half erect, as if they were half-quadruped,
half-biped, go shambling along, touching the ground with
the knuckles of one hand and then of the other.   In de-
scending the scale, from the most anthropoid Ape to the
true quadruped, we find the centre of gravity placed in-
creasingly  higher up—that is, farther  forward.    Birds
and Men are the only true bipeds; the former standing
on  their toes, the latter  on the  soles of the feet.   Terres-
trial Birds walk and run; while Birds of Flight usually 
HOW  ANIMALS MOVE.
163
hop.    The  Ostrich  can for  a  time  outrun  the Arabian
Horse;  and  the  speed  of  the Cassowary exceeds that of
the swiftest Greyhound.
Pig. 129.—Muscles of the Human Leg:
 sartorius, or " tailor's muscle," the
 longest  muscle in the body, flexes
 the leg upon the thigh ; rectus femo-
 ris and  vastus externus and internus
 extend the leg, maintaining an erect
 posture;  gastrocnemius,  or  "calf,"
 used chiefly in walking,  for raising
 the heel.  Another  layer underlies
 these superficial muscles.
Fig. 130.—Muscles of an Insect's Leg
  (Melolontha vulgaris): a, flexor, and
  b, extensor, of tibia; c, flexor of foot;
  d, accessory  muscle; e, extensor of
  claw; /, extensor of tarsus.   The
  joints are restricted to movements
  in one plane ; and therefore the mus-
  cles are simply flexors and extensors.
  All the muscles are within the skele-
  ton. 
164
COMPARATIVE ZOOLOGY.
CHAPTER XVIII.
THE  NERVOUS  SYSTEM.
  Nervous Matter exists in  the  form of cells, fibres, or
tubes.  In the cellular state it is grayish, and accumulated
                                      in  masses,  called
                                      ganglia, or centres,
                                      which alone origi-
                                      nate nervous force;
                                      the fibrous and tu-
                                      bular kinds are gen-
                                      erally white,  and
                                      arranged  in bun-
                                      dles,  called nerves,
                                      which serve only as
                                      conductors.   Most
                                      nerves  consist   of
                                      white fibres,  and
                                      go in pairs, each
                                      member  having  a
distinct ofiice: one carries impressions received from  the
external  world to the gray centres, and
hence  is called  an afferent, or sensory,
nerve; the other conducts an influence
generated in the centre to the muscles,
in  obedience to  which  they contract,
and hence  it is  called an efferent, or
motor, nerve.   Thus,  when  the finger  Fio
is pricked with  a pin, an  afferent nerve   tem of a star-fish :con-
  L               x _                     si sting of five ganglia,
conveys  the impression to the great cen-   g,  around the mouth,
      ,  .   .    . . .  .      ..   .         which send to each ray
tre—the brain, which immediately trans-   a pair of nerves.
Fig. 131.—Nerve-cells from Human Brain: A, associ-
 ated with nerve-tubes and blood-vessels; B, multi-
 polar nucleated cells.
 
THE NERVOUS SYSTEM.
165
^
 mits an order by  an efferent nerve to the muscles of the
 hand to contract.  If the former
 are cut, sensation is lost, but vol-
 untary motion remains;  if the
 latter are cut, the animal loses
 all control over the muscles, al-
 though sensibility is perfect;  if
 both are cut,  the  animal is said
 to  be paralyzed.   The nature of
 nerve-force, and exactly how the
 nerves terminate in the skin and
 muscles,  are unsolved problems.
               As to the velocity
               of  a  nervous im-  ,
                                  Fig.  133—Nervous System of a
               pulse, We knOW it   Mollusk, the Gasteropod Aplys-
               .                   ia: a, anterior ganglion; c, ce-
               is  far  leSS than   phalic; I, lateral; g, abdominal.
               that of electricity or light, and that it is
               more rapid in warm-blooded than in cold-
               blooded animals, being nearly twice as fast
               in Man as in the Frog.
                  Nervous matter  in the form  of  cells
               doubtless  exists in  the  very  lowest ani-
               mals,  although it is  invisible under the
               most  powerful microscopes.9*   But a nerv-
               ous system of centres and nerves for keep-
               ing up a communication between different
               parts of the body  is not  required in  such
               as have no distinct organs; we would look
               for it  only in those possessing a well-de-
               fined  muscular system.   In the Star-fish
Fig. 134.—Nervous  we detect  the first clear specimen of  cells
 pfiiarm(Spftt2teK-  and fibres  connected together  to  receive
 gustri); the first  an(j convey impressions.   It consists of a
 is the cephalic, or            j    r
 head, ganglion.    rmg around the mouth, made of five gan-
glia of  equal size, with radiating nerves.  The Mollusks 
166
COMPARATIVE ZOOLOGY.
are distinguished by an irregularly scattered nervous sys-
tem.  They have two or more ganglia around the gullet,
and  one or two more  in  the  posterior region;  all are
united by threads, and  send off nerves to the various or-
gans.  The articulated  animals generally have  a double
nervous cord lying along  the  ventral side, and studded
with ganglia of nearly uniform  size, except the first, which
is the largest of all, and represents the brain.  In the sim-
ple Earth-worm there is no trace of ganglia; in the Centi-
pede and Caterpillar there is a ganglion for each  segment;
but in the higher forms, as the Bee, several ganglia are
fused together in the head and thorax, indicating a con-
centration of organs for sensation and locomotion.
  In Yertebrates, the nervous system is more highly de-
veloped, more complex, and more concentrated than in
the lower  forms.   In fact, there are some parts, as the
brain, to which we find  nothing analogous in the Inverte-
brates;  and while the actions of the latter are mainly, if
not wholly, automatic,  those of backboned animals are
voluntary.   Its position, moreover, is peculiar,  the  great
mass  of the nervous matter being accumulated  on the
dorsal side, and inclosed by the  neural arches of  the skele-
ton.
  The brain and spinal  cord lie in the cavity of the  skull
and spinal column, wrapped in three membranes.   Both
consist of gray and white nervous matter; but in  the brain
the gray is on the outside, and the white within;  while the
white of the spinal cord is external, and the  gray internal.
Both are double, a deep fissure running from the forehead
backward,  dividing the brain into two hemispheres, and
the spinal cord resembling two columns welded  together;
even the nerves come forth in pairs to the right and left.
So that  a person may be said to consist of two individuals
acting simultaneously.   If the two halves of the brain do
not act in concert, the  man is said to be insane; if one 
THE NERVOUS SYSTEM.
167
..--&
3-.
4..

 half of the spinal cord is diseased, one side of the body is
 paralyzed.   The brain is the organ of sensation; the spi-
 nal cord is the organ of mere life and motion.   The brain
 may  be  removed,  and  yet  the
 animal, though  it  can not feel,
 will  live  for a time,  showino;
 that it is not absolutely essential
.to life; in  fact, the brain does
 nothing in  apoplexy  and  deep
 sleep.  But if the spinal cord be
 destroyed,  the  animal  dies,  for
 it can neither move nor breathe,
 as all the  afferent  and  efferent
 nerves terminate in the  cord.
   The Brain is that part of the
 nervous system contained in the
 skull.95  It increases in size and
 complexity as we  pass from  the
 Fishes, by the Reptiles and Birds,
 to Mammals.  Thus, the body of
 the Cod  is  5000  times  heavier
 than its brain—in fact, the brain
 weighs less than  the spinal cord;
 while  in Man,  the brain,  com-
 pared  with the bod}7, is  as 1 to
 36, and is 40 times heavier than  ^Xo^fX S^JfifJ
 the spinal cord.   The brains of
 the Cat weigh only 1 oz.; of the
 Dog,  6 oz.  5£ dr.; and of the
 Horse, 22 oz. 15 dr.  The  only
animals whose brains outweigh
Man's  are  the   Elephant   and
Whale;  the  maximum  weight
of the  Elephant's being 10 lbs.;  and of the Whale's, 5 lbs.;
while  the human does not exceed 4 lbs.   Yet the human
great longitudinal fissure;  6, an-
terior lobe; c, middle lobe; d,
medulla oblongata; e, cerebel-
lum ;  /, first spinal nerve; g,
brachial plexus of nerves supply-
ing the arms ; ft, dorsal nerves; i,
lumbar nerves;  k, sacral plexus
of nerves for the limbs; I,  cauda
equina: the figures indicate the
twelve pairs of cranial nerves, of
which 1 is olfactory, 2 are  optic,
and 8 auditory. 
168
COMPARATIVE ZOOLOGY.
brain is heavier in proportion to the body.  But quality
must be considered as well as quantity, else the Donkey
will  outrank the  Horse, and the  Canary-bird,  Man; for
their brains are relatively heavier.
  The main parts of the brain are the cerebrum, cerebel-
lum, and medulla oblongata.
  The cerebrum is a mass of wrhite fibrous matter covered
by a layer of gray cellular matter.   In the lower Verte-
                                  brates, the  exterior is
                                  smooth;  but in  most
                                  of the Mammals it is
                                  convoluted,  or folded,
                                  to increase the amount
                                  of the  gray  surface.
                                  The convolutions mul-
                                  tiply and deepen as we
                                  ascend the scale of size
                                  and intelligence, being
                                  very  complex  in the
                                  Elephant  and Whale,
                                  Monkey and Man. As
                                  a rule, they are pro-
                                  portioned to the intel-
                                  ligence of the animal;
                                  yet the brains of the
                                  Dog  and  Horse  are
                                  smoother  than  those
                                  of the Sheep and Don-
                                  key.    Evidently the
                                  quality of  the  gray
                                  matter must be taken
                                  into account.  Save in
                                  the bony Fishes, the
cerebrum  is the largest portion of the brain ;   in Man, it
is over eight times heavier than the cerebellum.
Fio. 136.—Brain of the Horse—upper view, one-
 half natural size: a, medulla oblongata; b, lat-
 eral and middle lobes of cerebellum; c, inter-
 lobular fissure;  d, cerebral hemispheres; e, ol-
 factory lobules. 
THE NERVOUS SYSTExM.
169
  The cerebellum, or " little brain," lies behind the cere-
brum, and, like it, presents an external gray layer, with a
white interior.  In Mammals, it is likewise finely convo-
luted, consisting of gray and white lamina?, and is divided
into two lobes, or  hemispheres.   In the rest of the Verte-
brates, the cerebellum  is  nearly  or quite  smooth;  and in
the lowest Fishes it is merely a thin plate of nervous mat-
ter.  In many Vertebrates, however, it is larger, compared
with the cerebrum, than in Man, since in Man the cere-
brum is extraordinarily developed.
  The medulla oblongata is the  connecting  link between
the cerebrum  and cerebellum and  the spinal cord.   In
structure, it resembles the spinal cord—the white matter
being external, and  the gray internal.  The former  lies
beneath or behind the brain, passing through the foramen
magnum of the skull, and merging imperceptibly  into the
cord.  The latter  is a continuous tract of gray matter in-
closed within  strands of white fibres,  and corresponds to
the ventral cord in Insects.  It usually ends in the lumbar
region of the  vertebral  column; but in Fishes it reaches
to the end of the tail.  In Fishes and Reptiles, the cord
outweighs the  brain; in Birds and Mammals, the  brain is
heavier than the cord.   In Man,  it weighs about an ounce
and a half.
  The parts of the brain  are always in pairs; but in rela-
tive development  and  position  they differ widely in  the
several  classes of  Vertebrates.   In Fishes and Reptiles,
they are arranged in a horizontal line; in Birds and Mam-
mals, they lie on top of  each other, till, in  Man, the axis of
the brain is at right angles with the spinal cord.  In look-
ing down upon the brain of a Cod, we see in front a pair
of olfactory lobes (which send forth the nerves of  smell),
behind  them  the  small cerebral hemispheres, then  the
large optic lobes (in which originate the nerves of sight),
and, last of all, the  thin  cerebellum.   Not till we reach 
170
COMPARATIVE ZOOLOGY.
Man and the Apes  do we find the  cerebrum so highly
developed  as to overlap both the olfac-
tory lobes  in front and the  cerebellum
                behind.
                  Functions of the Brain.
                —The cerebrum  is  the
                source of intelligence and
                will.   It has  no direct
                communication with the
                outside  world,  receiving
                its  consciousness of ex-
                ternal objects and events
                through  the spinal  cord
                and the nerves of special
                sense.  The nerves of smell
                and  sight  alone  come
                from the cerebrum.96
  The  cerebellum seems to preside over
the  muscular movements.   When re-
moved, the  animal desires to execute  the mandates of the
Pig. 137.—Brain of
 the Perch, upper
 view: a, cerebel-
 lum ; 6, cerebrum;
 c, olfactory gan-
 glia ;  i, olfactory
 nerves; g, supple-
 mentary lobes.
Srh
Fig. 138. — Brain of the
 Frog, upper view, X 4:
 /, olfactory nerves; Lol,
 olfactory lobes; He, cer-
 ebral hemispheres; Pn,
 pineal gland; Fho and
 Srh, third and fourth
 ventricles; Lop, optic
 lobes; C, cerebellum;
 Mo, medulla oblongata.
Fig. 139—A, C, upper and side views of the Brain of a Lizard.  B, D, upper and side
   views of the Brain of a Turkey: Olf, olfactory lobes; Hmp, cerebral hemispheres;
   Pn, pineal gland; Mb, optic lobes of the middle brain;  Cb, cerebellum; MO, me-
   dulla oblongata; ii, optic nerves; iv and vi, nerves for the muscles of the eye;
   Py, pituitary body. 
THE NERVOUS SYSTEM.
171
will, but can not; its motions are irregular, and  it acts as
if intoxicated.    It  is  largest in  animals capable of  the
Fig. 140.—Brain of the Cat (Felts do-      Fig. 141. —Brain of the Orang-utan,
 mesticd): a, medulla oblongata; b,        upper surface; one-third natural
 cerebellum; c, cerebrum.                size.

most complicated movements:  being  larger  in  the  Ape
than in the Lion, in the Lion than in  the Ox, in Birds
than in Reptiles.
Pig. 142. — Human Brain, side view: 1,   Fig. 143.—Human Brain, upper view,
 medulla oblongata; 3, cerebellum; 5,     one-third  natural size: 1, anterior
 frontal convolutions of cerebrum.          lobes; 2, posterior; 3, great median
                                    fissure.
brain above and the  cord below may be removed without
death  to the  animal, but the destruction  of the  medulla
 
172
COMPARATIVE ZOOLOGY.
is fatal.   Of the twelve  pairs of nerves issuing from the
contents of the skull (encephalon), ten come from the me-
dulla oblongata.   Among these are the nerves of hearing
and  taste, and those that control  the  lungs  and heart.
                                   Respiration ceases  im-
                                   mediately   when   the
                                   medulla is injured.
                                     The  spinal cord  can
                                   of itself generate nerve-
                                   force ;  but  it is mainly
                                   a conductor—propaga-
                                   ting through its central
                                   gray matter the impres-
                                   sions received  by the
                                   nerves to the brain, and
                                   taking  back  through
                                   its fibrous part the im-
                                   pulses of the brain.  In
                                   Man,  thirty-one pairs
                                   of nerves   arise  from
                                   the cord to supply the
                                   whole body, except the
                                   head.  Each nerve has
Fig. 144.—Relation of the Sympathetic and Spinal ajj anterior and  POSte-
  Nerves: c, fissure of spinal cord; a, anterior of                  "
  a dorsal spinal nerve; p,  posterior root, with rior root'.  its power of
  its ganglion; a', anterior branch; p', posterior     .
  branch; «, sympathetic; e, its double junction Causing  mUSClllar COn-
  by white and gray filaments.               ,   ,.    .   -, j   -, .
  J                                traction  is lodged in
the  former (hence  called motor), and the  power of  giv-
ing  rise to sensation resides in the latter (sensory).   The
fibres leading from  the  brain  to the cord cross one anoth-
er in the  medulla oblongata, so that if the right  cerebral
hemisphere be diseased, the left side of the body loses the
power of voluntary motion.
   The sympathetic nervous system is a double chain of
knots, or  ganglia, lying  along the sides  of the vertebral
 
THE NERVOUS SYSTEM.
173
column.   From these ganglia nerves are given off, which,
instead of going to  the skin and muscles, like the spinal
nerves, form net-works about those internal organs over
which the will has no control, as the heart, stomach, and
intestines.  Their apparent office is to stimulate these or-
gans  to constant activity.  By some  anatomists, the gan-
glia are considered as reservoirs of nervous force.

                     1. The  Senses.
  Sensation is  the consciousness of impressions on the
sensory nerves.   These impressions produce some change
in the brain; but what that change  is, is a darkness on
which no hypothesis  throws light.    Obviously, we feel
only the condition of our nervous system, not the objects
which excite that condition.
  All animals possess a general sensibility diffused over
the greater part of the body.97   But,  besides this (save in
the very  lowest  forms), they are endowed  with special
nerves for receiving the impressions  of light, sound, etc.
These nerves of sense, as they are called, although struct-
urally alike,  transmit  different sensations :  thus, the Ear
can not recognize light, and  the Eye  can not distinguish
sounds.  In the higher animals, the organs of sight, hear-
ing, and smell are situated in pairs  on each side of the
head;  that of  taste, in the mucous  membrane covering
the tongue; while the sense of touch  is diffused over the
skin.   Sight  and hearing are  stimulated,  each by one
agent only; while touch, taste, and smell may be excited
by various substances.  The agents awakening sight, hear-
ing, and touch are physical; those causing taste and smell
are chemical.   Animals differ widely  in the numbers and
keenness of their senses.  But there is no sense in any one
which does not exist in some  other.
  Touch  is the simplest and the only universal sense; no
animal is without it.   It is likewise the most positive and 
174
COMPARATIVE ZOOLOGY.
Fig. 145.—Various Antennse.
certain of the senses.  Wherever it exists, there is a tissue
containing a net-work of capillaries and the terminations
of sensory nerves.  In the Sea-anemone, Snail, and Insect,
                              it  is  most  acute  in  the
                              "feelers" (tentacles, horns,
                              and antennae) ;98 in the Oys-
                              ter, the edge of the mantle
                              is most  sensitive; in Fish-
                              es, the lips;  in Snakes, the
                              tongue ; in Birds, the beak
                              and under side of the toes;
                              in Quadrupeds, the lips and
                              tongue; and in  Monkeys
                              and Man, the lips and the
tips of the tongue and fingers.  In the most sensitive parts
of Birds and Mammals,  the true  skin is  raised up  into
multitudes of minute elevations, called papillai, contain-
ing loops of capillaries and nerve-filaments.  There  is a
correspondence between the delicacy of touch and the de-
velopment  of intelligence.  The Cat and  Dog are more
sagacious than hoofed  animals.  The Elephant  and Par-
rot are  remarkably intelligent, and are as  celebrated for
their tactual power.
  Taste is more  refined  than  touch, since  it  gives  a
knowledge of properties which can not be felt.   It is al-
ways  placed  at the entrance to the
digestive canal, as its chief purpose
is  to guide animals in their choice
of food.   No special organ of taste
can be detected in the Invertebrates,
although all seem to exercise a fac-
ulty in  selecting their  food.  Even
in Fishes, Reptiles, and  Birds this sense is very  obtuse,
for they bolt their  food.  But the higher animals  have it
well developed.   It is confined to the tongue, and  is most
Fig. 146.—Papillae of Human
 Palm, x 35, the cuticle be-
 ing removed. 
THE NERVOUS SYSTEM.
175
 delicate at the root.99  A state of solution and  an actual
 contact of the fluid are necessary conditions.
   Smell is the perception of odors, i. e., certain substances
 in the gaseous state.  Many Invertebrates have this sense:
 Snails, e. g., seem to be guided to their food by its scent,
 and Flies soon find  a  piece of meat.
 But it is impossible to  say what  or
 where the organ is.  Most probably it
 is united with the instrument of touch.
 In Vertebrates, it is  placed  at the en-
 trance to the  respiratory tube, in  the
 upper region of the  nose.   There the  ^tt^aJ"1^^"
 olfactory nerves, which issue from the   cavitv-
 front lobe of- the cerebrum, and pass through the ethmoid
 bone, or  roof of the nasal cavity, are distributed over a
 moist  mucous membrane.   The odorous substance, in a
 gaseous or finely divided state, is dissolved  in the mucus
 covering  this membrane.  In Fishes and Reptiles gener-
 ally,  this organ is feebly  developed:  Sharks, however,
 gather from a great distance around a  carcass.   In the
 Porpoises  and Whales  it is nearly or  entirely wanting.
 Among Birds, Waders  have the largest olfactory nerves;
 but Vultures seem to have the keenest scent.   It  is most
 acute  in  the carnivorous quadrupeds,  and in some wild
 herbivores, as the Deer.  In Man it is less delicate, but
 has a wider range than in any brute.
  Hearing is the perception of  sound.   The simplest
 form of the organ  is a sac filled with fluid, in  which float
 the soft  and delicate ends of the auditory  nerve.   The
 vibrations of  the fluid are  usually strengthened  by the
presence of minute hard granules,  called otoliths.  The
Invertebrates have no higher apparatus  than this; and it
is probable that they can  distinguish one noise from an-
other, but neither pitch nor intensit}\  In all animals the
organ is  double, but not always located  in the head.  In 
176
COMPARATIVE ZOOLOGY.
the Clam, it is found at the  base of the foot; some Grass-
hoppers have it in  the forelegs; and in many Insects it is
Pig. 148.—Brain and Auditory Apparatus of the Cuttle-fish: a, b, brain ; c, auditory
   apparatus;  d, the cavity in which it is lodged; e, f, g, eyes;  1, 2, 3, otoliths.
on the wing.  Lobsters, Crabs, and most  Insects have the
auditory sacs at the base of the antennas.100
   A complex organ of hearing, located in the head, exists
in all Vertebrates, save the very lowest Fishes.   As com-
plete in  Man, it consists of the following parts: 1st. The
                                external  ear  (which is pe-
                                culiar  to  Mammals);  the
                                auditory   canal,  about  an
                                inch long, lined with hairs
                                and a waxy secretion,  and
                                closed  at  the bottom by a
                                membrane,  called tympa-
                                num,   or  " drum  of  the
                                ear."     2d.   The  middle
Fig. 149.—Section of Human Ear: a, external ear, Containing three little
 ear, with auditory canal; 6, tympanic cavi- ,      , ,       ,.    .    .
 ty containing  the three bones; c, hammer,  OOUeS (the Smallest m the
 and its three  muscles, d, e,f; g, tympanic uor1„\  m„77^,.9 ™,,o  nnH
 membrane, or head of the drum; ft, Eusta-  Doajh  maVLeUS, incUS,  ana
 chian tube leading to the pharynx ;t,laby- s(apes  articulated togeth-
 rinth, with semicircular canals and cochlea    ■*                  °
 visible.                          er.  The  cavity communi-
cates with  the external air by means of the Eustachian
tube, which opens at the  back part of  the  mouth.   3d.
 
THE NERVOUS SYSTEM.
177
The internal  ear, or labyrinth, an irregular cavity in the
solid part of  the  temporal bone, and separated from the
middle ear  by a  bony partition, which is perforated  by
two small holes.   The labyrinth consists of the vestibule,
or  entrance;  the  semicircular  canals, or tubes;  and the
cochlea, or spiral canal.  While the other parts  are full of
air, the labyrinth  is filled  with  a liquid, and in this float
the ends of the auditory nerve.   The vibrations of the air,
collected  by the  external  ear, are concentrated upon the
tympanum,  and thence  transmitted through the chain of
little bones to the  fluid in the labyrinth.
  Now, the  essential organ of hearing is the  labyrinth,
which is, substantially, a bag filled with fluid and nerve-
filaments.   Fishes generally have but little  more.   In
Reptiles  there are  added a tympanum, chain of bones,
cochlea,  and  Eustachian tube; the tympanum being ex-
ternal.  Birds have, extra to Reptiles, an auditory  passage,
opening on  a level with the surface of the head, and sur-
rounded by a circle of feathers.  Mammals only  have  an
external ear.101
  Sight  is  the  perception of light.102   In all  animals it
depends upon the  peculiar sensitiveness of the optic nerve
to the vibrations of ether.   But while  in Vertebrates this
nerve comes from the middle mass of the brain, in Inver-
tebrates it is derived from  a  ganglion.  Many animals are
utterly destitute of visual organs, as  all the Protozoans,
and the lower Radiates and Mollusks, besides intestinal
Worms and the blind Fish of Mammoth Cave.   Around
the margin of the Jelly-fish,  and at the end of the rays of
certain Star-fishes  and Sea-urchins, are colored  spots, sup-
posed to be rudimentary eyes;  but as a lens is wanting,
there is no  image;  so that  the  creature can merely dis-
tinguish  light from darkness  and color without  form.
Such an eye is nothing but a collection of pigment gran-
ules on the expansion of a  nervous thread, and the percep-
                          12 
178
COMPARATIVE ZOOLOGY.
tion of light is the sensation of warmth, the pigment ab-
sorbing the rays and converting them into heat.
  Going higher, we find a lens introduced forming a dis-
                                 tinct  image.  The  Snail,
                                 for example, has two sim-
                                 ple  eyes,  called   ocelli,
                                 mounted on the tip of its
                                 long tentacles,  consisting
                                 of a globular lens,103 with
                                 a  transparent  skin (cor-
                                 nea) in front, and  a col-
                                 ored membrane (choroid)
                                 and a  nervous  net-work
                                 (retina)   behind.   Such
                                 organs  are the  only eyes
                                 possessed  by Myriapods,
                                 Spiders,  Scorpions,  and
                                 Caterpillars.   Adult In-
                                 sects  usually have three
                                 ocelli  on  the top of the
                                 head.    But  the  proper
visual organs  of  Lobsters,  Crabs,  and  Insects are  two
compound eyes,  perched  on  pedestals, or fixed on the
sides of the head.   They consist  of
an  immense number of ocelli  pressed
together so that they take an angular
form—four-sided in  Crustacea,  six-
sided  in   Insects.    They form  two
rounded protuberances variously  col-
ored—white, yellow, red, green, pur-
ple, brown, or  black.  Under  the mi-
croscope, the surface is seen to be di-
vided into a host  of facets,104 each be-
ing an ocellus complete in itself.   Each cornea is convex
on  one side, and either convex or  flat on the other, so that
Fig. 150.—Head of a Snail bisected, showing
 structure of tentacles: a, right inferior ten-
 tacle retracted within the body; 6, right su-
 perior tentacle fully protruded; c, left supe-
 rior tentacle partially inverted; d, left inferi-
 or tentacle ; /, optic nerve; g, retractor mus-
 cle ; ft, optic nerve in loose folds; i, retractor
 muscle of head ; k, nerve and muscle of left
 inferior tentacle ; I, m, nervous collar.
^P?!T
Fig. 151.—Head of the Bee,
 showing compound eyes,
 the three ocelli, or stem-
 mata, and the antennae. 
THE NERVOUS SYSTEM.
179
Behind  the  cornea, or
 it  produces a  focus like  a lens
 lens,  is  the  pigment,
 having a minute  ap-
 erture,   or  " pupil."
 Xext (in  place of  the
 "vitreous humor" of
 Vertebrates)  is a con-
 ical   tube — one  for
 each facet—with sides
 and bottom lined with
 pigment.   These tubes
 converge to the optic
 ganglion, the fibres of
 which  pass   through
 the tubes to the cor-
 nea.105 Vision b}7 SUcll FlG-152-~Eve of a Beetle (Melolontha):  A, section;
                   ,.        a, optic ganglion;  6, secondary nerves ; c, retina;
 a Compound eye  IS IlOt   d, pigment layer; e, proper optic nerves;  B, group
         •  t # i         ,    of ocelli; /, bulb of optic nerve; g, layer of pig-
 a  mosaic;  DUt  eaCll   mcnt; ft, vitreous humor; i, cornea.
 ocellus gives a complete  image, although a different per-
                                        spective   from   its
                                        neighbor.   The mul-
                                        tiplied  images are re-
                                        duced to one mental
                                        stereoscopic   picture,
                                        on  the  principle  of
                                        single vision  in our-
                                       selves.
                                          The   eyes  of  the
.,   ..„   c  ..   .„     *       ..        . Cuttle-fish  are  the
i?io. 153.—Section of Human Eye: a and b, upper and
  lower  lid;  c, conjunctiva, or mucous membrane, largest  and  the  most
  lining the inner surface; d, external membrane; e,    &
  sheath of optic nerve;  /, g, muscles for rolling the perfect among Inver-
  eye up or down ; ft, sclerotic; i,  transparent cor-    ,          ^-i
  nea; j, choroid ; k, I, ciliary muscle for adjusting teDrateS.    J.Hey  re-
  the eye for distance; m, iris and  pupil; n, canal; ---.V-i-   <.>,„  „„oa  „f
  o, retina; s,  vitreous humor; t, crystalline; v, an- bt;«luJ«   <<"«  c)c&  u±
  terior chamber; x, posterior chamber.          higher   animals  in
having a  crystalline lens with a chamber in front (open,
 
180
COMPARATIVE ZOOLOGY.
however, to the sea-water), and a chamber behind it filled
with " vitreous humor."
  The eye of Vertebrates  is formed  by the infolding of
the skin to create a lens, and an  outgrowth of the brain
                                    to  make  a sensitive
                                    layer.   It consists of
                                    a white spherical case
                                    (sclerotic)  made  of
                                    tough tissue, with a
                                    transparent fron t, call-
                                    ed the  cornea.   This
                                    case is kept in shape
                                    by two  fluids — the
                                    thin aqueous humor
                                    filling the cavity just
                                    behind  the  cornea,
                                    and  the   jelly - like
                                    vitreous  humor oc-
                                    cupying  the  larger
                                    posterior   chamber.
                                    Between the two hu-
                                    mors lies the double-
                                    convex    crystalline
                                    lens.    On the front
                                    face of the lens  is a
                                    contractile   circular
                                    curtain (iris), with a
                                    hole  in   the  centre
                                    (pupil);  and lining
                                    the sclerotic coat  is
                                    the  choroid  mem-
                                    brane,  covered  with
                                    dark pigment.    The
optic nerve, entering  at  the back of the eye  through the
sclerotic and choroid coats, expands into the transparent
Fig. 154.—Section of the Human Retina, X 400:1, in-
 ternal limiting membrane; 2, optic-nerve fibres;
 3, ganglion cells; 4, internal molecular layer; 5,
 internal granules; 6, external molecular layer; 7,
 external granules; 8, external limiting membrane;
 9, layer of rods and cones; 10, pigment layer. 
THE NERVOUS SYSTEM.
181
retina, which consists of several layers—fibrous, cellular,
and granular.   The most sensitive part is the surface ly-
ing next to the black pigment.  And here is a peculiarity
of the vertebrate eye:  the  nerve-fibres entering from be-
hind, turn back and look toward the bottom of the eye, so
that vision is directed backward; while invertebrate vision
is directly forward.   In Vertebrates only, the optic nerves
cross each other (decussate) in passing from the brain to
the eyes; so that the right  side of the brain, e. g., receives
the impressions of objects on the left side of the body.106
  Generally, the eyes of Vertebrates are on opposite sides
of the head; but in the Flat-fishes  both are on the same
side.   Usually, both eyes see the same object at once; but
in most Fishes the eyes are set so far back, the fields of
vision are distinct.   The cornea may be flat, and the lens
globular,  as in Fishes;  or the cornea very convex, and the
lens flattened, as in Owls.  Purely  aquatic animals have
neither eyelids nor tears; but nearly all others (especially
Birds) have three lids.107  The pupil is usually round; but
it may be rhomb-shaped, as in Frogs; vertically oval, as
in Crocodiles and Cats; or  transversely oval, as in Geese,
Doves, Horses, and Ruminants. Many quadrupeds, as the
Cat, have a membrane (tapetum) lining the bottom of the
eyeball with a brilliant metallic lustre, usually  green or
pearly: it is  this which makes the  eyes of such animals
luminous  in the dark.

              2. Instinct and Intelligence.
  The simplest form of  nervous excitement is mere sensa-
tion.  Above this we have sensation awakening conscious-
ness, out of which come those voluntary activities grouped
together under the name of Instinct;  and, finally, Intelli-
gence.
  The lowest forms of  life  are completely under law, for
their movements seem to be due solely to their organiza- 
182
COMPARATIVE ZOOLOGY.
tion.   They are automatons, or  creatures of  necessity.
Such,  also,  are  some  actions in  the  higher  animals, as
breathing, the beating of  the  heart, the contractions of
the iris, and all the first movements of an infant.108  But,
generally, the actions of animals are not the result of mere
bodily organization.
  The inferior orders are under the control of Instinct,
i. e., an  apparently untaught ability to perform  actions
which are useful to the animal.109   They seem to be born
with a measure of knowledge and skill (as Man is said to
have innate ideas), acquired neither by reason nor experi-
ment.  For what could have led  Bees to imagine  that by
feeding a worker-larva with royal jelly, instead  of bee-
bread, it would  turn out a  queen, instead of a  neuter?
In this case, neither the habit nor the experience could
be inherited, for the worker-bees are sterile.  We can only
guess that  the discovery has been communicated by the
survivors of an older swarm.  Uniformity is another char-
acteristic feature of instinct.  Different individuals of the
same species execute precisely the same movements under
like circumstances.  The career of one Bee is the career
of any other.   We do not find  one clever and  another
stupid.  Honey-combs  are  built now as they were before
the Christian era.  The creatures of pure instinct appear
to be  tied down, by the constitution of their nervous sys-
tem, to one  line of action, from which they can not spon-
taneously depart.  The actions vary only as the structure
changes.110  There is a wonderful fitness in what they do;
but there is no intentional adaptation of means to  ends.
  All animals, from the Star-fish to Man, are guided more
or less by instinct; but the best examples are furnished by
the insect-world, especially  by the  social Hymenopters
(Ants, Bees, and Wasps).   The  Butterfly carefully pro-
vides  for its young, which it  is destined never  to see;
many Insects  feed on  particular species of plants, which 
THE NERVOUS SYSTEM.             183
they select with wonderful sagacity; and Monkeys avoid
poisonous berries; Bees and Squirrels  store up food for
the future;  Bees, Wasps, and Spiders construct with mar-
velous precision ; and the subterranean  chambers of Ants
and the dikes of the Beaver show engineering skill; while
Salmon go  from the ocean up the  rivers to spawn; and
Birds of the temperate zones migrate with great regular-
ity.
  But in the midst of this automatism there are the glim-
merings of  intelligence and free-will.   We see some evi-
dence of choice and of designed adaptation.  Pure instinct
should be infallible.   Yet we notice mistakes that remind
us of mental aberrations.  Bees are not so economical as
has been generally supposed. A mathematician can make
five cells with less wax than the Bee uses for four; while
the Humble-bee uses three times as  much material as the
Hive-bee.  An exact hexagonal cell does not exist in nat-
ure.  Flies  lay eggs on the carrion-plant because it hap-
pens to have the odor of putrid meat.   The domesticated
Beaver will build a  dam across its apartment.  Birds fre-
quently make mistakes  in the construction and location of
their nests.   In fact, the process of cheating animals relies
on  the imperfection of instinct.  Nor are the actions of
the brute creation always  perfectly uniform; and so far
as animals conform to circumstances, they act from intel-
ligence, not instinct.  There is proof that some animals
profit by experience.  Birds do learn to  make  their nests;
and the older ones build the best.  Trappers know well
that young animals are more easily caught than old  ones.
Birds brought up from the egg, in cages, do not make the
characteristic nests of their species;  nor do they have the
same song peculiar to their species, if they have not heard
it.  Chimney-swallows certainly built their nests different-
ly in America three hundred years ago.  A Bee can make
cells of another shape, for it sometimes does; its actions, 
184
COMPARATIVE ZOOLOGY.
therefore, being elective and conditional, are iD a measure
the result of calculation.
  The mistakes and variations of instinct are indications
that animals  have something more — a limited range of
that principle of Intelligence so luminous in Man.  No
precise line can be drawn between instinctive and intel-
ligent acts; all we can say is, there is more freedom of
choice in the latter than the former; and that some ani-
mals are most instinctive, others most  intelligent.   Thus,
we  speak of the instinct of the Ant, Bee, and Beaver,
and  the  intelligence  of the Elephant,  Dog,  and Mon-
key.  Instinct loses its peculiar character as intelligence
becomes  developed.   Ascending  from  the  Worm and
Oyster to the Bee, we see the movements become more
complex  in character and  more special in their objects;
but instinct is  supreme.   Still  ascending, we observe a
gradual fading-away  of  the  instincts, till  they become
subordinate to  higher faculties — will and  reason.   We
can predict with considerable certainty the actions of ani-
mals guided  by pure instinct; but in  proportion as they
possess the power of adapting means  to ends, the more
variable their actions.  Thus, the architecture of Birds is
not so uniform  as that of Insects.
  We must credit brutes with a certain amount of obser-
vation and imitation, curiosity and cunning, memory and
reason.   Animals have been seen  to pause, deliberate, or
experiment, and resolve.  The Elephant and Horse, Dog
and Monkey, particularly, participate in the rational nat-
ure of Man, up to a certain point.  Thinking begins wher-
ever there is  an intentional adaptation  of means to ends;
for that involves the comparison and combination of ideas.
Animals have self-consciousness: a Cat never mistakes an-
other Cat for itself.  They interchange ideas:  the whine
of a Dog at the door on  a  cold  night certainly  implies
that he wants to be let in.  Even Bees and Ants, it is well 
THE NERVOUS SYSTEM.
185
known, confer by passing their antennae.   All the higher
animals, too, have similar emotions, as joy, fear, love, and
anger.
  While instinct culminates in Insects, the highest devel-
opment of intelligence is  presented in Man.111  In Man
only does instinct cease to be the controlling power.  He
stands alone in having the whole of his organization con-
formed to the demands  of his brain; and his intelligent
acts are characterized by the capacity for unlimited  prog-
ress.   The brutes  can  be improved  by  domestication;
but, left to themselves, they soon relapse into their  origi-
nal wildness.  Civilized  Man also goes back to savagery;
yet Man (though not all Men) has the ambition to  exalt
his mental and moral nature.  He has a soul, or conscious
relation to the Infinite, which leads him to aspire after a
lofty ideal.  Only he can form abstract ideas.  And, final-
ly, he is a completely self-determining agent, with a prom-
inent will and conscience—the highest attribute of the ani-
mal creation.  In all this, Man differs profoundly from the
lower forms of life.

              3. The  Voices  of Animals.
  Aquatic animals are  mute.113   A world of Radiates,
Mollusks, and Fishes, therefore, would be silent.   Insects
are about  the only Invertebrates capable  of producing
sounds.  Their  organs are usually external,  while  those
of higher animals  are internal.   Insects  of  rapid  flight
generally make the  most noise.  In some the noise is pro-
duced by  friction (stridulation); in others, by the passage
of air through the spiracles (humming).  The shrill  notes
of Crickets and  Grasshoppers are produced  by rubbing
the wings against each other, or against  the  thighs; but
the Cicada, or  Harvest-fly, has a special apparatus — a
tense membrane on the abdomen, acted upon  by muscles.
The buzzing of Flies and humming of  Bees are caused, in 
186
COMPARATIVE ZOOLOGY.
part, by the vibrations of the wings; but the true voice of
these Insects comes from the spiracles of the thorax.
  Snakes and Lizards  have no vocal cords, and can only
hiss.  Frogs croak,113 and  Crocodiles roar, by the vibra-
tion of the glottis.   The huge Tortoise of the Galapagos
Islands utters a hoarse, bellowing noise.
  The vocal apparatus in Birds is situated at the lower
end of the trachea, where it divides into the two bron-
chi.114  It consists mainly of a bony drum, with a cross-
bone, having a vertical membrane attached to its upper
edge.   The membrane  is put in  motion by currents of air
passing on either side of it.  Five pairs of muscles (in the
Songsters) adjust  the length of the windpipe to the pitch
of the glottis.   The various notes are produced by differ-
ences in the blast of air, as well  as by changes in the  ten-
sion of the membrane.  The range of notes is commonly
within an octave.   Birds of the  same family have a simi-
lar  voice.  All the Parrots have  a harsh utterance; Geese
and Ducks quack;  Crows, Magpies,  and Jays caw; while
the Warblers differ  in the quality, rather than the kind, of
note.115  The Parrot and Mocking-bird use the tongue in
imitating human sounds.   Some  species possess great com-
pass of voice.   The Bell-bird can be heard nearly three
miles; and Livingstone said he could distinguish the voices
of the Ostrich and the Lion only by  knowing that the for-
mer roars by day, and the latter  by night.
  The vocal organ  of Mammals, unlike that of  Birds, is
in the upper part of the larynx.  It consists of four  car-
tilages, of which  the largest (the thyroid) produces the
prominence  in the  human throat known  as "Adam's ap-
ple," and two elastic bands, called "vocal cords," just be-
low the glottis, or upper opening  of the  windpipe.   The
various  tones  are  determined   by  the tension of these
cords, which is effected by the raising or  lowering of the
thyroid prominence. The will can not influence the  con- 
THE NERVOUS SYSTEM.
187
traction of the vocalizing muscles, except in the very act
of vocalization.  The vocal sounds produced by Mammals
may be distinguished into the ordinary voice, the cry, and
the song.   The  second is the sound made
by brutes.  The Whale, Porpoise, Armadil-
lo, Ant-eater, Porcupine, and Giraffe  are
generally  silent.  The Bat's voice is prob-
ably the shrillest sound audible to  human
ears.   There is  little modulation  in brute
utterance.  The Opossum purrs, the Sloth
and  Kangaroo  moan, the Hog grunts  or
squeals, the Tapir  whistles, the  Stag bel-
lows, and the Elephant gives a hoarse trump-
et sound from its trunk and a deep groan
from its throat.   All sheep have a guttural
voice;  all the  Cows  low, from the Bison to
the Musk-ox;  all the Horses and Donkeys neigh; all the
Cats miau, from the domestic animal to the Lion; all the
Bears growl; and all the Canine family—Fox, Wolf, and
Dog—bark  or howl.  The  Howling-monkeys and  Goril-
las have a large  cavity, or sac, in the throat for resonance,
enabling them to utter a powerful voice; and one  of the
Gibbon-apes has the  remarkable power of emitting a com-
plete octave of  musical notes.  The human  voice, taking
the male and female together, has a range of nearly four
octaves.  Man's  power of speech, or the utterance of artic-
ulate sounds, is  due  to his intellectual development rath-
er than to any structural difference between  him and the
Apes.   Song  is  produced by the  glottis, speech by the
mouth.
Fig. 155.—Human
 Larynx, seen in
 profile: a, half
 of  the  hyoid
 bone;  e, tra-
 chea ; /, oesoph-
 agus ; g, epiglot-
 tis. 
188
COMPARATIVE ZOOLOGY.
CHAPTER XIX.
                     REPRODUCTION.
  It is a fundamental truth  that  every living organism
has had its origin in  some pre-existing organism.   The
doctrine of " spontaneous generation,"  or the supposed
origination of organized structures  out of inorganic parti-
cles, has not yet been sustained by facts.
  All  animals, without exception, arise from eggs.   But
while reproduction by eggs is common to all, it is only one
                                among several modes of
                                multiplication.  For the
                                lowest forms of life not
                                only generate by  eggs,
                                but also by self-division
                                and budding?16
                                  Self - division,    the
                                simplest mode  possible,
                                is a natural breaking-up
                                of the body into distinct
                                surviving  parts.   This
                                process is sometimes ex-
                                traordinarily  rapid,  the
                                increase of one animal-
                                cule (Paramecium)  be-
                                ing computed  at  268
Fig. 156.-Reproduction of Infusoria (Vorticel- millions in a month.  It
      Ice) by fission or self-division.
                                is most  common in the
Infusoria; but is occasionally exhibited even by the aquat-
ic and intestinal worms.
  Budding consists, in animals as in plants, in the growth
 
REPRODUCTION.
189
of buds, generally from the exterior of the body, possess-
ing all the essential parts of the parent stock.  The buds
may develop  into individuals  complete, but  not distinct,
forming, with others like itself, a compound animal, as
the Coral; or the buds may become detached, giving rise
to perfect independent individuals,  as the  Hydra (Fig.
186).   The latter mode closely resembles self - division.
Rarely, as in the Aphis, those  little green insects causing
" blight," the budding is internal, and so rapid that the
tenth generation would number one quintillion. Budding
in the higher animals  produces monstrosities, as  double-
heads,  double-thumbs, etc.
  Generation by Eggs is accomplished by the union of
two dissimilar cells—a germ-cell, or ovum; and a  sperm-
cell ; the embryo being evolved from the former.1"  An
egg, which  is  the  product of this
union, is the lowest possible  condi-
tion of animal life.   It is a globular
mixture  of  albumen  and oil.   A
freshly laid Hen's egg, boiled  hard,
well exhibits  the general structure
fc>'
The outside  shell consists of earthy  Pi0- 157._Theoretical Egg)
matter (lime) deposited in a net-work   orceii: v, vitelline mem-
  .    .                   .    #         brane; y, oleaginous pole;
of  animal matter.   It  is minutely   «, albuminous poie; p,
       .   n    .1            r         Purkiujean,  or germinal,
pOl-OUS, tO allOW the passage Of Vapor   vesicle; w, Wagnerian, or
and air to and fro.  Lining the shell   germiua1'dot-
is a double membrane (membrana putaminis) resembling
delicate tissue-paper.  At the larger end, it  separates to
inclose a  bubble of air for the use of the chick.  Next
comes the albumen, or " white," in spirally arranged lay-
ers, within which floats the yolk.  The yolk is prevented
from moving toward either end of the egg by two twisted
cords  of albumen, called chalazw;ue yet is allowed to rise
toward one side, the yolk being lighter than the albumen.
The yolk  is composed of oily granules (about -^^ of an 
190
COMPARATIVE ZOOLOGY.
inch in  diameter), inclosed in a  sac,  called  the vitelline
membrane, and disposed in concentric layers, like a set of
vases placed  one within the other.  That part of the yolk
which extends from the centre to a white spot (cicatricu-
la)  on  the outside can not  be hardened,  even  with  the
most prolonged  boiling.  The cicatricula, or embryo-spot
—the part for which all the rest was made—is a  thin disk
of cellular structure, in which the new life first appears.
It is always on that side which naturally turns uppermost,
Fig. 158.—Longitudinal section of Hen's Egg before incubation:  a, yolk, showing
   concentric layers; a', its semi-fluid centre, consisting of a white granular sub-
   stance—the whole yolk is inclosed in the vitelline membrane; b, inner dense
   part of the albumen ; b', outer, thinner part; c, the chalazse, or albumen, twisted
   by the revolutions of the yolk; d, double shell-membrane, split at the large end
   to form the chamber/; e, the shell; h, the white spot, or cicatricula, and under
   it the germinal vesicle of Purkinje, or nucleus, which is afterward ruptured, and
   becomes invisible.

for the  yolk can  turn upon  its axis;  it  is, therefore, al-
ways nearest to  the external air and to  the  Hen's body—
two necessary conditions  for its development.   There is
another  reason for  this  polarity  of the egg: the lighter
and most delicate part of  the yolk,  the cicatricula, is col-
lected where the upper cavity of the animal, inclosing the
nervous  system, is to be; while the  heavy oily portion re-
mains beneath,  where the lower  cavity, inclosing the or-
gans of nutrition, is afterward developed.
   The essential  parts of  any egg are the germ-cell, or cic- 
REPRODUCTION.
191
atricula,  yolk, and vitelline  membrane.  The shell  and
albumen are often wanting.  When the albumen is pres-
ent,  it is commonly covered by a membrane only, as in
Fig. 159.-Egg of a Shark.
Frogs; in Sharks, the envelope is  horny; and in Croco-
diles it is calcareous, as in Birds.
  The eggs  of all animals, in their essentials, are alike in
kind, but not in degree.   The egg of the Amoeba  is a
mere cell, with  a light spot in one part  of  the contents.
The egg of the  Hydra differs  in  having this light  spot
sharply defined, and inclosed in a  sac, or germinal vesi-
cle.  In the Rabbit's egg, there is another sac, called ger-
minal dot, inside the germinal vesicle; while the eggs of
most of the higher animals  show a nucleolus within the
germinal dot.  An egg, therefore, closely resembles a cell,
consisting of an  outer and  an inner sac  and a dot,  or a
series of hollow concentric spheres.
  The size of an egg depends  mainly  upon the quantity
of yolk it contains; and this is  proportioned to the grade
of development  which the embryo  attains when it  leaves
the egg.119  In the eggs of the Star-fishes, Worms, Insects,
Mollusks (except the Cuttle-fishes), and Mammals, the yolk
is very minute and formative, i.e., it is  converted into the
parts of  the future embryo.   In  the  eggs of  Lobsters,
Crabs, Spiders, Cephalopods, Fishes, Reptiles, and  Birds, 
192
COMPARATIVE ZOOLOGY.
the yolk is large and colored, and consists of two parts—
the formative, or germ-yolk, immediately surrounding the
germinal vesicle;  and  the nutritive, or food-yolk, consti-
tuting  the  greater part of the mass, by which the young
animal in the egg-life is nourished.  In the latter case, the
young  come forth more mature than where the food-yolk
is wanting.
  As to form, eggs are oval or elliptical, as in Birds and
Crocodiles; spherical, as in Turtles and Wasps; cylindric-
al, as in Bees and Flies;  or shaped like a hand-barrow,
with tendrils on the corners, as in the Shark.  The eggs
of some very low forms are sculptured or covered with
hairs or prickles.
  The number of eggs varies greatly in different animals,
as it is in proportion  to  the risks during development.
Thus,  the  eggs of  aquatic  tribes, being unprotected  by
the parent, and being largely consumed by many animals,
are  multiplied to  prevent extinction.   The spawn of a
single  Cod contains millions of eggs;  that of the  Oyster,
300,000.  A Queen-bee, during the five years of  her  ex-
istence, lays about a million eggs.
  Eggs are laid one by one, as by Birds; or in clusters, as
by  Frogs, Fishes, and  most  Invertebrates.  The spawn of
the  Sea-snails  consists of vast numbers of eggs adhering
together in masses, or in sacs, forming long strings.
   As a rule, the  higher  the rank, the more care animals
take of their eggs  and their young, and the higher  the
temperature needed for egg-development.  In the majori-
ty of cases, eggs are left to themselves.  The fresh-water
Mussel-shell (Unio) carries them between its gills,  and the
Lobster under its tail.  The eggs of many Spiders  are
enveloped  in a silken cocoon, which  the  mother guards
with jealous care.   Insects, as Flies  and  Moths, deposit
their eggs where the  larva, as soon as born, can  procure
its own food.  Most Fishes allow their spawn, or roe, to 
DEVELOPMENT.
193
float in the water; but a few build a kind of flat nest in
the sand or mud, hovering over the eggs until they  are
hatched; while the Acara of the  Amazons carries them
in its mouth.  The Amphibians, generally, envelop their
eggs in a gelatinous mass, which  they  leave to the ele-
ments; but the female of the Surinam Toad carries hers
on her back, where they are placed by the male.  The
great Amazon-Turtles lay their eggs in holes two feet
deep in the sand; while the Alligators simply cover theirs
with a few leaves and sticks.  Nearly all  Birds build nests,
those of the Perchers being most elaborate, as their chicks
are dependent for a time on the parent.180  The young of
Marsupials, as the Kangaroo,  which  are born in  an  ex-
tremely immature state, are nourished in a pouch outside
of the body.131  But the embryo of all other Mammals is
developed within the parent to a more  perfect condition
by means of a special organ, the placenta. It is a gen-
eral  law, that animals  receiving in the  embryo state  the
longest and most constant parental care ultimately attain
the highest grade of development.
                   CHAPTER XX.
                    DEVELOPMENT.

  Development is the evolution of a germ into a com-
plete organism.  The study of the changes within the egg
constitutes  the science of Embryology;  the transforma-
tions after  the egg-life are called metamorphoses, and in-
clude growth and repair.
  The  process of development is a passage from the gen-
eral to the  special, from the simple to the complex, from
the homogeneous to the heterogeneous, by a  series of dif-
                         13 
194
COMPARATIVE ZOOLOGY.
ferentiations.   It brings out first the profounder distinc-
tions, and  afterward those more external.  That is, the
most essential parts appear first; e. g., the  nervous system
and skeleton precede the digestive  apparatus.  And not
only does development tend to make the several organs of
an individual more distinct from one another, but also the
individual  itself more distinguished from other individuals
and from the medium in which it lives.  With advancing
development, the animal, as a  rule,  acquires a  more spe-
cific, definite form, gains the ability of maintaining a tem-
perature of its own, and increases in weight and locomo-
tive power.  Life is a tendency to individuality.
  Development of a Hen's Egg.—The first  change  is
the segmentation of the  formative part of the yolk (germ-
yolk) by a process  of self-division.   It separates into two
spheres,  which  subdivide into  four  more, and  so on till
the whole  is broken  up into  a myriad of  cells.   These
cells finally arrange themselves into  a layer (called blasto-
a                    b                    c
Fig. 160.—First Stages in Segmentation of a Mammalian Egg: A, first division into
   halves, with spermatozoa around it; B and C, progressive subdivision, ultimate-
   ly transforming the vitellus, or yolk, into a "mulberry mass" of globules, or em-
   bryo-cells.
derm), lining the vitelline membrane; and a round light-
colored disk in this layer is the germinal spot, or cicatric-
ula, already  mentioned.  This is the first trace of organ-
ization.  Soon  the  germinal disk thickens, and splits into
three layers: out of the upper one are ultimately formed 
DEVELOPMENT.
195
the instruments of thought, sensation, and motion, or the
brain,  spinal column, muscles, and  skin ;  in  the lower
originates  the  digestive system ;  while  the middle  gives
rise to the blood and the organs of  circulation.   The  next
Fio. 101.—Vertical Sections of an Egg, showing progressive stages of development:
     a, primitive streak; b, the furrow, becoming a closed canal in the last.

phase is the appearance of a faint straight furrow passing
through the middle of the external layer, called the prim-
itive stn'pe, which corresponds to the axis  of the future
body.1"   The walls  of the  furrow gradually  rise, and at
last  meet, forming a  canal, larger at one  end  than the
other,  which is filled with  a fluid—the beginning of the
brain and spinal marrow.  Beneath the furrow, a delicate
cartilaginous thread appears (called notochord)—the rep-
resentative of the backbone.  At the same time, the mar-
gin of the germ extends farther and farther over the yolk,
till it  completely incloses it.   So that now we see two
cavities — a small one,  containing the  nervous system;
and  a  larger one below, for the digestive organs.   Pres-
ently, numerous rows
of dark-yellow corpus-
cles  are  seen on the
middle   layer,   which
are  subsequently  in-
closed, forming a net-
work  of  capillaries,
called  the vascular area.  A dark spot indicates the  situ-
ation of the heart, which is the  first distinctly  bounded
cavity of the circulatory system.   It is a short tube lying
lengthwise just  behind the head, with a feeble pulsation,
Pig. 162.—Rudimentary Hearts, human:  1, venous
  trunks; 2, auricle; 3, ventricle; 4, bulbus ar-
  teriosus. 
196
COMPARATIVE ZOOLOGY.
causing the blood to flow backward  and  forward.   The
tube  is  gradually bent together, until it forms a  double
cavity, resembling the  heart  of a  Fish.   On  the fourth
day of incubation, partitions begin to grow, .dividing the
cavities  into  the  right and  left  auricles  and  ventricles.
         A.                                          A
Figs. 163-167.—Embryo in a Hen's Egg during the first five days: A, mucous layer;
   B, vascular layer; C, blastoderm, or serous membrane, in the last figure forming
   the amniotic sac;  D, vitelline membrane;  e, thickened blastoderm, the first rudi-
   ment of the dorsal part (in the last figure it marks the place of the luugs);  h,
   heart:  a, b, its two chambers • » branchial arteries; m, aorta; i, liver; p, al-
   lautois. 
DEVELOPMENT.
197
The septum between the auricles is the last  to  be finish-
ed;  being closed  the moment  respiration  begins.   The
Fig. 168.—Hen's Egg, more highly developed. The embryo is enveloped by the am-
   nion, and has the umbilical vesicle, or remnaut of the yolk, hanging from its un-
   der surface;  while the allantois turns upward, and spreads out over the internal
   surface of the shell-membrane. (From Dalton's "Physiology.")
blood-vessels  ramify in  all directions through the yolk,
making it a spongy mass, and all perform the same  office;
it is not  till the fourth or
fifth day that  arteries can
be distinguished from veins,
by being thicker and by car-
rying  blood only from the
heart.123
  The embryo lies with its
face, or ventral surface, to-
ward the yolk, the head and
tail  curving  toward  each
other.   A delicate transpar-
ent membrane (a part of the
upper layer of the  blasto-
derm) rises like a hood over
the  back of the  embryo till it forms a closed sac,  called
the amnion.  It is filled with a thin liquid, which  serves
Fig. 169. — Mammalian Embryo, with al-
 lantois fully formed: 1, umbilical vesi-
 cle, containing the last of the yolk;  2,
 amnion; 3, allantois, on which the fringes
 of the placenta are developing.  (From
 Dalton's "Physiology.") 
198
COMPARATIVE ZOOLOGY.
to protect the embryo.   Meanwhile, another important or-
gan is forming on the other side.  A portion of the lower,
or internal, blastodermic layer elongates  downward, and
then upward, spreading out over the whole inner  surface
of the shell, so that it surrounds both embryo and amnion.
This is  the allantois.  It is full of  blood-vessels, and it
serves as  the respiratory organ until the  chick  picks the
shell and breathes by its lungs.124  The chorion is  the outer-
most part of the allantois—in other words, the envelope of
the ovum;  and  the placenta of Mammals is the  shaggy,
vascular edge of the  chorion.
  The alimentary canal is at first a straight tube closed at
both ends, the middle  being connected with the yolk-bag.
As it grows faster than the body, it is thrown into a spi-
ral coil; and at  several points it dilates, to form the crop,
stomach, gizzard, etc.  The mouth  is developed from an
infolding of the skin.  The liver is an outgrowth from
the digestive tube, at first a cluster of cells, then  of folli-
cles, and finally  a true gland.   The lungs are  developed
on the  third day as minute buds from the upper part of
the alimentary canal, or pharynx.   As they grow in size,
they pass from a smooth to a cellular condition.
  The skeleton  at the beginning consists, like  the noto-
chord, of a gelatinous  material,  which gradually becomes
condensed and cellular, turning to cartilage.   Then mi-
nute canals containing blood-vessels arise, and earthy mat-
ter (chiefly phosphate of lime)  is deposited between the
cells.  The primary bone  thus  formed is compact: true
osseous  tissue, with canaliculi, laminae, and Haversian ca-
nals, is  the result  of subsequent absorption.126   Certain
bones, as  those of the face and cranium, are not preceded
by  cartilage, but by connective tissue :  these  are  called
membrane bones.  Ossification, or bone-making, begins at
numerous distinct points, called centres;  and, theoretical-
ly,  every centre stands  for a bone, so that there are as 
DEVELOPMENT.
199
 many bones in  a  skeleton as centres of ossification.  But
 the actual number in the adult animal is much smaller, as
 many of  the centres coalesce.126  The development of the
 backbone (as, in fact, the  growth of the whole  chick) is
 not from the head or from the tail, but from a central
 point  midway between: there the first vertebrae appear,
 and from thence they multiply forward and backward.
   The limbs appear  as buds on the  sides of the body;
 these  lengthen  and expand  so as to resemble paddles—
 the wings and  legs looking precisely alike; and, finally,
 they  are  divided  each  into  three segments, the last one
 subdividing into digits.  The feathers are developed from
 the outside cells of the epidermis:  first, a  horny cone is
 formed, which elongates and spreads out into  a vane, and
 this splits up into  barbs and barbules.
   The muscles are formed either by the growth in length
 of a single  cell, or by the coalescence of a row of cells:
 the cell-wall thus  produces a long  tube—the  sarcolemma
 of a  fibre, and  the granular contents arrange themselves
 into linear series, to make fibrillse.1"
   Nervous tissue  is derived  from the multiplication and
 union  of embryo-cells.  The white fibres at first resemble
 the gray.   The  brain and spinal marrow are developed
 from  the primitive stripe—that pale-white line on the cic-
 atricula, which almost from the beginning is conical, fore-
 shadowing head and  tail.   Soon the brain, by two con-
 strictions,  divides  into fore - brain,  mid - brain, and hind-
 brain.   The fore-brain throws out two lateral hemispheres
 (cerebrum),  and  from these protrude forward the two ol-
factorj7 lobes.   From  the  middle - brain grow the  optic
lobes; and the hind-brain, afterward separated into  cere-
bellum and medulla oblongata, is the origin  of  the ear-
sacs.
   Modes  of Development.—The  structure and  embry-
ology of a Hen's egg  exhibit many facts which are  com- 
200
COMPARATIVE ZOOLOGY.
mon to all animals.  But every grand division of the Ani-
mal  Kingdom has its characteristic method of developing.
  Protozoans differ from all higher forms in not undergo-
ing segmentation.
  The egg of the  Sea-anemone, after segmentation, be-
comes a solid pear-shaped body, covered with cilia.  Soon
one end is indented;  then the indentation deepens until it
becomes a cavity, and the edge rolls inward till it extends
half-way to the bottom.  Little ridges are then seen in the
interior, which  finally  become  so many partition-walls;
while minute protufberances around the opening, or mouth,
are the beginnings of the tentacles.  There is no distinc-
                             tion  into  nervous and  di-
                             gestive regions.   The egg
                             of the  Oyster  divides in
                             two,  as if to form two in-
                             dividuals,  but soon two tri-
                             angular disks appear, the
                             rudiments  of shells,  and
                             the two pulsating sacs are
                             fused together  into  one
                             heart.   Then  the embryo
                             revolves  in the  egg,  and
becoming fringed with cilia, it quits the egg, and  closes
the valves, which were wide open.  The embryo of an In-
sect  shows from the first a right and  left side ; but the
first indication that it is an Articulate is  the development
of a series of indentations dividing the body into succes-
sive  rings, or joints.  Next, we  observe that the back lies
near the centre of the egg, the ventral side looking out-
ward ; *. e., the  embryo is  doubled upon itself  backward.
And, finally, the appearance of three pairs of legs proves
that it will be an Insect, rather  than a Worm, Crustacean,
or Spider.
  The Vertebrate  embryo  lies with its stomach toward
Fig. 170. — Development of Sea-anemone,
 slightly magnified: 1, first stage after seg-
 mentation ; 2, shows the beginning of the
 digestive cavity; 6, its completion; a, ten-
 tacles ; b, partitions. 
DEVELOPMENT.
201
the yolk, reversing the position of the Articulate; but the
grand characteristic is the primitive stripe, which is nearly
confined to  the  eggs of Vertebrates.  This is connected
with another,  the  setting apart of two  distinct regions—
the nervous  and nutritive.  There are three modifications
of Vertebrate  development: that of Fishes and Amphib-
ians, that of True Reptiles and  Birds, and that of Mam-
mals.  The amnion and allantois are wanting in the aquat-
ic Vertebrates; while the placenta (which  is the allantois
vitally connected with the parent) is peculiar to Mammals.
In Mammals, the whole yolk is segmented; in Birds, seg-
mentation is confined to the  small white speck seen in
opening the shell.
   At the outset, all animals, from the Sponge  to Man, are
indistinguishable from one another.    They  are  mainly
drops of fluid, a little more transparent on one  side than
the other; and, in all cases, this almost homogeneous  glob-
ule must develop three well-defined parts — a germinal
dot, germinal vesicle, and yolk.   But  while  Vertebrates
and Invertebrates can  travel together on  the same road
up to this point, here they diverge—never to  meet again.
For every grand group early shows that it has a peculiar
type of construction.   Every egg is  from the first impress-
ed  with the power  of developing in one  direction  only,
and never does it lose its fundamental characters.   The
germ of the Bee is divided into segments, showing that it
belongs to the Articulates; the germ of the Lion has the
primitive stripe—the mark of the coming Vertebrate.   The
blastodermic layer of the Vertebrate egg rolls  up into two
tubes — one to hold  the viscera, the other  to  contain the
nervous  cord; while that of the Invertebrate egg forms
only one such tubular division.  The features which de-
termine  the subkingdom to which  an animal  belongs are
first developed, then the characters  revealing its class.
   There are differences also in  grade of development as 
202
COMPARATIVE ZOOLOGY.
well as type.  For a time there is no essential  difference
between a Fish and a Mammal: they have the same nerv-
ous, circulatory, and digestive systems.  The first depart-
ure is the alteration of the heart of the Mammal, giving
it four cavities;  while the heart of the Fish remains in its
rudimentary condition.  We may call this arrested devel-
opment.   There are many such cases, in  which the eni'
bryo of an animal represents the  permanent  adult condi-
tion of some lower form.   In  other  words, the  higher
species, in the  course of their  development, offer like-
nesses, or analogies, to  finished lower species.   The hu-
man germ, at first, can not be distinguished from that of
any other animal: for aught  we can see, it may turn out
a Frog or a Philosopher.  The appearance of a primitive
stripe excludes  it at once from all Invertebrates.  For a
time, it assumes a  structure seen  only  in  the  Fish, and
then another, found only in Mammals.   Later  still, it is
not unlike the embryos of the Ox, Dog, and Monkey suc-
cessively ; then  it looks like any other infant, and finally
it acquires the  peculiarities of the race to which it be-
longs.128  All the members of a group,  therefore, do not
reach the same degree of perfection, some remaining in
what corresponds to the immature stages  of the  higher
animals.   Such may be  called  permanently embryonic
forms.
  Sometimes an embryo develops an organ in a  rudiment-
ary condition, which is lost or useless in the adult.  Thus,
the Greenland Whale, when grown up, has not  a tooth in
its head, while in the embryo life it has teeth in both jaws;
unborn Calves have canines and upper  incisors; and the
female Dugong has tusks which never cut the gum. The
" splint-bones" in the Horse's foot are unfinished metatar-
sals.
  Animals differ widely  in  the degree of  development
reached at ovulation and at birth.  The eggs of Frogs 
DEVELOPMENT.
203
are laid  before  they can hardly be said to have become
fully formed as eggs.  The eggs of Birds are  laid when
there is scarcely a trace within them of the germinal spot;
while the eggs  of Mammals are retained by the parent
till after the egg - stage is passed.129  Ruminants and  ter-
restrial Birds are born with the power of sight and loco-
motion.  Most Carnivores, Rodents, and perching Birds
come into the world blind and helpless; while the  human
infant is  dependent for a much longer time.

                  1. Metamorphosis.
   Few animals come forth from the egg in perfect condi-
tion.  The vast  majority pass through a great  variety of
forms before reaching maturity.  These metamorphoses
(which are merely periods  of growth) are not peculiar to
Insects, though more apparent  in them.   Man  himself is
developed on the same general principles as the Butterfly,
but  the  transformations are concealed  from view.  The
Coral, when hatched,  has  six  pairs of partitions; after-
ward, the spaces are divided  by six more pairs; then
twelve intermediate  pairs are introduced ; next,  twenty-
four, and so on.  The embryonic Star-fish has  a long
body, with  six  arms on a side, from  one end of which
the young Star-fish  is apparently budded off.  Soon  the
twelve-armed body dies, and the young animal is  of age.
Worms are  continually growing by  the addition of new
segments.  Nearly all Insects undergo complete metamor-
phosis, i. e.,  exhibit four distinct stages of existence—egg,
larva, pupa, and  imago.  The worm-like larva130 may be
called a locomotive-egg.  It has little resemblance to  the
parent in structure or habits, eating and growing rapidly.
Then it enters the pupa state, wrapping itself in a cocoon,
or case, and remaining apparently dead till new  organs
are developed;  when it escapes a  perfect winged  Insect,
or imago.131   Wings never exist in the larva; and Insects 
204
COMPARATIVE ZOOLOGY.
which undergo no  apparent metamorphosis, as  Lice, are
wingless.   The Grasshopper develops from the young lar-
         Fig. 171.—Butterfly in the Imago, Pupa, and Larva States.

va to the winged adult  without changing its mode of life :
the worm-like stage is passed  within the  egg.   In the de-
Fig. 172.—Metamorphosis of the Mosquito (Culex pipiens): A, boat of eggs; B, some
   of the eggs highly magnified, d, with lid open for the escape of the larva, C ; D,
   pupa; E, larva magnified, showing respiratory tube, e, anal fins, /, antennae, g;
   F, imago; a, antenna;; 6, beak. 
DEVELOPMENT.
205
velopment of the common Crab, so different is the out-
ward form of the newly hatched embryo from that of the
adult,  that the former has been  described as  a  distinct
species.
  The most remarkable example of metamorphosis amono-
Vertebrates is furnished by the Amphibians.   A Tadpole
—the larva of the Frog—has a tail, but no legs; gills, in-
stead of lungs; a heart precisely like that of the Fish; a
horny beak for eating vegetable food, and  a  spiral  intes-
Fig. 173.—Metamorphosis of the Newt.
tine to digest it.   As  it matures, the hinder  legs show
themselves,  then the front pair;  the  beak falls off;  the
tail and gills waste away;  lungs  are created; the digest-
ive apparatus is changed to suit an animal diet; the heart
is altered to the Reptilian type by the  addition of another
auricle;  in  fact,  skin, muscles, nerves, bones, and blood-
vessels vanish, being absorbed atom by atom, and a new
set is substituted.  Moulting, or the periodical renewal of
epidermal parts, as the shell  of the Lobster, the skin of
the Toad, the scales of  Snakes, the feathers of Birds, and 
206
COMPARATIVE ZOOLOGY.
the hair of Mammals,  may be  termed a metamorphosis.
The change from milk-teeth to a permanent set is another
example.
  An animal  rises in  organization as development ad-
vances.   Thus, a  Caterpillar's  life has  nothing  nobler
about it than  the  ability to eat, while the Butterfly ex-
pends the power garnered up by the larva in a gay and
busy life.   But there are  seeming reversals of this law.
Some mature animals appear lower in the scale than their
young.  The larval Cirripede has  a pair  of magnificent
compound eyes and  complex antennae; when adult, the
antennae are gone, and the eyes are reduced to a single,
simple minute eye-spot.   So the germs of the sedentary
Sponge  and Oyster are free and active.   The adult ani-
mal, however,  is always superior in alone possessing the
power of reproduction.

               2.  Alternate Generation.
  Sometimes a metamorphosis extending over several gen-
erations is required to evolve the perfect animal; " in oth-
er words, the parent finds no resemblance to himself in
any of his progeny,  until  he comes down to  the  great-
grandson."   Thus, the Jelly-fish,  or Medusa, lays eggs
which are hatched  into larvae resembling Infusoria—little
transparent oval bodies covered  with cilia, by which they
swim about for a time till  they find a resting-place.  One
of them, for example,  becoming fixed, develops rapidly;
it  elongates and spreads  at the upper end;  a month  is
formed  opening into a digestive  cavity;  and tentacles
multiply till the mouth is surrounded by them.  At this
stage it resembles a Hydra.   Then slight wrinkles appear
along the body, which grow deeper and deeper, till the
animal looks like "a pine-cone surmounted by a tuft of
tentacles;" and then like a pile of  saucers (about a dozen
in number)  with scalloped edges.   Next, the pile breaks 
DEVELOPMENT.
207
up into separate segments, which are, in fact, so many dis-
tinct animals;  and each  turning over  as it is set free so
as to bring the mouth below, develops into an adult Me-
dusa,  becoming more and more convex,  and furnished
with tentacles, circular canals,  and other  organs exactly
like those of the progenitor that laid the original egg.
  Here we see a  Medusa producing eggs  which develop
into Infusoria-like larvae, and these grow  into stationary
Fig. 174.—Alternate Generation: a, b, c, ova of an Acaleph (Chrysaora); d, e, f. Hy-
   dras ; g, h, Hydras with constrictions; i, Hydra undergoing fission; k, one of the
   separated segments, a free Medusa.

forms resembling Hydras.  The Hydras then produce not
only  Medusae by budding in  the  manner described, but
also other Hydras like themselves  by budding and by
eggs.  All  these  intermediate  forms are transient states
of the Jelly-fish; but the  metamorphoses can not be said
to occur in  the same individual.  While a Caterpillar be-
comes a Butterfly, this Hydra-like individual produces a
■number of Medusae.  Alternate generation is almost con-
fined to the low Radiates (Fig. 190).

                3. Growth and Repair.
  Growth is increase of bulk, as Development is increase
of structure.  It occurs whenever the process of repair
exceeds that of  waste, or  when new  material  is added
faster than the tissues are destroyed.   There is a specific 
208
COMPARATIVE ZOOLOGY.
limit of growth for all animals, although many of the low
cold-blooded forms, as the Trout and Anaconda, seem to
grow  as long  as they live.  After the body has attained
its maturity, i. e., has fully developed, the tissues cease to
grow; and nutrition is concerned solely in supplying the
constant waste, in  order to preserve the size and shape of
the organs.  A child eats to grow and repair; the adult
eats only to repair.132  Birds develop rapidly, and so spend
most  of their  life  full - fledged; while Insects  generally,
Fishes, Reptiles, and Mammals mature at a comparatively
greater age.   The perfect Insect rarely changes its size,
and takes but  little food; eating and growing are almost
confined to larval life.   The crust of the Sea-urchin,
which is nevsr shed, grows by the addition of matter to
the margins of the plates.  The shell of the Oyster is en-
larged by the  deposition of new laminae, each extending
beyond the other.   At every  enlargement, the  interior is
lined  with a new  nacreous  layer; so that the number of
such layers in the oldest part of the shell indicates the
number of enlargements.  When the shell has reached its
full size, new  layers are added to the inner surface only,
which increases  the  thickness.  It is the margin of the
mantle  which provides for the increase  in length  and
breadth; while the thickness  is derived  from the  whole
surface.   The edges of the  concentric  laminae are the
"lines of  growth."   The Oyster is  full-grown in about
five years.  The bones of Fishes and Reptiles are contin-
ually  growing; the long  bones of higher animals increase
in length so long as the ends (epiphyses) are separate from
the shaft.   The limbs of  Man, after birth, grow more rap-
idly than the trunk.
  The power  of regenerating lost parts is greatest  where
the organization is lowest, and while the  animal is  in the
young or  larval state. It is really a process of budding.
The head  of  the  Hydra, if separated, will reproduce a 
DEVELOPMENT.
209
 body  and tail; if the tail is cut off, it will add a body
 and head.  Certain Worms may be cut into several pieces,
 and each part will regain what is needed to complete the
 mangled organism.  The Star-fish can reproduce its arms:
 the Holothuria, its stomach;  the Snail, its tentacles;  the
 Lobster, its claws; the Spider, its legs; the Fish, its fins;
 and the Lizard, its tail.  Nature makes no mistake by put-
 ting on a leg where a tail belongs, or joining an immature
 limb to an adult animal.133  In  Birds and Mammals,  the
 power is limited to the reproduction of certain tissues, as
 shown  in the healing of wounds.   Very rarely an entire
 human bone, removed by disease or surgery, has been re-
 stored.  The nails and hair continue to grow in extreme
 old age.

               4.  likeness and Variation.
   It is a great law of reproduction  that all  animals tend
 to resemble their  parents.  A member of one  class never
 produces a member of another class.  The likeness is very
 accurate as to general structure and form.  But it does
 not  descend to every individual feature and trait.   In
 other words, the tendency to  repetition is qualified by a
 tendency to variation.   Like produces like, but not exact-
 ly.   The similarity never amounts  to identity.  So that
 we have two opposing forces—the hereditary tendency to
 copy the original  stock, and a distinct tendency to devi-
 ate from it.
  This is one of the most universal  facts in  nature. Ev-
 ery development ends in diversity.   Every  body knows
 that no two individuals of a family, human or brute, are
 absolutely alike.   There are always individual  differences
 by which they can be  distinguished.  Evidently a parent
does not project precisely the same line of influences upon
each of its offspring.
  This variability  makes possible an indefinite modifica-
                          14 
210
COMPARATIVE ZOOLOGY.
tion  of the forms of life.  For the variation extends to
the whole being, even to every organ and mental char-
acteristic as well as to form and color.  It is very slight
from generation to generation;  but it can be accumulated
by choosing from a  large number of individuals those
which  possess any given variation  in a  marked degree,
and  breeding from these.  Nature  does  this by the very
gradual process of "natural selection;" Man hastens it, so
to speak, by selecting extreme varieties.  Hence we have
in our day remarkable specimens of Poultry, Cattle, and
Dogs, differing widely from the wild races.
  Sometimes we notice that children resemble, not their
parents, but their grandparents or remoter ancestors. This
tendency to revert to an ancestral type is called atavism.
Occasionally stripes appear on  the  legs and shoulders of
the Horse, in imitation of the aboriginal HorSe, which was
striped like the Zebra.  Sheep have a tendency to revert
to dark colors.
  The laws governing inheritance are unknown.  No one
can say why one peculiarity is transmitted from father to
son,  and not another; or why it appears in one member
of the family, and not  in all.  Among the many causes
which  tend to modify animals after birth, are the quality
and  quantity of food, amount of temperature  and light,
pressure of the  atmosphere, nature of the soil or water,
habits of fellow-animals, etc.
  Occasionally  animals occur, widely different in struct-
ure,  having a very close external  resemblance.   Barna-
cles were long mistaken for Shells,  Polyzoans for Polyps,
and  Lamprey-eels for Worms.   Such forms are homo-
morphic.  Members  of  one group often  put on the out-
ward appearance of  allied species  in the  same locality:
this  is called mimicry.   " They  appear like  actors  or
masqueraders dressed up and painted for amusement, or
like swindlers endeavoring to pass themselves off for well- 
DEVELOPMENT.
211
 known and respectable members of society."   Thus, cer-
 tain Butterflies on  the Amazons have such a  strong odor
 that the Birds let them  alone;  and Butterflies of another
 family in  the same  region  have been found disguising
 themselves for protection by assuming the same form and
 color of wing.  So we  have bee-like Moths, beetle-like
 Crickets, wasp-like Flies, and ant-like Spiders; harmless
 and venomous Snakes copying each other, and Orioles de-
 parting from their usual gay coloring to imitate the plum-
 age, flight, and voice of quite another style of Birds.  The
 species which are imitated are much more abundant than
 those which mimic them.   There  is also  a general  har-
 mony  between the colors of an animal  and those of its
 habitation.  We have the white Polar Bear, the sand-col-
 ored  Camel, and the  dusky Twilight -moths.   There are
 Birds  and Reptiles so  tinted and  mottled as  exactly to
 match the rock, or ground, or  bark  of  a tree they  fre-
 quent ; and there  are Insects  rightly named  " Walking-
 sticks" and "Walking-leaves."  These  coincidences are
 not always  accidental, but often intentional on the part
 of nature, for the benefit of  the imitating  species.  Gen-
 erally, they wear the livery of those  they  live on, or ape
 the forms more favored than themselves.

        5. Homology, Analogy,  and Correlation.
  The tendency to repetition in the development  of  ani-
 mals leads to some remarkable affinities.  Parts or organs,
 having the like origin and development, and therefore the
 same essential structure,  whatever their form or function,
 are said to be homologous; while parts or organs ana-
 tomically different,  but corresponding in use, are called
analogous.
  The  following  are examples  of  homology:  the arms
and legs of Man ;134 the  upper  and lower  set of  teeth;
the parts of the vertebral column, however modified;  the 
212
COMPARATIVE ZOOLOGY.
scapular and pelvic arches; the humerus and femur; car-
pus and tarsus;  the right and left sides of most animals;
the dorsal and anal fins of Fishes; the  arms of Man, the
fore - legs  of a Horse, the paddles of a Whale, the wings
of a Bird, the front flippers of a Turtle, and the pectoral
fins of a Fish ; the proboscis of a Moth, and the jaws of a
Beetle; the  shell of a Snail,  and both valves of a Clam;
the follicles of  Invertebrates, and  the compact liver of
Vertebrates.136   The wings of the Bird, Flying Squirrel,
and Bat are hardly homologous, since the wing of the first
is developed from the fore-limb only; that of the Squir-
rel is an extension of the skin between the fore and hind
limbs; while in the Bat the  skin stretches between  the
fingers, and then down the side to the  tail.  The legs of
a Lobster  and  Lizard; the wings of a Butterfly and Bird;
the gills of a Fish, and the lungs of other Vertebrates, are
analogous.   The air-bladder of a Fish is homologous with
a lung, and analogous to the air-chambers of the Nautilus.
The wings of Birds and Bats are both homologous and
analogous; as  likewise the contractile protoplasm of the
Amoeba, and the muscular tissue of the Vertebrate.
  In the midst of the great variety of form and structure
in the animal world, a certain harmony reigns.   Not only
are different species so related as to suggest a descent
from  the  same ancestor, but the parts of any one organ-
ism are so closely connected and mutually dependent, that
the character of one must receive its  stamp from the char-
acters of all the rest.  Thus, from a single tooth it ma}'' be
inferred that the animal had a skeleton and spinal cord,
and that it was a carnivorous hot-blooded Mammal.  Cer-
tain structures always co-exist.   Animals with two occipi-
tal condyles, and non - nucleated blood - corpuscles, suckle
their  young, i. e.,  they  are  Mammals.    All  Ruminant
hoofed beasts  have horns and cloven-feet.  If the hoofs
are  even, the  horns are  even,  as in the  Ox; if odd, as in 
DEVELOPxMENT.                     213
the  Rhinoceros,  the  horns  are  odd, i.  e., single,  or two
placed one behind the  other.   Creatures with feathers  al-
ways  have beaks.   Pigeons with  short  beaks have  small
Fig. 177.                                 Fig. 178.
                       HOMOLOGIES OF LIMBS.
Fig. 175.—Arm and Leg of Man, as they are when he gets down on all fours.  Fig.
   176.—Fore and Hind Legs of Tapir. Fio. 177—Fore Leg of Seal and Hind Leg
   of Alligator.   Fig. 178__Wing of the Bat.  S, scapula; I, ilium, or shin-bone of
   pelvis; H, humerus; F, femur ;  O, olecranon, or tip of the elbow ;  P, patella;
   U, ulna;  T, tibia; R, radius ; Fi, fibula; Po, pollex, or thumb; Ha, hallex, or
   great toe. Compare the fore  and hind limbs of the same animal, and the fore
   or hind limbs of different animals.  Note the directions of the homologous seg-
   ments. 
214
COMPARATIVE ZOOLOGY
feet;  and those with long beaks, large feet.   The long
limbs of the Hound  are associated with a long head.   A
white spot in the forehead of a Horse generally goes with
white feet.  Hairless Dogs are deficient in teeth.  Long
wings usually accompany long tail-feathers.  White Cats
with blue eyes  are deaf.   A Sheep with numerous horns
is likely to have long, coarse wool.   Homologous parts
tend to  vary in the same manner; if one is diseased,  an-
other is more likely to sympathize  with it than one  not
homologous.  This association of parts is called correla-
tion of growth.

    6. Relations of Number, Size, Form, and Rank.
  The Animal  Kingdom has been likened to a pyramid,
the species diminishing in number as  they ascend in  the
scale of complexity.   This is not strictly true.   The num-
ber of living species known is about  400,000, of which
nearly nineteen-twentieths are  Invertebrates.   The Ani-
malcules (not  reckoned in  this count) are innumerable.
But next, the Articulates are  the  most numerous, then
Vertebrates, Mollusks, and  Radiates.    Of Vertebrates,
Fishes are most abundant;  then follow Birds, Mammals,
and Reptiles.
  The largest species usually belong to  the highest classes.
The aquatic members of a group are generally larger than
the terrestrial, and the marine than the fresh-water.  The
extremes of size are an Infusorium,  401oo of an inch in di-
ameter, the smallest animal ever measured, and the Whale,
one hundred feet  long, the largest animal ever created.
The female  is frequently larger than the  male, as of  the
Nautilus, Spider, and Eagle.   The higher the class,  the
more uniform the size.  Of all animals, Insects  and Birds
are the  most constant in their dimensions.
  Every organism has its own  special  law of  growth:  a
Fish and an Oyster, though born in the same locality, de- 
DEVELOPMENT.
215
velop into very different forms.   Yet a symmetry of plan
underlies the  structure of all  animals.   In the embrvo
this  symmetry of the  two ends,  as well as  the two sides,
is nearly perfect; but it is subsequently interfered with to
adapt the animal to its special conditions of life.   It is a
law  that an animal grows equally in those directions in
which the incident forces are equal.  The Crinoid, rooted
to the rocks,  is subjected to like conditions on all sides,
and, therefore, it has no right and left, or  fore and hind
parts.  The lower forms, generally, are more or less geo-
metrical figures: spheroidal, as the Sea-urchin;  radiate,
as the Star-fish; and spiral, as many Foraminifers. The
higher animals are subjected to a greater variety of con-
ditions.    Thus, a Fish, always going through the water
head foremost, must show considerable difference between
the head and  the hinder end; or a Turtle, moving over
the ground with the same surface always down, must have
distinct dorsal and ventral sides.
  Nevertheless, there  is a striking  likeness between  the
two  halves or any two organs situated on  opposite sides
of an axis.  And, first, a bilateral symmetry is most con-
stant.  It is best exhibited by the Articulates and Verte-
brates ;  but nearly all  animals can be clearly divided into
right and left sides — in  other words, they appear to  be
double.    A vertical plane would divide into two equal
parts our brain, spinal cord, vertebral column, organs of
sight, hearing, and smell; our teeth, jaws, limbs, lungs,
etc.  In  fact,  the  two  halves of every  egg are  identical.
There are many exceptions:  the heart and liver of  the
higher Vertebrates are eccentric; the nervous system of
Mollusks  is scattered;  the  hemispheres of  the  human
brain are sometimes unequal; the corresponding bones in
the right and  left arms are not precisely the same length
and weight; the Narwhal has an immense tusk on  the left
side, with none to speak of  on the other:  Rabbits have 
216
COMPARATIVE ZOOLOGY.
been born with one  ear, and Stags with  one horn ; the
Rattlesnake has but one lung; both eyes of the Flounder
and Halibut are on the  same side; the claws of the Lob-
ster differ;  and the valves of the Oyster  are unequal.
But all these animals and their organs are perfectly sym-
metrical in the embryo state.
  Again, animals  exhibit  a  certain correspondence be-
tween the fore and hind parts.136   Thus, the two ends of
the Centipede repeat  each other.  Indeed, in some Worms,
the eyes are developed  in the last segment as well as the
first.  So a Vertebrate may be considered,  not only as two
individuals placed  side by side, but also as two individu-
als put end to end—the head and  arms representing one,
and the legs the  other.  In  the embryo of Quadrupeds,
the four limbs are closely alike.   But in the  adult, the
fore and hind limbs differ more than the right and left
limbs, because the functions are more dissimilar.  An ex-
treme want of symmetry is seen in Birds  which combine
aerial and land locomotion.
  There is also a tendency to  a vertical symmetry, or
up-and-down arrangement — the part  above a horizontal
plane being a reversed  copy of the part below. A good
example is the posterior half of a Cod, while the tail of  a
Shark shows  the want of it.  This symmetry decreases as
we ascend the scale. In most animals there is consider-
able difference between the  dorsal and ventral surfaces;
and in all the nervous  system is more symmetrically dis-
posed than the digestive.
  Every animal is perfect in its  kind and in its  place.
Yet we recognize  a gradation of life.  Some animals are
manifestly superior to some others.  But it is not so easy
to say precisely what  shall guide us  in  assorting living
forms into high and low.  Shall  we make  structure the
criterion of rank?  Plainly the simple Jelly-fish is  be-
neath complicated Man.  An ounce of muscle is worth  a 
DEVELOPMENT.                 217
pound of protoplasm, and a grain of nervous matter is of
more  account than a ton of flesh.   The intricate and fin-
ished  build of the Horse elevates him immeasurably above
the stupid Snail.  The repetition of similar parts, as in the
Worm, is a sign of low life.  So also a prolonged posteri-
or is a mark of inferiority, as the Lobsters are lower than
the Crabs, Snakes than Lizards, Monkeys than Apes. The
possession of a head distinct from the region behind it is
a sign of power.  And in proportion as the fore-limbs are
used for head purposes, the animal ascends the scale: com-
pare the Whale, Horse, Cat, Monkey, and Man.
   But shall the Fish, never rising  above the " monotony
of its  daily swim," be allowed to outrank the  skillful Bee ?
Shall the brainless, sightless, almost heartless Amphioxus,
a Vertebrate, be allowed to stand nearer to Man than the
Ant?  What  is the possession of a backbone to intelli-
gence ? No good reason can be given why we might not
be just as intelligent beings if we carried, like the Insect,
our hearts in our backs and our spinal cords in our breasts.
So far as its activity is concerned, the brain may be as ef-
fective if spread out like a map as packed into its present
shape.  Even animals of the same type, as Vertebrates,
can not be  ranked according to complexity.  For while
Mammals, on the whole, are  superior to Birds, Birds to
Reptiles, and Reptiles to Fishes, they are not so  in every
respect.  Man himself  is not altogether at  the  head of
creation.  We carry about in our bodies embryonic  struct-
ures.   Thus, the embryo  Bird has its bones full of mar-
row as we have all our  life long;  but afterward  they be-
come  hollow air-sacs.  So feathers are more complex than
hairs.  That  structural  affinity and vital dignity are not
always parallel, may be seen by comparing an Australian
and an Englishman.18'
   Function  is the test of worth.   Not  mere work, how-
ever;  for we  must consider its quality  and scope.  An 
218
COMPARATIVE ZOOLOGY.
animal may be said to be more perfect in proportion as
its relations to the external world are  more varied, pre-
cise, and fitting.  Complexity of organization, variety, and
amount of power are secondary to the degree  in which
the whole organism is adapted to the circumstances which
surround it, and to the work which it has to  do.  Ascent
in the animal scale is not a passage from animals  with
simple organs to animals with complex organs,  but from
simple individuals with  organs of complex  function to
complex individuals with organs of simple function: the
addition as we ascend being not function, but of parts to
discharge those functions; and the advantage gained, not
another thing done, but the same thing done better.  Ad-
vance in rank is exhibited, not by the possession of more
life (for some  animalcules are ten times more lively  than
the busiest Man), but by  the setting apart of  more organs
for special purposes.  The higher the animal, the greater
the number of parts combining to perform each function.
The power  is increased  by this division of labor.   The
most important  feature in this specialization is  the tend-
ency to concentrate the nervous energy toward  the  head
(cephalization).   It increases as wTe pass from the Oyster
up to Man.
  As a rule, fixed species are inferior  to the free, water
species to land species, fresh-water animals to marine, arc-
tic  forms to tropical, and the herbivorous to the carniv-
orous.   Precocity is a sign of inferiority: compare the
chicks of the  Hen and the Robin, a Colt with  a Kitten,
the comparatively  well - developed Caterpillar  with the
footless grub of the Bee.  Among Invertebrates, the  male
is frequently inferior, not only in size,  but also in grade
of organization.  Animals having a wide range as to cli-
mate, altitude, or depth  are commonly inferior to those
more restricted: Man is a notable exception.
  There is some relation between the duration of life and 
DEVELOPMENT.
219
the size, structure, and rank of animals.  Vertebrates not
only grow to a greater size, but also live longer than In-
vertebrates.  Whales and Elephants are the longest lived;
and Falcons, Ravens, Parrots, and Geese, Alligators and
Turtles, and Sharks and Pikes, are said to live a century.
The life of Quadrupeds generally reaches its limit  when
the molar teeth are worn down:  those of the Sheep last
about 15  years; of the Ox, 20;  of the Horse, 40; of the
Elephant, 100.   Many inferior species die as soon as they
have  laid their  eggs, just as herbs perish as soon as they
have flowered.

               7. The Struggle for life.
   Every animal  is striving to increase in  a geometrical
ratio.   But each lives, if at all, by  a struggle at  some
period of its life.  The meekest creatures  must fight, or
die.
   " There is no exception to  the rule  that every organic
being naturally increases at so high a  rate that,  if not de-
stroyed, the earth would soon be covered by the progeny
of a single pair."  If the increase of the human  race were
not checked, there would  not be  standing-room for the
descendants of Adam and Eve.  A pair of Elephants, the
slowest  breeder of all known animals, would become the
progenitors, in five centuries,  of 15,000,000 of Elephants,
if death did not interfere.   In  fifteen years,  a pair of
Birds would increase to 2,000,000,000.  Evidently a vast
number must perish, and  a far greater host of eggs fail
to mature.  Whatever the  average number of individuals
in any country, twice that number must die annually.  A
single Cod, laying millions of eggs, if  allowed to have its
own way, would soon pack the ocean.
  Yet, so nicely balanced are the forces of nature, the
average number of  each  kind remains about the same.
The total extinction of any  one  species is exceedingly 
220
COMPARATIVE ZOOLOGY.
rare.  The number of any given species is not determined
by the number of eggs  produced, but by its conditions.138
Aquatic  birds  outnumber  the land birds,  because  their
food never fails, not because they are more  prolific.  The
Fulmar-petrel lays but one egg, yet it is believed to be the
most numerous bird in the world.
  The main checks to the high rate of increase are: cli-
mate (temperature and moisture), acting directly or  indi-
rectly by reducing food; and other animals, either rivals
requiring the same food and locality, or enemies, for the
vast majority of animals are carnivorous.   Offspring are
continually varying from their parents, for better or worse.
If feebly adapted to the conditions of existence, they will
finally go to the wall.  But those forms having the slight-
est advantage over others inhabiting the same region, be-
ing hardier or stronger, more agile  or sagacious, will sur-
vive.  Should this advantageous variation  become heredi-
tary and intensified, the new variety will gradually extir-
pate or replace other kinds.  This is  what Mr.  Darwin
means by Natural Selection, and Herbert Spencer by the
Survival of the Fittest. 
      PART II.




SYSTEMATIC ZOOLOGY. 
  Facts are stupid things  until brought into connection with  some general
law.—Agassiz.
  No man becomes a proficient in any science who does not transcend sys-
tem, and gather up new truth for himself in the boundless field of research.
—Dr. A. P. Peabodt.
  Never ask a question if you can help it;  and  never  let  a  thing go un-
known for the lack of asking a question if you can't help it.—Beecher.
  He is a thoroughly good  naturalist who knows his own parish thoroughly.
—Charles  Kingsley. 
THE CLASSIFICATION OF ANIMALS.        223
                   CHAPTER XXI.
             THE CLASSIFICATION OF ANIMALS.
  The Kingdom of Nature is a literal Kingdom.  Order
and  beauty,  law and  dependence, are seen everywhere.
Amidst the great diversity  of  the forms  of life, there is
unity; and this suggests that  there is one general plan,
but carried out in a variety of ways.
  Naturalists have ceased to believe that each animal or
group is a distinct, circumscribed  idea.   " Every animal
has a something in common  with all its fellows: much
with  many of  them; more with a few;  and, usually, so
much with several, that it  differs but little from them."
The object of  classification  is to bring together the like,
and to separate the unlike.  But how shall this be done ?
To arrange a library in alphabetical order, or according to
size, binding, date,  or language, would be unsatisfactory.
We must be guided by some internal character.  We must
decide whether a book is poetry or prose: if poetry, wheth-
er dramatic,  epic, lyric,  or  satiric; if  prose, whether his-
tory,  philosophy, theology,  philology, science, fiction,  or
essay.   The  more  we subdivide these groups,  the more
difficult the analysis.
  A classification of animals founded on external resem-
blances, as size, color, or adaptation  to similar habits  of
life, would be  worthless. It would bring together Fish-
es and Whales, Birds  and Bats, Worms  and  Eels.  Nor
should it be  based on any  one character, as  the quality
of the blood, structure  of the  heart,  development of the
brain, embryo-life, etc.;  for no character is of equal value
in every tribe.  A natural classification must rest on those 
224
COMPARATIVE ZOOLOGY.
prevailing characters which are the most constant.139 And
such a classification can not be linear.   It is impossible to
arrange all animal forms from the Sponge to Man in a
single line, like the steps of a ladder, according to rank.
Nature passes in so many ways from one type to another,
and so multiplied are the relations between  animals, that
one series is out of the question.  There is a number of
series,  and series  within  series, sometimes  proceeding in
parallel lines, but more often divergent.  The animals ar-
range themselves in radiating groups, each group being
connected, not with two groups merely, one above and the
other below, but with several.   Life has been likened to a
great tree with countless branches spreading widely from
a common trunk,  and deriving their origin from a com-
mon root; branches bearing all manner of flowers, every
fashion of leaves,  and all kinds of fruit, and these for ev-
ery use.
  The groups into which we are able to cast the various
forms of animal development are very  unequal and  dis-
similar.  We must remember that a genus, order, or class
is not of equal value throughout the kingdom.  Moreover,
each division is allied to  others in different degrees—the
distance between any two being the measure of that affin-
ity.  The  lines between some are sharp and clear, between
others indefinite.  Like the islands of an archipelago, some
groups merge  into one another through connecting reefs,
others are sharply separated by unfathomable seas,  yet all
have one  common basis.  Links have been found reveal-
ing a relationship, near or distant, even between animals
whose forms are very unlike.   How different a Fish and
a Lizard!  yet  there is  a  strange  creature (the Axolotl of
Mexico) whose organization is intermediate between  the
two, so that it is difficult to determine to  which group it
belongs.   The Slow-worm is a transition between Lizards
and Snakes; while the extinct Ichthyosaurus bridges  the 
THE CLASSIFICATION OF ANIMALS.        225
chasm between Crocodiles and Fishes.   Birds seem iso-
lated ; but in ancient times there were flying Reptiles;
while the Ornithorhynchus, Kangaroo, and Bat  stand on
the border-line between Mammals and the feathered tribe.
Even between the grand Vertebrate and Invertebrate di-
visions there  flits a ghost-Hke form—the Amphioxus, half
Worm, half Fish.
  We have, then, groups subordinate to groups, and inter-
locking, but not representing  so many successive degrees
of organization.   For, as already intimated, complication
of structure  does  not rise in continuous  gradation from
one group to another.  Every type starts at a lower point
than that at which the preceding class closes;  so that the
lines overlap.  While one class, as  a whole, is higher than
another, some members of the higher class may be inferi-
or to some members of the lower one.  Thus, certain Star-
fishes are nobler than certain Mollusks;  the Nautilus is
above the Worm,  and the Bee is more worthy than the
lowest Fish.   The groups  coalesce by their inferior  spe-
cies ; e.g., the Fishes do not graduate into Reptiles through
their higher forms, but the two come closest together low
down in the scale.  Man appears to be the goal of crea-
tion ; but even within the Vertebrate series, every step of
development, say of the Fish, is away from the goal.  The
highest Fish is the one farthest from Man.
  A number  of animals  may, therefore, have the same
grade of  development, but conform to entirely different
types.  While a fundamental unity underlies the whole
Animal Kingdom, suggesting a common starting-point,
we  recognize  four or five  distinct  plans of structure.140
Thus, animals like the Coral, unlike all others, have the
alimentary canal opening into the body-cavity, have no
separate  nervous and vascular regions, and the parts of
the  body radiate from a centre.   Such form  a subking-
dom called Coelenterata.  Animals, like the Star-fish, hav-
                          15 
226
COMPARATIVE ZOOLOGY.
ing also a radiating body, but a closed alimentary canal,
and a distinct, symmetrical nervous system, constitute the
subkingdom Fchinodermata.1*1  Animals,  like the Snail,
with a soft, unsymmetrical body, well-developed digestive
apparatus, and  scattered nervous system, form the  sub-
kingdom Mollusca.  Animals, like  the Bee,' with a sym-
metrical body composed of numerous segments, a nervous
system consisting of a double chain of ganglia along the
lower side of the body, and limbs on the same  side as this
nervous cord, form the subkingdom Articulata. Animals,
like the Ox, having a double nervous system, one (the  gan-
glionic) lying on the upper side of  the alimentary canal,
the other and  main part (spinal)  lying  along the back,
and completely shut off from the other organs  by  a parti-
tion of bone or gristle, known as the " vertebral column,"
and having limbs, never more  than four, always on the
side opposite the great  nervous cord, constitute  the  sub-
kingdom Vertebrata.
  Comparing these great divisions, we see that the Verte-
brates differ from all the others chiefly in having a double
body-cavity and a double nervous system, the  latter lying
above the alimentary canal;  while Invertebrates have one
cavity  and one  nervous  system, the latter being placed
either below  or around the alimentary canal.  The Ar-
ticulate type differs from the Molluscan  mainly in being
jointed.  The Echinoderms and Coelenterates are built on
the common type of a star; but they differ from each oth-
er in the presence or absence of distinct alimentary, circu-
latory, and nervous systems.
  But there are types within types.  Thus, there  are five
modifications of the Vertebrate type — Fish, Amphibian,
Reptile, Bird, and Mammal; and these are again  divided
and subdivided, for Mammals, e. g., differ among them-
selves.  So that in  the  end we  have a constellation of
groups within groups, founded  on peculiar characters of 
THE CLASSIFICATION OF ANIMALS.        227
less and less importance, as we descend from the general
to the special.
   Individuals are the units of the Animal Creation.  An
animal existence, complete in all its parts, is an individual,
whether separate, as Man, or living in a community, as the
Coral.1"
   Variety: when two or more individuals differ by a sin-
gle peculiarity only, such as size,  color, or outline, one is
called a variety of the other; as the various races of Men
and breeds of  Cattle.  Varieties are usually local. A cross
between distinct races is called  mongrel.
   Species is  the smallest group of individuals which can
be defined by several constant characteristics.  They are
so alike, that  it is possible for them to  have descended
from one pair; and they always transmit to their offspring
some peculiarity of their organization.  A cross between
two  distinct species, as the Horse  and Ass, is called a hy-
brid; as the Mule.
   Genus is a group  of species having the same essential
structure.   Thus, the  closely allied species, Cat, Tiger, and
Lion, belong to one genus.
   Family, or Tribe, is a group of genera having a simi-
lar form.   Thus, the  Dogs  and Foxes belong to different
genera, but betray a family likeness.
   Order is a  group of families, or genera, related to one
another by a common structure.  Cats, Dogs, Hyenas, and
Bears are linked together by important anatomical feat-
ures ; their teeth, stomachs, and  claws show carnivorous
habits.
   Class is a  still larger group,  comprising all animals
which  agree simply in a special  modification of the type
to which  they belong.   Thus, Fishes, Amphibians, Rep-
tiles, Birds, and Mammals are so many aspects of the Ver-
tebrate type.
   Subkingdom. is a primary division of the Animal King- 
228
COMPARATIVE ZOOLOGY.
dom, which includes all animals formed upon  one of the
four or five types of structure; as Vertebrate.
  These terms were invented by Linnaeus, except Family
and Subkingdom, which were added by Lamarck.  To Lin-
naeus we are also indebted for a scientific method of nam-
ing animals.   Thus, a Dog in Zoology is called Canis
familiaris, which is the union of a generic and specific
name, corresponding to the surname and Christian name
in George Washington, only the specific name  comes last.
It will  be understood  that these are abstract terms, ex-
pressing simply the relations of resemblance:  there is no
such thing as genus or  species.
  Classification is a process of comparison.  He is the best
naturalist who most readily and correctly recognizes like-
ness founded on structural characters.  As it  is easier  to
detect differences than resemblances, it is much  easier  to
distinguish the class to which an animal belongs than the
genus, and the  genus than the species.   In passing from
species to classes, the characters of agreement become few-
er and fewer, while the  distinctions are more and more
manifest; so that animals of the same class are more like
than unlike, while members of distinct classes are more
unlike than like.
   To illustrate the method of zoological analysis by search-
ing for  affinities and differences, we will take an  example
suggested by Professor Agassiz.  Suppose we see together
a Dog, a Cat, a Bear, a Horse, a  Cow, and a Deer.  The
first feature which strikes us  as  common  to any two  of
them is the horn  in the Cow and Deer.   But how shall
we associate either of the others with these ? We examine
the teeth, and find those of the Dog, the Cat, and the Bear
sharp and cutting; while those of  the Cow, the Deer, and
the Horse have flat surfaces, adapted to grinding and
chewing, rather than cutting  and tearing.   We  compare
these features of their structure with the habits of these 
THE CLASSIFICATION OF ANIMALS.        229
animals, and find that the first are carnivorous—that they
seize and tear their prey; while the others are herbivorous,
or grazing, animals, living only on  vegetable substances,
which  they chew and grind.  We compare, further, the
Horse  and Cow, and find  that the Horse has front teeth
both in the upper and the lower jaw, while the Cow has
them only in the lower; and going still further, and com-
paring the internal with the external features, we find this
arrangement of the teeth in  direct  relation to the differ-
ent structure of the stomach in the two animals—the Cow
having a stomach with four pouches, while the Horse has
a simple stomach.  Comparing the Cow and Deer, we find
the digestive apparatus the same in both; but though both
have horns, those of the  Cow are hollow, and last through
life ; while those of the Deer are solid, and are shed every
year.  Looking  at the feet, we see that the  herbivorous
animals are hoofed; the carnivorous, clawed.  The Cow
and Deer have cloven feet, and are ruminants; the Horse
has a single hoof, and does not chew the cud. The Dog
and Cat walk on the tips of their  fingers and  toes (digiti-
grade) ; the Bear treads on the palms  and soles  (planti-
grade).  The claws of the Cat are retractile; those of the
Dog and Bear are fixed.
  In this way we determine  the exact place of each  ani-
mal.  The Dog  belongs to the kingdom Animalia, sub-
kingdom Vertebrata, class Mammalia, order Carnivora,
family Canidai,  genus Canis,  species Familiaris, variety
Hound (it may be), and its individual name, perhaps, is
" Rover."   The Cat differs  in belonging  to  the  family
Felidoe, genus Felis, species Catus.  The Bear belongs to
the family Ursidaz, genus Ursus, and species Ferox, if the
Grizzly is meant.  The  Horse, Cow, and Deer belong to
the order TJngulata; but  the Horse  is  of  the  family
Equidce, genus Eguus,  species Caballus;  the Cow is of
the family Bovidce, genus Bos, species Taurus ; the Deer 
230
COMPARATIVE ZOOLOGY.
is of the family Cervidae, genus Cervus, species Virgini-
anus, if the common Deer is meant.
   The following diagram roughly represents (for the rela-
tions of animals can not be expressed on a plane  surface)
the relative positions of the subkingdoms and classes ac-
cording to  affinity  and rank.  It will be seen that the Ver-
tebrate, Articulate, and Molluscan types stand at the angles
of an isosceles  triangle.  The lowest Worms  are simpler
than the lowest Mollusks;  yet Articulates, on the whole,
are, perhaps, higher than the Mollusks:  the  former have
more outward expression; the latter, more concentration.*
 Mammalia.
Vertebrata.
 Amphibia.
  Pisces.
Aves.
Reptilia.
                Cephalopoda.
        Gasteropoda.
Lamellibranchiata.  Mollusca.
Tunicata.
Brachiopoda.
Polyzoa.
Insecta.
      Myriapoda.
Articulata.  Arachnida.
        Crustacea.
Annelida.
Echinoidea.  Holothuroidea.
   Echinodermata.
   Asteroidea.
        Crinoidea.
   Anthozoa.
 Coslenterata.
   Hydrozoa.
   Spongida.
          Infusoria.
      Protozoa.
   Rhizopoda.
         Gregarinida.
  * The student should master the distinctions between the great groups, or
classes,  before proceeding to a minuter classification.  "The essential mat-
ter, in the first place," says Huxley, "is to be quite clear about the different
classes,  and to have a distinct knowledge of all the sharply definable modifi-
cations of animal structure which are discernible in the Animal Kingdom." 
PROTOZOA.
231
               Subkingdom.—Protozoa..
   This division was proposed by Von Siebold in 1845 to
 contain that vast cloud of microscopic beings on the verge
 of the Animal Kingdom whi-ch could not be received into
 the  other subkingdoms.  It is artificial and provisional.
 The classes composing  it are not founded on a common
 type, but are distinguished by the absence rather than the
 presence of positive characters. The time may come when
 the microscope will  resolve these nebulae, so that we can
 give them a natural classification.   Probably some of them
 are transitory stages in the  history of higher organisms.
 Many stand parallel to  the Protophytes of the Vegetable
 World, and no definite line can be drawn between them.
   Protozoans  agree in being minute, aquatic, and exceed-
 ingly simple in structure, their bodies consisting mainly
 or wholly of the contractile,  gelatinous matter called pro-
 toplasm, or sarcode — the  first  homogeneous substance
 which has the power of controlling chemical and phys-
 ical forces.  No traces  of nervous or muscular fibres, cir-
 culator}7 or  digestive organs  have been discovered.  Yet
 they take and  assimilate food, grow and multiply, which
 are the essential signs of life.  The usual methods of re-
 production are self-division and budding.
  The subkingdom may be divided into four classes:  Gre-
 garinida, Rhizopoda, Infusoria,  and Spongida.  The per-
 fectly homogeneous Monera of Haeckel would rank lower
 than the Gregarinae; but as  they are doubtfully referred
 to the Animal Kingdom, we do not include them.

                 Class I.—Gregarinida.
  The Gregarinae, discovered by Dufour  in 1828, are the
simplest animal forms of which we have any knowledge.
They closely resemble a cell, or microscopic egg; the only
organ is a nucleus, suspended in extremely mobile granu- 
232
COMPARATIVE ZOOLOGY.
lar matter; and the most conspicuous signs of life are the
contraction and lengthening of the worm-like body.  They
feed by absorption, and are all parasites, living in the ali-
          K


        Fig. 179.—Gregarina gigantea, highly magnified: a, nucleus.

mentary canal of higher animals; particularly in the Cock-
roach and Earth-worm.   The name is derived from the
fact that  they occur  in large numbers crowded together.
By some  authors  they are associated  with  the parasitic
Worms.

                 Class II.—Rhizopoda.
  The Rhizopods are  characterized by the power of throw-
ing out at will  delicate processes of their bodies,  called
pseudopodia, or false feet, for prehension or locomotion.
They possess no cilia.   The representative forms are Amoz-
bw, Foraminifera, and Polycystina.
  An Amoeba is a naked, fresh-water  Rhizopod; an in-
definite bit  of protoplasm, as structureless as a speck of
                           jelly, save that it is made of
                           two distinct layers, and has
                           a  contractile  cavity  inside.
                           It has no particular form, as
                           it  changes  continually.   It
                           moves by putting forth short,
                           blunt processes,  and eats  by
Fig. iso.-Amceba princeps, x 150; the wrapping its body around the
   same animal in various shapes.    particle  0f f00d.  The size
ranges from ^V  to s^0() of an inch in diameter.    Speci-
mens can be obtained by scraping the mucous matter
from the  stems and leaves in stagnant  ponds.
 
CCELENTERATA.
233
  A Foraminifer differs from an Amoeba in having an
apparently simpler  body, the protoplasm being  without
layers or cavity; but it has the property of secreting an
envelope,  usually of carbonate of lime.   The shell thus
formed is sometimes of extraordinary complexity and sin-
gular beauty.  It is generally perforated  by innumerable
minute  orifices (foramina) through which the animal pro-
trudes its myriad of glairy, thread-like arms.  The major-
ity  are  compound, resembling chambered shells, formed
by a process of budding, each new individual being add-
ed so as to make a straight series, a spiral, or a flat coil.
Fig. 181.—Rhizopods: a, a mouothalamous, or single-chambered, Foraminifer (La-
   gena striata); b, a polythalamous, or many-chambered, Foramiuifer (Polystomella
   crispa, with pseudopodia extended); c, a Radiolarian, one of the Polycystines
   (Podocystis Schomburgkii).

As a rule, the many-chambered species have calcareous,
perforated shells; and the  one-chambered have an imper-
forated membranous,  porcelanous, or  arenaceous envel-
ope.  The former are marine.   There are few parts of the
ocean where these microscopic shells do not occur, and in
astounding numbers.   A single ounce  of  sand from the
Antilles was  calculated to contain over  three millions.
Their remains  constitute  a great  proportion of the so-
called sand-banks which block  up many harbors.  Yet they
are the descendants of an ancestry still more prolific; foi
the chalk-cliffs of England, the building-stone of Paris, 
234             COMPARATIVE ZOOLOGY.
and the blocks in the Pyramids of Egypt are largely com-
posed of extinct Foraminifers.
  A Polycystine differs from  a Foraminifer in secreting
a siliceous,  instead  of a calcareous, shell,  studded with
spines;  and the central part of the  body is made up of
many cells, and surrounded by  a strong membrane.  While
Foraminifers live mostly at the bottom of the sea, Polycys-
tines generally float on the surface.  They are also more
minute, but as widely diffused.  They enter largely into
the formation of some strata of  the earth's  crust, and
abound especially in the rocks of Barbados and at Rich-
mond, Va.

                  Class III.—Infusoria.
  This  unassorted group of living  particles derived its
                        name  from  the fact that they
                        were  first  discovered in  vegeta-
                        ble infusions.   Every drop of a
                        stagnant pool is  crowded with
                        them.  They are all single and
                        microscopic,
                        yet of various
                        sizes, the differ-
ence  between the smallest  and largest
being  greater than  the difference  be-
tween a Mouse and an Elephant.  Some
are fixed  (as Vorticella); but the major-
ity are free, and  constantly in motion,
propelled by  countless cilia, as a galley
by its oars.   The delicate body consists
of unorganized sarcode (i. e.,  there  are
no cellular  tissues, but  the  whole  body
represents a  single cell), covered  by  a
membrane,  or  skin,  and  containing  a
semi-fluid  substance, a contractile  cavity, and   several
Fig. 182.—A Compound Monad
     (Uvella), X 1000.
Fig. 183. — Infusorium
 (Paramecium aurelia),
 X 300:  m, mouth;  v,
 contractile vesicles; n,
 nucleus. 
C02LENTERATA.
235
granules.  On one side is a slight depression, or "mouth,"
leading to a short, funnel-shaped throat.  A mouth and a
rudimentary digestive cavity are  the distinctive features
of these Protozoans.   They  multiply so rapidly (chiefly
by self-division), that a Paramecium, the most common
form, may become the parent of 1,364,000 in 42 days.
  There are two main groups:  Flagellata, or Monads,
provided  with one  or  two flagella, or  long bristle-like
cilia;  and  Ciliata, which are  furnished with  numerous
vibratile cilia.

                  Class IV.—Spongida.
  An  ordinary Sponge  is a compound  animal, or, more
properly, an aggregated colony of individual cells,  sup-
ported on a skeleton of horny fibres, which  are so united
as to form a net-work of tubes.  The essential part is the
glairy, gelatinous substance investing this  elastic  frame-
work.  It consists of myriads of mouthless, sarcode  bodies,
which  in some respects resemble  Amoebae, but approach
Fig. 184.—Hypothetical Section of a Sponge:  a, superficial layer; b, inhalent pores:
   c, ciliated chambers; d, exhalent aperture, or osculum; e, deeper substance of
   the Sponge.
the flagellate  Infusoria in being uniciliated, and the Hy-
drozoa in having two layers  of  cells in the body-wall, and
in producing  true eggs.  While in other Protozoa aggre-
gation is a result of growth,  and the parts are not mutual-
ly dependent,  in Sponges the parts  work for the life of the
whole, giving the mass a kind of individuality.  Differen- 
236
COMPARATIVE ZOOLOGY.
tiation is carried to a higher degree, as we find ectoderm
and  endoderm, fibrous tissue, ciliated tracts, and a canal-
system for circulation.
  When freshly taken from its element, a Sponge is hard
and  glistening on the outside, and strongly resembles a
piece of liver.  While living, constant currents of water
issue from the large orifices, fed by smaller streams enter-
ing by the minute pores, the currents being caused by cilia
Fig. 185.—Horny Skeleton of a Sponge.
lining  the  passages.   So that "the  Sponge  represents a
kind of subaqueous city, where  the  people are arranged
about the streets and roads, in such  a manner that each
can easily appropriate his food from the water as it passes
along."  The apertures, or "gates,"  can be closed  at the
will of the  animal.
  A few species are wholly gelatinous, having no skele-
ton ; some  are  calcareous, aping  the Corals; many are
entirely siliceous, resembling spun glass, as the beautiful 
CC3LENTERATA.
237
Venus flower-basket (Euplectella); but the majority have
a fibrous, horny skeleton, which in some forms is strength-
ened by siliceous needles (spicula). Excepting a few small
fresh-water species (as Spongilla), Sponges are marine.  In
the former, the gelatinous part is greenish; in the latter, it
is brown, red, or purple.  In preparing the sponge of com-
merce, this is rotted by exposure, and washed out.  The
best fishing-grounds are the eastern end of  the Mediter-
ranean and around  the Bahama Islands.

             Subkingdom.—Coslenterata.
  These radiate  animals are distinguished  by  having a
distinct body-cavity, the walls of it consisting of two lay-
ers  of cellular tissue, an outer (ectoderm) and inner (endo-
derm);  and thread  cells, which are  minute sacs contain-
ing a fluid, and connected with barbed filaments capable
of being thrown out for stinging purposes.  Most are pro-
vided with hollow  tentacles around  the  mouth.  All are
aquatic, and nearly  all are marine. There are two classes,
represented by  the Hydra and Sea-anemone.   Both repro-
duce by budding and by eggs; but in the former the eggs
are developed from the  exterior of the body,  while in the
latter they are internal.

                 Class I.—Hydrozoa.
  These Ccelenterates have no separate  digestive sac, so
that the body is a  simple  tube, or cavity, into which the
mouth opens.   A nervous system is  not  apparent.  Such
are  the  fresh-water Hydra and the oceanic Jelly-fish
(Acaleph or Medusa).
  The body of the Hydra is tubular, soft, and  sensitive,
of a greenish or reddish color, and  seldom over half  an
inch long.   It  is found spontaneously attached by one
end  to  submerged  plants, while the free end contains
the  orifice, or mouth, crowned with tentacles, by which 
238             COMPARATIVE ZOOLOGY.
Fig. 186—Hydra: 2, with tentacles fully extend-
      ed ; 3, creeping; 5, budding.
                                 the creature feeds and
                                 creeps.  Closely related
                                 to  the Hydra are  the
                                 compound  Sertularians,
                                 often  mistaken for del-
                                 icate  sea-weeds.   The
                                 commonest  species  on
                                 our Atlantic coast (Dy-
                                 namena) is of  a pale-
                                 yellow color, and hangs
                                 in   fringes  from  sea-
                                 weeds, shells, and rocks.
                                   The ordinary Acaleph
                                 has a soft,  gelatinous,
                                 semi - transparent, bell-
                                 shaped body, with tubes
                                 radiating from the cen-
tral cavity to the circumference, where they are connected
by  a circular canal,
and  with the margin
fringed with stinging
tentacles.  The radia-
ting parts are in multi-
ples of four.  Around
the  rim  are  minute
colored  spots, called
" eye - specks,"   sup-
posed to be the earli-
est  indications of the
organs of  sight.    In
fine   weather,  these
k' sea - blubbers "   are
seen  floating  on  the
sea, mouth downward,
moving about by flap-    Fig. 187.—Sertularia growing on a Shell.
 
CCELENTERATA.
239
Fig. 188.—Medusa (Pelagia noctiluca), a free Lucer-
            narian.  Mediterranean.
Fig. 189.—Portuguese Man-
  of-war (Physalia), I natu-
  ral size.  Tropical Atlantic.
Fig. 190.—Jelly-fish (Aurelia aurita), with young in various stages.
67 
240
COMPARATIVE ZOOLOGY.
Pig. 191.—A Meausa, seen in
 profile  and from below,
 showing central polypite,
 radiating  and  marginal
 canals.
ping their sides, like the opening and shutting of an um-
brella, with great regularity.   They  are frequently  phos-
                    phorescent  when  disturbed.   Some
                    are  quite  small, resembling  little
                    glass bells;  the  common Aurelia is
                    over  a foot in  diameter when full
                    grown; while  the Cyanea, the  giant
                    among Jelly-fishes, sometimes meas-
                    ures  five  feet, with  tentacles  forty
                    feet long.  When dried, nothing is
                    left but a film of membrane weigh-
                    ing only a few grains.
                       There are two representative types:
                    the Lucernaria, the Umbrella-acaleph,
                    having a  short  pedicel  on  the back
                    for attachment;  tentacles disposed in
                    eight  groups around  the margin, the
eight  points  alternating with  the four partitions of  the
body-cavity and  the four corners of  the mouth; not  less
than eight radiating ca-
nals, and no membra-
nous veil. The common
species on the Atlantic
shore,  generally found
attached  to  eel-grass,  is
an inch in diameter, of
a green  color.   Disco-
phora, the ordinary Jel-
ly-fish, is free and oceanic.  It differs from the Lucernaria
in its usually larger size and solid disk, four radiating ca-
nals, which ramify and open into  a  circular vessel,  and a
" veil," or shelf, always running around the  mouth of the
disk.143
Fio. 192___Lucernaria auricula attached to a
 piece of sea-weed; natural size.  The one
 on the right is abnormal, having a ninth
 tuft of tentacles. 
COZLENTERATA.
241
                  Class II.—Anthozoa.
  These marine animals, which by their gay tentacles con-
vert the bed of the ocean into a flower-garden, or by their
secretions build up coral-islands,
have a body like a round gelat-
inous bag.  One end, the  base,
is  usually  attached; the  other
has the  mouth in  the  centre,
surrounded by  numerous hol-
low tentacles, which  are  cov-
ered with nettling  lasso - cells.
This  upper edge is turned  in
so as to  form a sac within a
     like  the  neck  of a  bottle
sac,
Fig. 193.—Horizontal Section of Ac-
 tinia through the stomach, show-
 ing septa and compartments.
turned  outside in.  The inner
one, which is the  digestive cavity, does not reach the bot-
tom, but opens into the general body-cavity.   The space
between these two concentric tubes is divided by a series
of vertical  partitions, some of  which extend  from  the
body-wall to the  digestive sac, but others fall  short of it
                                  Instead,  therefore,  of
                                  the radiating tubes of
                                  the Acaleph, there are
                                  radiating  spaces.   No
                                  members  of this  class
                                  are microscopic. All are
                                  long - lived  compared
                                  with the Hydrozoa, liv-
                                  ing for several years.
                                     1. Soft-bodied Pol-
                                  yps.—The best-known
                                  representative  of  this
                                  group is the Actinia, or
Fig. 194.—Actinia expanded, seen from above,               T+l™,^ „
           showing mouth.            Sea-anemone. It leads a
                           16
 
242
COMPARATIVE ZOOLOGY.
Fio. 195__A Ctenophore (Pleu-
  robrachia pileus);  natural
  size.
single life, and is capable of a slow locomotion.  Muscular
fibres run around the body, and others cross these at right
                     angles.   The tentacles, which often
                     number over 200, and the partitions,
                     which are in reality  double, are in
                     multiples of six.  At  night or when
                     alarmed, the tentacles are drawn in,
                     and  the  aperture  firmly closed, so
                     that the  animal looks like a round-
                     ed lump of fleshy  substance plaster-
                     ed on the rock.  It feeds on Crabs
                     and  Mollusks.   It abounds on ev-
                     ery shore, especially of tropical seas.
                     The size varies from one-eighth of
                     an inch to a foot in diameter.
                        The Ctenophora (as  the  Pleuro-
brachia, Cestum, and Bero'e) likewise secrete no hard de-
posit.   They  are  transparent  and  gelatinous, swimming
on the  ocean  by means  of eight
bands of comb-like fringes, which
work like paddles.  The body is
not contractile  as in the  Jelly-
fishes.  They  are  considered the
highest of Ccelenterates, having a
complex nutritive apparatus and
a definite nervous system.
  2. Coral Polyps.—The majori-
ty of Anthozoa secrete a calcare-
ous or  horny  frame-work  called
" coral."  With few exceptions,
they are fixed and composite, liv-
ing in colonies formed by a con-
tinuous, process of budding. Their
structures take a variety of shapes: often dome-like, but
more frequently imitating shrubbery and clusters of leaves.
Fig. 196.—Organ-pipe Coral (Tubv-
 pora musica). Indian Ocean. 
CC3LENTERATA.
243
 The members of a coral community are organically con-
 nected ;  each feeds himself, yet is not independent of the
 rest.  We can speak of the individual Corals, a, b, c, but
 we must write them down abc.  The compound  mass is
 " like a living sheet of animal matter, fed and nourished
 by numerous mouths and as  many  stomachs."  Life and
 death go on together, the old Polyps dying below as new
 ones are developed above.  The living part of an Astrcea
 is only half an inch thick.  The growth of the  branching
 Madrepore is about three inches a year.   The  prevailing
 color of the Coral Polyps is green;  and the  usual size
 varies from that of a pin's head to  half  an inch, but the
 Mushroom - coral (which is a  single  individual) may be a
 foot in diameter.
   Corals are of two kinds:  those deposited within the tis-
 sues of the animal (sclerodermic), and those  secreted by
 the outer surface at  the foot of the  Polyp (sclerobasic).
 The Polyps  producing the former  are Actiuoid, resem-
 bling the Actinia in structure.144  The skeleton of a single
 Polyp  (called corallite, Fig. 95) is a copy of  the animal,
 except the stomach and tentacles, the earthy matter being
 secreted  within  the outer wall and  between each pair of
 partitions.   So that a corallite is a short tube with vertical
 septa radiating toward the centre.146  A  sclerobasic coral
 is  a true  exoskeleton, and is distinguished  by being smooth
 and solid.  The Polyps, having eight fringed tentacles, are
 situated on the outside of this as a common axis, and are
 connected  together by the fleshy ccenosarc covering  the
 coral.
   (1) Sclerodermic Corals.—Astrcea is  a  hemispherical mass
 covered with large cells.  Meandrina, or " Brain-coral,"
is  also globular;  but the mouths of  the Polyps open into
each other, forming furrows.   Fungia,  or " Mushroom-
coral," is disk-shaped,  and differs from other kinds in be-
ing the secretion of a  single  gigantic Polyp, and in not 
244
COMPARATIVE ZOOLOGY.
Fig. 197.—Madrepora aspera, living and expanded; natural size.  Pacific
being fixed.  Madrepore is neatly branched with pointed
extremities, each ending in a small cell about a line in
diameter.   Porites, or "Sponge-coral," is also branching,
but  the ends are  blunt,  and the surface comparatively
Fig. 198—Ctenactia echinata, or " Mushroom-coral;" one-fourth natural size. Pacific. 
C03LENTERATA.
245

          Fig. 199.— Astrcea pallida; natural size.  Feejee Islands.
smooth.   Tubipora, or " Organ - pipe coral," consists  of
smooth red  tubes  connected at  intervals by cross plates.
Fig. 200.__Diploria cerebriformis, or " Brain-coral;" one-half natnral size. Bermudas. 
246
COMPARATIVE ZOOLOGY.
Fig. 201.—Astrcea rotulosa.  West Iudies.
The  Astrcea, Meandrina,  Madrepore,  and  Porites  are
the  chief  reef-forming corals.   They will  not  live in
waters the mean  temperature  of  which  is  below  68°
Fig. 202.—Cell of Madrepore Coral,
 magnified. The cap-like depres-
 sion at the top of a coral skele-
 ton is called calicle.
Fig. 203—Fragment of Red Coral (Coral-
 lium rubrum), showing living cortex
 and expanded Polyps.  Mediterranean. 
ECHINODERMATA.
247
Fahr.,  nor at greater  depth than twenty  fathoms.   The
most  luxuriant  reefs  are in  the  Central and Western
Pacific and around the West  Indies.
  (2) Sclerobasic Corals.—Corallium rubrum,  the precious
coral of commerce, is  shrub-like, about a foot high, solid
throughout, taking a  high polish,  finely grooved on the
Fig. 204.—Sea-fan (Oorgonia) and Sea-pen (Pennatula).
surface, and of a crimson or rose-red color.  In the living
state the branches are covered with  a red coenosarc stud-
ded with Polyps.  Gorgonia, or " Sea-fan,"  differs from
all the other repi-esentative forms in having a horny axis
covered with calcareous spicules.  The  branches arise in
the same vertical plane, and unite into a beautiful net-
work.

             Subkingdom  Echinodermata.
  The Echinoderms, as Star-fishes and Sea-urchins, are dis-
tinguished by the possession of a distinct nervous system
(a ring around the mouth); an alimentary canal, complete-
ly shut off from the body-cavity, and  having both oral and
anal apertures;  a peculiar system of circular and radiat- 
248
COMPARATIVE ZOOLOGY.
ing canals, and a symmetrical arrangement of all the parts
of the body around a central axis in multiples of five.14'
Fig. 205.—Forms of Echinoderms, from Radiate to Annulose type.
There  are four principal  classes, all exclusively marine
and solitary, and all having the power of secreting more
or less calcareous matter.

                  Class I.—Crinoidea.
  The Crinoids, or " Sea-lilies," are fixed to the sea-bottom
by means of a hollow, jointed, flexible stem.  On the top
of the stem is the body proper, resembling a bud or ex-
panded flower, containing the digestive apparatus,  with
the surrounding arms, or tentacles.  The mouth looks up-
ward.  There  is a complete internal skeleton for strength
and support, the entire animal, body, arms, and stem, con-
sisting of thousands of stellate pieces connected together
by irritable matter.  Crinoids were  very abundant in the
old geologic seas, and many limestone strata were created
out of their remains.  They are now nearly extinct: dredg-
ing in the deep parts of the Caribbean  Sea and Atlantic
Ocean has brought to light two or three living representa-
tives. 
ECHINODERMATA.
249
>.i#r:
W I
Fig. 206.__A living Crinoid (Pentacrinus asteria); one-fourth natural  size.  West
                                Indian Seas. 
250             COMPARATIVE ZOOLOGY.


                 Class II.—Asteroidea.
  Ordinary Star-fishes  consist of a flat central  disk, with
five or  more arms,  or  lobes, radiating from it, and con-
taining  branches of  the viscera.  The skeleton is leathery,
hardened by small  calcareous plates (11,000 by  calcula-
tion), but somewhat flexible.  The mouth is below; and
the rays are furrowed  underneath, and pierced with nu-
merous  holes, through  which pass the sucker-like  tenta-
cles—the organs of  locomotion and prehension.   The red
spots  at the ends of the rays  are supposed to be eyes.  The
usual color of Star-fishes is yellow, orange, or red.  They
Pig. 207.—Under-surface of Star-fish (Goniaster reticulatus), showing ambulacral
                 grooves aud protruded suckers. 
ECHINODERMATA.
251
Fig. 208.—Ophiocoma Russei, an Ophiura; natural size. West Indies.
abound on every shore, and are often seen at low tide half
buried in the sand, or slowly gliding over the rocks.  Cold
fresh water is instant death to them.  They have the pow-
er of reproducing lost parts to a high degree.  They are
very voracious, and are the worst enemies of the Oyster.
  About 150  species are known.   These may be divided
into three groups: (1) species having four rows of.feet,
represented by the common five-fingered Asterias; (2)
species having two rows of feet, as the many-rayed Solas-
ter, or "Sun-fish,"  and the  pentagonal Goniaster;  (3)
species having long slender arms,  which are not prolon-
gations of the body, and are not provided with  suckers,
as the  Ophiura, or  " Brittle - star," and Astrophyton, or
" Basket-fish."  The last are of inferior  rank, and resem-
ble inverted stemless Crinoids.  The digestive sac is con-
fined to the disk; and the madreporic plate is underneath. 
252             COMPARATIVE ZOOLOGY.
                 Class III.—Echinoidea.
  The Sea-urchin is encased in a thin hollow shell cov-
ered with spines, and varying in shape from a sphere to
a disk.147  The mouth is underneath, and contains a dental
apparatus more  complicated than that of any other creat-
ure from the Sponge to Man.  It leads to a digestive tube,
which extends spirally to the summit of the  body.  The
spines are for burrowing and locomotion, and are  moved
                                      by small muscles,
                                      each being  articu-
                                      lated by ball-and-
                                      socket  joint to a
                                      distinct  tubercle.
                                      When stripped  of
                                      its spines, the shell
                                   ^ (or " test") is seen
                                      to be formed of a
                                      multitude of pen-
                                      tagonal plates, fit-
                                      ted  together  like
                                      a mosaic.148  Five
                                      double  rows   of
                                      plates,     passing
                                      from pole to pole,
like the ribs of  a melon, alternate with five  other  double
rows.  In  one set, called the ambulacra, the plates  are
perforated for the  protrusion of tubular feet, or suckers,
as in the Star-fish.   So that altogether there  are twenty
series  of plates — ten ambulacral, and ten  interambula-
cral.  The shell  is not cast, but grows by the enlargement
of each individual plate, and  the addition of new ones
around the  mouth and the opposite  pole.  Every part of
an  Echinus, even sections of the spines, show the  princi-
ple of radiation.  If the arms  of a Star-fish were  turned
Fig. 209.—Under-surface of a Sea-urchin (Echinus escu-
  lentus), showing rows of suckers among the spines.
  British seas. 
ECHINODERMATA.
253
backward so as to meet, we would have a very close imi-
tation of a  Sea-urchin, the ventral surface  correspond in «■
to the ambulacral areas.  Echini live near the shore,  in
rocky holes or under sea-weed.   They are less active than
Star-fishes, but, like them, feed on Shells and Crabs.  They
reproduce by minute red eggs.
  Regular Echini, as  the common Cidaris,  are  nearly
globular,  and the oral and  anal openings are opposite.
Irregular Echini, as the Clypeaster, are flat, and the anal
orifice is near the margin.

               Class  IV.—Holothuroidea.
  These worm-like " Sea-slugs," as they are called, have a
soft elongated body, with a tough contractile skin contain-
ing scattered granules.  One end, the head, is abruptly
terminated,  and  has a simple aperture  for a mouth, encir-
cled with feathery tentacles.   There are five longitudinal
Fig. 210.—Sea-slugs (Holothuria).
rows of ambulacral  suckers, but only one is used for lo-
comotion.  The mouth opens into a pharynx leading to
a long intestinal canal.   Holothurians have the singular
power of  ejecting all their internal organs, surviving for
some  time the loss of these essential parts, and afterward
reproducing them.   They  occur  on nearly every  coast, 
254
COMPARATIVE ZOOLOGY.
especially in tropical waters, where they sometimes attain
the length of three or four feet.  As found on the beach
after  a storm, or when the tide is out, they are leathery
lumps, of a reddish,  brownish, or yellowish color.  They
may be likened to a Sea-urchin devoid  of a shell,  and
long drawn out, with the  axis horizontal, instead  of  ver-
tical.

               Subkingdom Mollusca.
  A Mollusk is a  soft-bodied animal, without  internal
skeleton, and without joints, covered with  a moist, sensi-
tive, contractile skin, which loosely envelops the creature,
like a mantle.  In some cases the skin is naked ; but gen-
erally it is protected by a calcareous covering (shell). The
length of the body is less in proportion to its bulk than in
other animals.  The  lower classes have no distinct head.
The nervous system consists, in the true Mollusks, of three
well-developed pairs of ganglia, which are principally con-
centrated  around the entrance to the alimentary  canal,
forming a ring around the  throat.  The other ganglia  are,
in most cases, scattered irregularly through the body,  and
in such the body is unsymmetrical.   The digestive  system
is greatly  developed, especially the liver, as in most aquat-
ic animals.   Except in the Cephalopods, the muscles are
attached to the  skin.  Only the higher Mollusks  have a
distinct heart (auricle and  ventricle), and this is always on
the arterial side.  While in neighboring groups, as Fora-
minifers, Corals, Star-fishes, and Articulates, we find repe-
titions of similar parts, in Mollusks every part or organ is
single.  The total number of living species probably ex-
ceeds 20,000.  The  great majority are water - breathers,
and marine; some  are fluviatile or lacustrine;  and a  few
are terrestrial air-breathers.  All bivalves,  and nearly all
univalves, are aquatic  Each zone of depth in the sea has
its particular species. 
MOLLUSCA.
255
  There are six classes of Mollusks.  Of these, the first
three form a group  by themselves, called Molluscoidea,
distinguished by having only one or two nervous ganglia,
an imperfect circulatory apparatus, and generally no or-
gans for prehension and locomotion.  Of the other three,
Lamellibranchs are headless, while Gasteropods and Ceph-
alopods only have a distinct head, and an apparatus for
mastication.

                   Class  I.—Polyzoa.
  These minute Mollusks resemble the Polyps in appear-
ance, living in clusters, each individual inhabiting a del-
icate cell, or tube, and having a simple mouth surrounded
with ciliated tentacles.  The colony often takes a plant-
like  form; sometimes  spreads,  like fairy-chains or lace-
work, over other bodies; or covers rocks and sea-weeds in
Fig. 211.—Polyzoans: 1. Hornera lichenoides; natural size. 2. Branch of the same,
           magnified.  3. Dtscopora Skenei; greatly enlarged.
patches with a delicate film.   The majority secrete carbo-
nate of lime.  Though an extremely low form of the Mol-
luscan type, a Polyzoan shows its superiority to the Coral,
which it imitates, in possessing a distinct alimentary canal 
256
COMPARATIVE ZOOLOGY.
and a well-defined nervous system.   The cells of a group
never have connection with a common tube, as in Coelen-
terates.   There are both marine and fresh-water species.

                  Class II.—Tunicata.
  The Tunicates are  either single or compound, and  are
found in all seas.   The most common form (the  solitary
Ascidians) are inclosed in a leathery, elastic bag, one end
of which is fastened to the rocks, while the other has two
orifices, for the inlet and exit of a current of  water for
nutrition and  respiration.  They are without head, feet,
arms, or shell.   Indeed, few animals seem more helpless
                             and apathetic than these ap-
                             parently  shapeless  beings.
                             The tubular heart exhibits
                             the curious phenomenon of
                             reversing its action  at brief
                             intervals, so that  the blood
                             oscillates backward and for-
                             ward in the  same vessels, aa
                             was supposed to be the case
      fig. 212—An Ascidian.        'm  the human system before
the time of Harvey.  Another peculiarity is the presence
of cellulose in the skin.  While the Ascidians are  fixed
and  single, the Salpians are  free, and alternately single
and  social.   They are usually seen swimming in long
chains (the offspring of one individual).   Each member of
this colony produces solitary young, which are unlike their
parents, and these again give birth to aggregated forms.

                Class III.—Brachiopoda.
  These Mollusks have a bivalve shell, the valves being
applied to the dorsal  and ventral sides of the body.   The
valves are unequal, the ventral being usually larger, and
more convex; but  they are  symmetrical,  i. e., a  vertical
 
MOLLUSCA.
257
line  let fall  from  the hinge  divides the  shell into  two
equal parts.   The  ventral valve has, in the great major-
ity, a prominent beak, perforated  by a foramen,  or hole,
through which a fleshy foot protrudes  to  attach  the  ani-
mal to submarine rocks.  The valves are opened and shut
by means of muscles, and in some cases they are hinged,
having  teeth  and  sockets
near the beak.  The mouth
faces the  middle   of the
Fig. 214.—Dorsal Valve of a Brachiopod
  (Terebratula), showing, in descending
  order, cardinal process, dental sockets,
  hinge-plate, septum, and loop support-
  ing the ciliated arms.
 Fig. 213.—A Brachiopod (Terebratulina
   septentrionalis).  Atlantic coast.
 margin  opposite the beak;
 and on  either side of  it is
 a long fringed " arm,"  gen-
 erally coiled up, and  sup-
 ported  by a bony frame-work.   The  animal,  having no
 gills, respires either  by the arms  or the mantle.   Brachi-
 opods were once very abundant, over 2000 extinct species
 having  been described; but less  than  a hundred species
 are now living.1-49  They are all marine, and fixed; but of
 all Mollusks, they enjoy the greatest range of climate and
 depth.

              Class  IV.—Lamellibranchiata.
  Lamellibranchs are all ordinary bivalves,  as the Oyster
and  Clam.   The shells differ from  those  of Brachiopods
in being placed on the right and left sides of the body, so
that the hinge is on the back of the animal, and in being
nnequilateral and equivalved.160  The umbo answers to the
                           17 
258
COMPARATIVE ZOOLOGY.
Fig. 215. — Pearl Oyster
 (Meleagrina margariti-
 fera); one-fourth nat-
 ural size.  Ceylon.
a hinge
beak, but it is not perforated.  It is the point from which
the growth of the valve  commences.  Both Brachiopods
and  Lamellibranchs are headless; but  in  the  latter, the
                  mouth points the same way as the umbo,
                  i. e., toward the  anterior part.   The
                  length of the shell is measured from its
                  anterior to its  posterior margin, and its
                  breadth from the dorsal side where the
                  hinge is to the opposite, or ventral, edge.
                  The valves are united to the  animal by
                  one muscle  (as in the Oyster), or two
                  (as in the Clam), and to  each other by
           In some species, as the Fresh-water  Mussel,
the hinge is simply an elastic ligament,
passing on  the outside from one valve
to the other  just  behind the beak, so
that it is on the stretch when the valves
are closed, and another placed between
the edges  of  the  valves,  so that it is
squeezed as they shut, like the spring in
a watch-case.   Such bivalves are said to
be edentulous.  But in the majority, as
the Clam,  the valves also articulate by
interlocking parts called  teeth.    The
valves are, therefore, opened  by  the
ligaments, and closed  by the muscles.
The margin of the shell on which the ligament and teeth
are situated is termed  the hinge-line.
  Lamellibranchs breathe by four plate-like gills (whence
the name), two on each side underneath the mantle (Fig.
78).   In the higher forms,  the  mantle is  rolled  up into
two tubes, or siphons, for the inhalation and exhalation of
water.  They  feed on infusorial particles filtered from the
water.  A few are fixed, the Oyster, e. g., habitually lying
on its left valve, and the Salt-water Mussel hanging to the
Fig. 216.— Salt-water
 Mussel (Mytilus pel-
 lucidus).   Atlantic
 coasts. 
MOLLUSCA.
259
rocks by a cord of threads called " byssus;" but the rest
have  a "foot,"  by which  they creep about.  Unlike  the
Oyster, also, the majority live in an erect position, resting
         Fig. 217__Lamellibranch (Mactra): a, foot; b, c, siphous.

on the edges of their shells.  Over 4000 living species are
known.   These  are  fresh-water and  marine, and range
from the shore to a depth of a thousand feet.
  The chief characters for distinguishing Lamellibranchs
are the muscular impressions,161 whether one  or two; the
presence of a pallial sinus, which indicates the possession
of siphons; the structure of the hinge; and the symmetry
of the valves.
  The following are the leading types  of structure:
  1. Mantle open; no siphon-tubes, and therefore no pal-
lial sinus; one muscular impression; foot wanting, or very
small; shell unequivalve and edentulous:  as  the Oyster
(Ostrea), Scallop (Pecten), and Pearl
Oyster (Avicula).1™
  2. Mantle open infront,and closed
behind, except one small aperture;
no pallial sinus; two muscular im-
pressions;  foot large; shell equi-
valve: as the Sea Mussel (Mytilus),
and Fresh-water Clam (Unio).
  3. Mantle having three openings; FiG.2is.-cockie(Cardi,«mco»-
                 °      i           tatum); one - third natural
tubes wanting,  or very  short;  no   size, china seas.
 
260
COMPARATIVE ZOOLOGY.
pallial sinus; two muscular impressions: as " the giant of
the bivalve race" (Tridacne) and Cockle (Cardium).163
  4. Mantle, with three openings; siphons large;  pallial
sinus and muscular impressions well marked: as the com-
mon Clam  (Venus),16*  and  the  burrowing Razor-shell
(Solen).

                Class  V.— Gasteropoda.
  The Snails are,  with rare exceptions, all  univalves.'65
The body is coiled  up in a conical shell, which is usually
spiral, the whorls passing obliquely (and generally from
right to  left)168  around a  central  axis, or  "columella."
When  the columella is hollow (perforated), the  end is
called  the  " umbilicus."   When  the whorls  are  coiled
around the  axis in the same plane, we have a discoidal
Fia. 219.—Whelk (Buccinum), showing operculum, o, and siphon, «.
shell, as the Planorbis.  The mouth, or "aperture,"  of
the shell is " entire " in most vegetable-feeding Snails, and
notched  or  produced into a canal for the siphons in the
carnivorous species.   The former are generally land and
fresh-water forms,  and the latter all marine.   In some
Gasteropods, as the River - snails,  a  horny or calcareous 
MOLLUSCA.
261
plate (operculum) is secreted on the foot, which closes the
aperture when the  animal withdraws into its shell.  In
locomotion, the shell is carried with the apex directed
backward.
  The body of most Gasteropods is nnsymmetrical, the
organs not  being  in  pairs, but single, and  on one  side,
instead of  central.  The mantle is continuous round the
body, not bilobed, as in Lamellibranchs.   A few, as the
common Garden-snail, have a lung; but the vast majority
breathe by gills.  The  head is more or less distinct, and
provided with two tentacles, with auditory sacs at their
bases;  two eyes, which are often on stalks; and a strap-
like tongue covered with minute teeth.  The heart is situ-
ated, in the majority, on the right side of the back.  All,
except the  Pteropods, move by means of a ventral disk, or
foot.
  Gasteropods are now the reigning Mollusks, comprising
three-fourths of all  the living species, and are  the types of
the subkingdom.  They have  an extraordinary  range in
latitude, altitude, and depth.
  Omitting a few rare and aberrant forms,167 we may sep-
arate the class into the following orders:
  1. Pteropods.—These are small, marine, floating Mol-
lusks, whose main  organs  of motion resemble a pair of
wings or fins coming out of the neck,
whence the common name, " Sea-but-
terflies." Many have a delicate, trans-
parent shell.   The head has six ap-
pendages,  armed with  several hun-
dred thousand microscopic suckers—
a prehensile apparatus  unequaled in FlG 220._a pteropod (fly-
complication.   Pteropods  OCCUr in  aUa tridentata).  Atlantic.
every latitude, but generally in mid-ocean, and  in the
arctic regions are the food of Whales and Sea-birds.
  2. Opisthobranchs.—These low Gasteropods are, for the
 
262
COMPARATIVE ZOOLOGY.
most part, naked  Sea-slugs, a  few only having  a small
shell.   The feathery  gills  are behind the heart (whence
the name).  They are found in all seas from the arctic to
the torrid, generally on rocky coasts.
Fig. 221___A Tritonian (Dendronotm arborescens).      three-fourths natural
             British seas.                   size. Indian Ocean.
or tough skin.  Examples: Sea-lemon (Doris), the beauti
ful Tritonia, the  painted jEolis, the Sea-hare (Aplysia),
which  discharges  a purple fluid, and the Bubble - shell
(Bulla).
   3. Pulmonates.—These air-breathing Gasteropods, rep-
resented by the familiar Snail, have  the simplest form of
lung — a cavity lined with a delicate net-work of  blood-
vessels, which  opens  externally on the right side of the
neck.  This entrance is closed by a valve, to  shut out the
water in the aquatic tribes, and the hot, dry air of summer
days in the land  species.   They  are  all fond of  moisture,
and are more or less slimy.  Their shells are lighter (being
thinner, and containing  less earthy matter) than those of
marine Mollusks,  having to be carried on the back with-
out the support of the water.   Their eggs are laid singly;
while the eggs  of  other orders are laid in chains.
   They are found in all zones, but most numerous where
lime and moisture abound.  All  feed on vegetable matter.
A few are  naked,  as the Slug; some are terrestrial; others
live in fresh water.   The Land-snails, represented by the
common Helix, the gigantic Bulimus, and the Slug (Li-
max),  are  distinguished  by their four "horns,"  the short 
MOLLUSCA.
263
front pair being the true tentacles, and the  long hinder
pair being the eye-stumps.   They have a saw-like upper
 Fig. 223.—.4, Land-snail (Helix); B, C, D, Slugs (Umax); E, F, G, Pond-snails
                 (Limncea, Paludina, and Planorbis).

 jaw for biting leaves,  and a  short  tongue covered  with
 minute teeth.   The  Pond-snails, as Limnaia and Planor-
 bis, differ in having no eye-stalks, the
 eyes being at the base of the tentacles.
 They are obliged to  come frequently to
 the surface of the water to breathe.
   4. Prosobranchs.—These are aquatic
 Gasteropods, breathing by gills situated
 in  front of the heart.   They are  the
 most highly organized  and the  most
 abundant  of the crawling Mollusks.
 Nearly all  are  marine,  and all have a
 gng]l                                    Fig. 224.—Bulimus oblon-
                                         gus; one-half natural
  Among  the lower  forms are the sin-   8ize- Guiana.
gular  Chiton, covered with  eight shelly plates; Limpet
(Patella), well  known  to every sea-side visitor;  and the
 
264
COMPARATIVE ZOOLOGY.
Fig. 225.— Cowry (Cyprceacapensis); two-    Fig.  226. — Haliotis, or  "Pearly Ear-
   thirds natural size.  South Africa.               shell."  Pacific coasts.
Fig.  227. — Spindle-     Fig.  228. — Cassis rufa,  or     Fig. 229.—Auger-shell
  shell (Fusus colus);       " Helmet-shell;" one-fourth       (Terebra maculata);
  one - half  natural       natural size.  Indian Ocean.       one - half    natural
  size.  Ceylon.                                            size.  China seas.
Fig. 230.—Cone-shell (Conus    Fig. 231.—Chiton squa-    Fig. 232__Volute (Voluta
  marmoreus);  two-thirds      mosus; one-half natn-     musica);  one-half nat-
  natural size.  China seas.       ral size. West Indies.     ural size.  West Indies. 
MOLLUSCA.
265
Fig. 233.—Top-shell (Turbo marmo-
  ratus);  one - fourth natural size.
  China seas.
Fig. 234__Strombus gigas, or "Winged-
  shell;"  one-fifth  natural size.  West
  Indies.
Fig. 236. — Paludina,  a Fresh-water       Fio. 236.— Key-hole Limpet (Fissurella
               Snail.                              listeri).  West Indies.
Fig. 237.—Ear-shell (fl. tubercidata), and Dog-whelk (Nassa reticulata).  England. 
266
COMPARATIVE ZOOLOGY.
beautiful Ear-shell (Haliotis), frequently used for orna-
ments and inlaid-work.
  In the higher Prosobranchs, the gills are comb-shaped
and the sexes are distinct.  The group  includes all the
spiral univalve sea-shells, and a few fresh-water shells.
Many have the  aperture entire,  which is closed with  an
operculum:  as the dull-colored Paludina and Melania
from  fresh  water, and  the  pyramidal  Trochus,  pearly
Turbo,  screw-like Turritella, common  Periwinkle  (lit-
torina), and  globular Natica from the sea.  Others, the
highest of the race, have the margin of the aperture notch-
ed or produced into a canal, and are carnivorous and ma-
rine: such are nearly all the sea-shells, remarkable for
their  beautiful  forms,  enameled  surfaces, and brilliant
tints, as the Cowry (Cyprcea), Volute, Olive, Cone, Harp,
Whelk  (Buccinum),  Cameo-shell (Cassis), Rock-shell
(Murex), Trumpet-shell (Triton), Spindle-shell (Fusus),
and Wing-shell (Strombus).

                Class VI.—Cephalopoda.
  The Cephalopods stand at the head of the subkingdom,
some of them outranking the highest Articulates; but they
are not so typical as the Gasteropods.   The head is set off
from the body by a slight constriction, and furnished with
a pair of  large, staring  eyes, a mouth armed with a  rasp-
ing tongue and a parrot-like beak, and eight or more ten-
tacles or arms.  The body is symmetrical, and wrapped in
a muscular mantle.
  The nervous system is more concentrated than in  other
Invertebrates; the  cerebral ganglia are  even inclosed in
a cartilaginous cranium.  All the five senses are present.
The class is entirely marine (breathing by plume-like gills
on  the  sides of the body), and carnivorous.  The naked
species  are  found in every sea.   Those with chambered
shells (as Nautilus, Ammonite, and Orthoceras) were once 
MOLLUSCA.
267
very abundant:  more than 2000 fossil species are known,
but only one living representative—the Pearly Nautilus.
  1. Tetrabranchs. — This order is characterized  by the
possession of four gills, forty or more short tentacles, and
an external, chambered shell.  The partitions, or septa, of
the shell are united by a tube called " siphuncle," and the
animal lives in the last and largest chamber.168  The living
Nautilus has a smooth, pearly shell, a head retractile with-
in the mantle or " hood,"  and  calcareous mandibles well
fitted for masticating Crabs, on which it feeds.  This strag-
Fig. 238.—Pearly Nautilus, with shell bisected; one-half natural size. Indian Ocean.
gler of a mighty race dwells in the  deep parts of the
Indian Ocean, crawling  on the bottom; and while the
shell is well known, only two or three specimens of the
animal have ever been obtained.
   2. Dibranchs.—These are the  most active of Mollusks,
and the  tyrants  of  the lower tribes.  Among them are
the largest  of invertebrate animals.   They  are naked,
having  no external shell covering  the body, but  usual-
ly  a horny  or  calcareous part  within.   They have  a
distinct head, prominent eyes, horny mandibles,  eight or
ten  arms furnished with suckers,169 two gills,  a  com- 
268
COMPARATIVE ZOOLOGY.
                             plete tubular funnel, and an
                             ink - bag  containing  a  pe-
                             culiar  fluid  (sepia), of  in-
                             tense blackness, with which
                             the water is darkened to fa-
                             cilitate  escape.   They have
                             the power of changing color,
                             like the  Chameleon.   They
                             crawl  with  their  arms  on
                             the bottom of the  sea, head
                             downward,  and  also  swim
                             backward or forward, usual-
                             ly  with the  back downward,
                             by means of fins,  or squirt
                             themselves backward by for-
                             cing water forward through
                             their breathing funnels.
                               The  Paper Nautilus (Ar-
                             gonauta) and the Poulpe (Oc-
                             topus) have eight arms.  The
female  Argonaut secretes a  thin, unchambered shell  for
Fig. 239.—Cuttle - fish (Sepia officinalis);
 one-fifth natural size.  Atlantic coasts.
           2                                 3
Fig. 240. —Paper Nautilus (Argonauta argo): 1, swimming toward a by ejecting
   water from funnel, b; 2, crawling on the bottom; 3, coiled within its shell,
   which is one-fourth natural size.  Mediterranean. 
ARTICULATA.
269
carrying its eggs.   The Squid (loligo)  and Cuttle-fish
(Sepia) have ten arms, the additional pair being much
longer than the others.   Their eyes  are movable, while
those of the Argonaut and Poulpe are fixed.  The Squid,
so much used for bait  by cod-fishermen, has an  internal
horny  " pen,"  and  the  Cuttle  has  a spongy - calcareous
" bone."  The extinct Belemnite had  a similar structure.

              Subkingdom Articulata.
  This is larger than all the other subkingdoms put to-
gether, as it includes the jointed animals, such as Worms,
Crabs, and  Insects.  These  differ widely from the Mol-
luscan type in having a  symmetrical form, and in showing
a repetition of similar parts, not only in the shelly exterior,
but equally among the internal  organs.
  The skeleton is outside, and  consists of articulated seg-
ments  or rings.   The limbs, when present, are likewise
jointed and hollow.  The jaws move from side to side.
The nervous system consists mainly of a double chain of
ganglia running along  the  ventral surface of the  body
under  the alimentary canal.   The brain is in the form of
a ring encircling the gullet.  The  alimentary canal and
the circulatory apparatus are nearly  straight tubes  lying
lengthwise — the one through  the  centre, and the  other
along the back.   There is a remarkable correspondence,
especially in the lower  forms,  between the joints of  the
body and the ganglia of the nervous cord, the respiratory
organs and the chambers of the dorsal heart.  Each ring
of a Worm has  a complete circulatory,  respiratory, and
nervous apparatus."0
  As we advance from the lowest  forms (Worms),  in
which  the  body is elongated,  the rings numerous,  the
skin soft, and  the legs  imperfect, we shall find the  rings
fewer,  the  skin firmer, and the  legs more elaborately
formed, as in the Centipede; till in the Bee, Spider, and 
270
COMPARATIVE ZOOLOGY.
Crab, we see an increasing consolidation of the exoskele-
ton, and more  perfect limbs, and a  tendency to concen-
trate the body in front, and thin out behind.  Along with
these progressive steps, we notice the muscular powers be-
coming more energetic, and the nervous ganglia condens-
ing into larger masses.
  The subkingdom is divided into five classes: the aquat-
ic Worms and Crustaceans, and the air-breathing Spiders,
Myriapods, and Insects.161  The various forms of articulate
life arise from the unequal development of  the body-seg-
ments  and variation in the  number  and form of append-
ages.
                   Class I.—Annelida.
  The  Annelids,  or Worms, are distinguished  from  all
other  Articulates by  the absence of hollow articulated
Fig. 241.—Marine Worm (Cirratulus grandis), with extended cirri. Atlantic. 
ARTICULATA.
271
limbs,  and by  the  fact that no one  part of the body is
highly honored above the rest.   The  body is soft, and
composed of  a  succession of rings (from 20 to 500), which
are repetitions  of each other.   The  first segment, called
the head, differs little from the rest.  The legs, when ex-
isting,  are merely  bunches of  short, stiff bristles (setce),
each terminating in a hook or blade.  Many of the sea-
worms have tentacles.  Touch is probably the  only sense.
d, segments of the body.                magnified.
The blood is often reddish,  but seldom contains corpus-
cles.  Strange to say, the circulatory apparatus is closed,
and more highly developed  than in Insects.   They are
mostly marine, and carnivorous.
  There are three representative orders:  1. The low and
abnormal Annuloida, which are without  joints or setae,
and are mainly parasitic, as the Tape-worm (Tcenia),
Hair-worm (Gordiacea), and Trichina.1™   2. The Abran- 
272
COMPARATIVE ZOOLOGY.
chiates, which  breathe  by the skin.  The majority are
fresh-water, as the Leech (Sanguisuga)  and Earth-worm
(lumbricus).   3. The Branchiates, whose organs of res-
piration are tuft-like gills on the back or head.  Such are
the sea-worms Arenicola, Nereis, and Serpula.
  The Rotifers,  or  " Wheel - animalcules,"  are  minute
aquatic Articulates, whose exact position  is doubtful, hav-
ing some of the features of both Worms  and Crustaceans.
They resemble Infusoria externally, but  have a complete
alimentary canal and well-developed nervous system.  At
the head is a disk furnished with cilia for locomotion and
prehension, and at the other end is a pair of "toes," which
act like forceps.  None are over -gV of an inch long.

                 Class II.—Crustacea.
  This class includes all Articulates having  jointed legs
and gills.163   Among them are the largest, strongest, and
most  voracious of the subkingdom, armed with powerful
claws and a hard cuirass bristling with spines. Although
constructed  on a common type, Crustaceans exhibit a
wonderful diversity of external form:  contrast, for exam-
ple, a Barnacle and  a Crab.   We will select  the Lobster
as illustrative of the  entire group.
  Every Crustacean  consists of twenty-one segments, of
which seven belong  to the head,  seven to the thorax, and
seven to  the abdomen.164  In the Lobster, however, as in
all  the higher  forms, the joints  of  the head and  thorax
are welded together into a single  crust, called  the cephalo-
thorax.  On the front of this shield is a pointed process,
or rostrum; and attached to the last joint of the  abdo-
men (the  so-called "tail") is the sole representative of a
tail—the telson.   This skeleton is  a  mixture of chitine
and calcareous matter,  and corresponds to our epiderm-
is.165
  On the under-side of  the body we find numerous ap- 
ARTICULATA.
273
pendages, feelers, jaws, claws, and legs beneath the ceph-
alo-thorax, and flat  swimmerets under  the abdomen.  In
fact, as a rule, every segment carries  a pair  of movable
appendages.   The seven  segments of  the head are com-
pressed into  a very small space, yet have  the following
members:  the eye-stalks; the short and long antennae;
the mandibles, or jaws,
between  which  the
month opens; the two
pairs of maxillae ; and
a  pair  of   modified
limbs,   called " foot-
jaws."     The thorax
carries two more pairs
of foot-jaws,  and five
pairs of  legs.    The
foremost  legs,   " the
great claws," are ex-
traordinarily    devel-
oped, and terminated
by   strong   pincers
(chelai).  Of  the  four
slender pairs succeed-
ing, two are  furnished
with claws,  and two
are pointed.   The last
pair of swimmerets, to-
gether with the telson,
form the  caudal fin—
the  main instrument  of locomotion;   the  others  (called
"false feet") are  used by the female for carrying her eggs.
The eyes  are raised on stalks so as  to  be movable (since
the head is fixed to  the thorax), and are compound, made
up  of  about  2500 square facets.   At the  base of each
small antenna is  a minute sac, whose mouth  is guarded by
                           18
Fio. 244.—Under-side of the Cray-fish, or Fresh-
 water Lobster (Astacus fluviatilis): a, first pair
 of antennae: b, second pair; c, eyes; d, audi-
 tory tubercles; e, foot-jaws ; /, g, first and fifth
 pair of thoracic legs; h, false abdominal feet; i,
 anus ; k, caudal fin. 
274
COMPARATIVE ZOOLOGY.
hairs: this is the organ of hearing.168   The gills, twenty on
a side, are situated at the bases of the legs and inclosed
in two  chambers, into  which  water  is freely  admitted,
in fact,  drawn by means of a curious valve at  the outlet
which works like  the " screw " of a propeller.  The heart
is a single oval cavity, and drives arterial blood—a dusky
fluid full of  corpuscles.  The alimentary canal consists of
a short gullet, a gizzard-like stomach, and a straight intes-
tine.
  Crustaceans  pass through a series of strange  metamor-
phoses before reaching their adult form.   They also peri-
odically cast the shell, or molt, every  part of the integu-
ment being renewed; and another remarkable endowment
is the spontaneous rejection of limbs  and their complete
restoration.  Many species are found in fresh water, but
the class is essentially marine and carnivorous.
  No natural classification of this varied group has been
discovered.  It will be  convenient to divide it into four
orders:
  1. Cirripeds, distinguished by being fixed, by having  a
shelly covering, and by their feathery  arms (cirri).  Such
are Barnacles (Lepas) and Acorn-shells (Balanus), so com-
mon on rocks and timbers by the sea-shore.
  2. Entomostracans, which agree in having a horny shell
and no abdominal limbs; represented  by the little Water-
fleas (Cyclops) of our ponds, the King-crabs (Limulus),
abounding on  every sea-coast, and the extinct Trilobites.
The abdomen of the King-crab is  reduced to a mere
spine, the appendages about the mouth are used for loco-
motion, and  the eyes are smooth.
  3. Tetradecapods, small, fourteen-footed species; as the
Wood-louse, or Sow-bug (Oniscus), so common in damp
places, and  the Sand-flea  (Gammarus), seen by the  sea-
side in summer.
  4. Decapods, having  ten  legs, as  the  Shrimp  (Cran- 
Fig. 245.—Barnacles, or Pedunculate Cirripedes (Lepas anatifera).
gon), Cray-fish and Lobster (Astacus), and Crab (Cancer).
Crabs differ from Lobsters chiefly  in being formed for
creeping at the  bottom of the sea instead of swimming,
and in the reduction of the abdomen or " tail" to a mere
Fig. 246.—Acorn-shells (Balanus) on   Fig. 247—Water-fleas: 1, Cyclops communis;
 the Shell of a Whelk (Buccinum).      2, Cypris unifasciata; 3, Daphnia pulex.
 
276
COMPARATIVE ZOOLOGY.
rudiment  which folds into a groove under the enormous
thorax.  They are the highest and largest of Crustaceans:
they have been found  at Japan measuring ten feet be-
tween the tips of the claws.
Fig. 248.—Lobster (Homarus Americanus).
Fig. 249__Swimming Crab, 
ARTICULATA.
277
                 Class III.—Arachnida.
  The Arachnids are  closely related to the Crustaceans,
having the body  divided into a cephalo-thorax and abdo-
men.167   To  the  former are attached eight legs  of  sev-
en joints each; the latter has no locomotive appendages.
The head carries  two, six, or eight eyes, smooth and sessile
(i. e., not faceted and  stalked,  as in the Lobster), and ap-
proaching the eye of the Vertebrates in the completeness
and perfection of their apparatus.  The antennae, if pres-
ent, are only two, and these  are not " feelers," but modi-
fied to serve for the prehension of food.  They are all air-
breathers, having spiracles which open either into air-sacs
or tracheae.   The young of the higher forms undergo no
metamorphosis after leaving the egg.
  Arachnids number  nearly 5000  species.  The typical
forms are divided into three groups :
  1. Acarina, represented by the Mites and Ticks.  They
have an  oval or rounded body, without any marked artic-
ulations,  the head, thorax, and  abdomen being apparently
merged into one.   They have no brain, only a single  gan-
glion lodged in the abdomen.
They  breathe  by  tracheae.
The mouth  is  formed  for
suction, and  thev  are  gener-  _   OK.   .  „.t /7.
               *      °      Fig. 250. — A Mite (Demodex folliculo-
ally  parasitic.    The  Mites   rum), one of the lowest Arachnids;
  J                           a parasite on human hair; X 125.
(Acarus) are among the low-
est of  Articulates.   The  body is soft and minute.   The
Ticks (Ixodes)  have a leathery skin,  and are sometimes
half  an inch long.   The  mouth is furnished with a beak
for piercing the animal it  infests.
  2.  Pedipalpi, or Scorpions, characterized by very large
maxillary palpi ending in forceps, and a prolonged, joint-
ed abdomen.   The nervous  and  circulatory  systems  are
more highly organized than  those  of Spiders; but  the 
278
COMPARATIVE ZOOLOGY.
long, tail-like abdomen and the abnormal jaws place them
in a lower rank.  The abdomen consists  of twelve seg-
ments : the anterior half is as large as the  thorax, with no
well-marked division between; the other part is compara-
tively slender, and ends in a hooked sting, which is perfo-
rated by a tube leading to a poison-sac.  The antennae are
transformed into small, nipping claws, and the eyes gen-
Fig. 251__Scorpion (under-surface) and Centipede.
erally number six.  Respiration is carried on by four pairs
of pulmonary sacs which open on the under-surface of
the abdomen.  The heart is a  strong artery, extending
along the middle of the back, and divided into eight sepa-
rate chambers.  Scorpions are confined to the warm-tem-
perate and tropical regions, usually lurking in dark, damp
places.
  The Harvest-men (Phalangium), frequently seen about
our houses, belong to this order.  They have a short, thick
body and extremely long legs, and breathe by tracheae.
  3. Araneina, or  Spiders.   They are  distinguished by 
ARTICULATA.
279
their  soft, unjointed abdomen, separated from the thorax
by a  narrow constriction, and provided at  the  posterior
end with two or three pairs of appendages,  called " spin-
nerets," which  are  considered  homologous with  the legs.
The office of the spinnerets is to reel out the silk from the
silk-glands, the tip  being perforated by  a myriad of little
Fig. 252.—A, female Spider; B, male of same species; C, arrangement of the eyes.
tubes, through which the silk escapes  in excessively fine
threads.   An  ordinary thread, just visible to the naked
eye, is the union of a thousand or more of these delicate
streams of silk.168   These primary threads are drawn out
and united by the hind legs.
  The mandibles are vertical, and end in a powerful hook,
in the  end of which opens  a  duct from a poison-gland in
the head.  The maxillae, or " palpi," which in Scorpions
are changed to formidable  claws, in Spiders resemble the
thoracic feet, and are often  mistaken for a fifth pair.  The 
280
COMPARATIVE ZOOLOGY.
brain is of larger size, and the whole nervous system more
concentrated than in the preceding order.  There are gen-
   i    i    erally eight simple eyes, rarely six.   They
             breathe both by tracheae and lung-like sacs,
             from two to four  in number, situated under
             the abdomen.   All the species  are carnivo-
             rous.
fig. 253.—spinner-    Tlie instincts of Spiders are of a  high
 M^Sform order.  They  are, perhaps, the most  wily
 organs.       0£ Articulates.   They  display remarkable
skill and industry in the construction of their webs; and
some species (called " Mason Spiders")  even excavate a
subterranean pit, line  it with  their silken tapestry, and
close the entrance with a lid which moves upon a hinge.168

                Class IV.—Myriapoda.
  Myriapods differ from  Crustaceans and Spiders in hav-
ing the thorax merged in the  abdomen, while the  head
is free.  In  other words, the body is divided into similar
segments, so that thorax and abdomen are scarcely distin-
guishable.   They  resemble  Worms in  form and  in the
simplicity of their nervous  and circulatory systems; but
the skin is stiffened with chitine, and the' legs  (indefinite
in number)  are  articulated.   The legs resemble those of
Insects, and the  head appendages follow each other in the
same order as in Insects—eyes,  antennae, mandibles,  max-
illae, and palpi.  They breathe  by tracheae, and have two
antennae and a variable number of eyes.
  There are two orders:
  1. Chilognatha,h&v'mg a cylindrical body,each segment
furnished with two pairs of  legs.  They  are of slow loco-
motion, harmless, and* vegetarian.   The  Thousand-legged
Worm (Julus) is a common  representative.
   2. Chilopoda, characterized by having a flattened  body
composed of about twenty  segments, each carrying one
 
ARTICULATA.
281
pair of legs, of which the hindermost is converted into
spines.  They have longer antennae than the preceding,
and the mouth is armed with two formidable fangs con-
nected with poisonous glands.   They are carnivorous and
active.  Such is the Centipede (Scolopendra).

                  Class V.—Insecta.
  Insects  are distinguished by  having head, thorax, and
abdomen distinct, three pairs  of jointed legs, one pair of
antennae, and generally two pairs of wings.   The number
of-segments in the body never  exceeds twenty. The head,
apparently one, is  formed by  the  union of seven  pieces.
The thorax consists of three,  the prothorax, mesothorax,
and metathorax, each bearing  a pair of legs;  the wings, if
present, originate from the last two segments.  The abdo-
men is normally composed of  nine segments, more or less
movable upon  one another.  The skin  is hardened with
chitine, and to it, as in all Articulates, the muscles are  at-
tached.   The organs of sense  are confined to the cephalic
division of the body, the motor organs to the thoracic, and
the vegetative to the abdominal.  All the appendages are
hollow.
  The antennae are  inserted between  or in  front of the
eyes.  There is a great variety of forms, but all are tubu-
lar and jointed.  They are supposed to be organs of touch,
and also seem to  be sensitive to sound.  The eyes are
usually compound, composed of a large number of hexago-
nal corneas, or facets (from fifty  in the Ant to many thou-
sands in the  winged  Insects).  They are never  placed  on
movable pillars as  the Lobster's.   Besides these, there are
three simple eyes, called ocelli.   The  mouth may be fit-
ted for biting (masticatory), as in Beetles,  or for sucking
(suctorial), as in Butterflies. The masticatory type, which
is the more complete, and of which the other is but a mod-
ification, consists of four  horny jaws (mandibles and max- 
282
COMPARATIVE ZOOLOGY.
L-*-
illai) and an upper and an under lip {labrum and labium).
                                      Sensitive palpi (max-
                                      illary and labial) are
                                      developed  from  the
                                      lower jaw and lower
                                      lip.   The  labium  is
                                      also prolonged into a
                                      ligula, or tongue.
                                        The  legs  are in-
                                      variably  six  in  the
                                      adult, the  fore-legs
                                      directed forward and
                                      the hinder pairs back-
                                      ward.  Each consists
                                      of a hip, thigh, shank,
                                      and foot.170  The lar-
                                      vae have also "false
                                      legs," without joints,
                                      on the abdomen,upon
                                      which  they  chiefly
                                      rely  in  locomotion.
                                      The  wings  are  ex-
                                      pansions of the crust
                                      stretched over a  net-
                                      work of horny tubes.
                                      The venation, or ar-
                                      rangement  of these
                                      tubes  (called veins
                                      and veinlets), partic-
                                      ularly  in  the  fore-
                                      wings,  is peculiar  in
                                      each genus. In many
                                      Insects,    especially
                                      Hymenopters, the ab-
domen of the female ends  in a tube  which is the sheath
Fig. 254.—Under-surface of a Beetle (Harpalus cali-
 ginosus): a, ligula;  b, paraglossae; c, supports of
 labial palpi; d, labial palpus ; e, mentum; /, in-
 ner lobe of maxilla; g, outer lobe; h, maxillary
 palpus; i, mandible;  k, buccal opening; I, gula,
 or throat; m, buccal sutures ; n, gular suture; o,
 presternum; p, episternum of prothorax; p\ epi-
 meron ; q, q', q", coxae; r, r', r", trochanters; 8,
 8', s", femora, or thighs;. t, V, t", tibae; v, ventral
 abdominal segments; w, episterna of mesothorax;
 x, mesosternum; y, episterna of metathorax; y',
 epimeron;  z, metasternnm. 
ARTICULATA.
283
of a sting, as in the Bee, or of an ovipositor, or " borer,"
as in the Ichneumon, by means of which the eggs are de-
posited in suitable places.
   Cephalization is carried to  its maximum in this class,
and we have animals of the  highest instincts under the
articulate type.   The "brain" is formed of several gan-
glia massed together, and lies across the upper side of the
throat just behind the mouth.   The main cord, which lies
along the ventral side of the body, with a swelling for
each segment, corresponds  to the sympathetic system of
Vertebrates.  A  true brain and spinal cord are unrepre-
sented among invertebrated animals.   The digestive ap-
paratus consists of a pharynx, gullet (to which a crop is
added  in the Fly, Butterfly, and Bee tribes), gizzard, stom-
ach, and  intestine.   There are no absorbent  vessels, the
chyme simply transuding through the walls of the  canal.
The blood, usually a colorless liquid, is driven by a chain
of hearts along the back, /. e., by a pulsating tube divided
into valvular  sacs, ordinarily eight, which allow the cur-
rent to flow only toward the head.  As it leaves this main
pipe (aorta), it escapes into the cavities of the body, and
thus bathes all the organs.  Although the blood does not
circulate in a closed system of blood-vessels,  as in Verte-
brates, yet it always takes one set of  channels  in  going
from the heart, and another in  returning.  Respiration is
carried on by tracheae, a system of tubes opening at the
surface by a row of apertures  (spiracles), generally nine
on each  side of the body.
   The sexes are  distinct, and the larvae are hatched from
eggs.  As a rule, an Insect, after reaching the  adult, or
imago, state, lives from six months to a few hours, and dies
after the process of reproduction.   Growth takes place
only during larval life, and all metamorphoses occur then.
Among  the social tribes, as Bees  and Ants, the  majority
(called " workers") do not develop either sex. 
284
COMPARATIVE ZOOLOGY.
  Insects (the six-footed Articulates) comprise four-fifths
of the whole Animal Kingdom, or about 200,000 species.
They are grouped into seven orders:
Lower series:  body usually flattened;  prothorax large and 1 Neuroplers,
    squarish ;  mouth-parts usually adapted for biting; met-  Orthopters,
    arnorphosis incomplete;  pupa often inactive; larva flat-  Hemipters,
    tened, often resembling the adult.                  J Coleopters.
Higher series:  body usually cylindrical; prothorax small;
    mouth-parts more generally formed for sucking; meta-
    morphosis complete; pupa inactive; larva usually cylin-
    drical, very unlike the adult.

  1. Neuropters have a comparatively long, slender body,
and four large, transparent wings,  nearly equal in size,
membranous and  lace-like.  Such are the brilliant  Drag-
on-flies, or Devil's Darning-needles (libellula), well known
by  the enormous head and thorax, large,  prominent eyes
Dipters,
Lepidopters,
Hymenopters.
Fig. 255.—Dragon-fly (Libellula).
(each furnished with 12,000 polished  lenses), and Scor-
pion-like abdomen; the delicate and  short-lived May-flies
(Ephemera);  Caddis-flies (Phryganea), whose larvae  live
in a tnbular case made of minute stones, shells,  or  bits 
ARTICULATA.
285
of wood; the Horned  Corydalis (Corydalus),  of  which
the male has formidable mandibles twice as long as the
head; and the White Ants (Termes) of the tropics.
  2. Orthopters have four wings:  the  front pair some-
what thickened, narrow, and overlapping along  the back;
the hind pair broad, net-veined, and folding up like a fan
upon the abdomen.  The hind legs are usually large, and
fitted for leaping, all the species being terrestrial, although
some fly as well as leap.  The eyes are  small, the mouth
remarkably developed for cutting and grinding.  The lar-
vae and pupae are active, and resemble  the imago.   They
are  all vegetarian.   Each famfly produces characteristic
sounds (stridulation).  The representative forms are Crick-
ets (Gryllus), Locusts (locusta), Grasshoppers (Acrydium),
Walking-sticks (Phasmd), and Cockroaches (Blatta).
  3.  Ilemipters, or " Bugs," are chiefly characterized by
a suctorial mouth,  which is  produced  into  a long, hard 
286              COMPARATIVE ZOOLOGY.
     Fig. 257__Metamorphosis of an Hemipter, Water-boatman (Notonecta).


beak.   The four wings are irregularly and sparsely vein-
ed, sometimes wanting.  The body is flat above, and the
legs slender.  The larva differs from the imago in wanting
Fig. 258.—Seventeen-year Locust (Cicada septendecim): a, pupa;  6, the same, after
   the imago, c, has escaped through a rent in the hack; d, holes in a twig, where
   the eggs, e, are inserted. 
ARTICULATA.
287
wings.  In some species, the fore-wings are opaque at the
base,  and transparent at the apex, whence  the name of
the order.  Some feed on the juices of animals, others on
plants.  Here belong the wingless Bed-bug (Cimex) and
Louse (Pediculus), the Squash-bug (Coreus), Water-boat-
man (Notonecta), Seventeen-year Locust (Cicada), Cochi-
neal (Coccus), and Plant-lice (Aphis).
  4. Coleopters, or " Beetles."  This is the largest of the
orders, the species numbering about  90,000.   They are
easily recognized by the elytra, or thickened horny fore-
wings, which are not used  for flight,  but serve to cover
the hind pair.   When in repose, these elytra are  always
united by a straight  edge along the whole length.  The
hind wings, when not in use, are folded transversely.  The
mandibles are well developed, and the integument general-
ly is hard.  The legs are strong, for the Beetles are among
Fig. 259.—a, imago, and 6, larva, of the Goldsmith Beetle (Cotalpa lanigera); c,
               pupa of June-bug (Lachnosterna fusca).

the most powerful running Insects.  The larvae are worm-
like, and the pupa is motionless.  The highest  tribes  are
carnivorous.   The most prominent forms are the  savage
but  beautiful  Tiger Beetles (Cicindela);  the common
Ground  Beetles (Carabus), whose hind wings  are often
absent; the Diving  Beetles (Dytiscus), with boat-shaped
body, and hind legs changed into oars; the Carrion Bee-
tles (Silpha), distinguished by their black, flat bodies and 
288
COMPARATIVE ZOOLOGY.
Fig. 260.—Sexton Beetles (Necrophorus vespillo), with Inrva and nymph.  They are
         burying a mouse, preparatory to laying their eggs in it.
club-shaped antennae;  the Goliath  Beetles (Scarabaius),
the giants of the order;  the  Snapping-bugs (Elater); the
Lightning - bugs  (Pyrophorus);  the spotted Lady - birds
(Coccinella); the showy  Long-horned  Beetles (Cerambyc-
idce)',  and the destructive Weevils  (Curculionidce), with
pointed snouts.
  5. Dipters, or "Flies," are characterized by the rudi-
mentary state  of the hinder pair of wings.   Although
having, therefore, but one available pair, they are gifted
with the power of very rapid flight.   While a Bee moves
its wings 190 times  a second, and a  Butterfly 9 times, the 
ARTKTLATA.
289
House-fly makes 330 strokes.  A few species are wingless.
The eyes are large, with numerous facets; the tongue ter-
minates in a fleshy knob, and the other parts of the mouth
are fitted for suction, being generally converted into fine

ikF&i,

Fig. 261.—Metamorphosis of the Mosquito (Culex pipiens).
                   19 
290
COMPARATIVE ZOOLOGY.
lancets; the  thorax is globular;  and  the legs slender.
The larvae are footless grubs.  The Dipters number about
24,000.  Among them are the Mosquitoes (Culex); Hes-
v  \V   111  f)Q
        wrg*
 Fig. 262.—Metamorphosis of the Flesh-fly (Sarcophaga carnaria):  a, eggs; 6, young
      maggots just hatched; c, d, full-grown maggots; e, pupa; /, imago.

 sian-fly (Cecidomyia), so destructive  to  wheat;  Daddy-
 long-legs (Tipula),  resembling a gigantic Mosquito; the
 wingless Flea (Pulex); besides the  immense families rep-
 resented by the House-fly (Musca) and Bot-fly (CEstrus).
   6.  Lepidopters, or "Butterflies" and  "Moths," are
                              known chiefly by their four
                              large wings, which are thick-
                              ly covered on both sides by
                              minute, overlapping scales.
                              The scales are  of  different
                              colors, and   are often ar-
                              ranged in patterns of ex-
                              quisite beauty.   They are in
 reality  modified hairs, and every family has its particular
 form of scale.   The
 head is  small,  and
 the body cylindrical.
 The legs are not used
 for locomotion.  All
 the mouth  parts are
 nearly obsolete except
 the  maxillae,  which
 are fashioned  into a
 "proboscis"for pump-
 ing up  the  nectar  of
 n        rni   -i       Fl»- 264.—Part of the wing of a Moth (Saturnia),
HOWei'S.   1 ne  larvae,   magnified to show the arrangement of scales.
Fig. 263,
-Scales from the wings of vari-
  ous Lepidopters.
 
ARTICULATA.
291
called "caterpillars,"  have a  worm-like form,  and from
one  to  five pairs of  abdominal legs, in addition  to  the
six on the thorax.
The   mouth    is
formed  for mas-
tication, and  (ex-
cept  in  the  larvae
of Butterflies)  the
lip has a spinner-
et connected with
silk-glands.
   There are three
groups:  the   gay
Butterflies, having
knobbed or hooked  antennae, and flying in the sunshine
only; the dull-colored Sphinges, with antennae thickened
Fig. 265.— Vanessa polychloros, or "Tortoise-shell But-
                terfly."
Fig. 266__Moth and Larva of Attacus pavoniowmajor. 
292
COMPARATIVE ZOOLOGY.
in the middle, and  flying at twilight; and the nocturnal
Moths, which generally prefer the night, and whose anten-
nae are thread-like  and often feathery.   Generally, when
    Fig. 267.—Fruit-moth (Pyralis pomona):  b, larva; o, chrysalis; c, imago.

at rest, the Butterflies keep their wings raised vertically,
while the  others hold  theirs horizontally.   The  pupa of
the former is unprotected,  and  is usually  suspended  by
                            a bit of  silk:1'1 the pupa of
                            the Moths is inclosed  in a
                            cocoon.
                              From  22,000  to  24,000
                            Lepidopterous  species  have
                            been  identified.   Some  of
                            the most common Butterflies
                            are the swallow-tail Papilio,
                            the white Pieris, the sulphur-
                            yellow Colias ; the Argynnis,
                            with silver spots on the un-
                            der side of the hind  wings;
                            the Vanessa, with  notched
Fig. 268.-Head of a Caterpillar, from wjnors.  The  Sphinges exllib-
  beneath: a,  antennae; 6, horny jaws;      °          i   o
  c, thread of silk from the conical fusu- it  little variety.   They have
  lus, on either side of which are rudi-                .
  mentary paipi.                 narrow, powerful wings, and
 
ARTICULATA.
293
are sometimes  mistaken for Humming - birds.   The " po-
tato-worm"  is  the  caterpillar of a  Sphinx.   The most
conspicuous Moths are the large and beautiful Attacus,
distinguished  by a  triangular,  transparent spot  in the
centre of the wing; the white Bombyx, or " silk-worm;"
the reddish-brown Clisiocampa, whose larva, " the Amer-
ican Tent - caterpillar," spreads  its web  in many an ap-
ple and cherry  tree; the pale, delicate Geometrids; and
the small  but  destructive  Tineids,  represented by the
Clothes-moth.
   7. Hymenopters, comprising at least 25,000  species, in-
clude the highest, most social, and, we may add (if we ex-
cept the Silk-worm), the most useful, of Insects.   They
have a large  head, with compound eyes and three  ocelli,
mouth fitted  both for  biting and suction,1" legs formed
for locomotion as well  as support, and four wings equally
transparent, and interlocking by small hooks during flight.
The females  are usually provided with a sting, or borer.
The larvae  are footless, helpless grubs, and generally nurt-
Fig. 269.—Honey-bee (Apis mellifica): a, female; 6, worker; c, male.
nred in cells, or nests.  Such are the Honey-bees (Apis),
Humble-bees (Bombus), Wasps ( Vespa), Ants (Formica),
Ichneumon-flies, and  Gall-flies.   Those living in societies
exhibit  three  castes: females, or "queens;"  males, or
"drones;" and neuters, or sexless "workers."  There is but
one queen in a hive,  and she is treated with the greatest
distinction, even when dead.   She dwells in a large, pear-
shaped cell, opening downward.  She lays three broods of
e<r«-s: from the first come  forth  workers, the second pro- 
294
COMPARATIVE ZOOLOGY.
duces males, and the last females.  The drones, of which
there are  about 800 in an ordinary hive, are marked by
their great size, their large eyes meeting on the top of the
head, and  by being stingless.  The workers, which number
twenty to  one drone, are  small and active, and provided
with stings and hollow pits in the  thighs, called "baskets,"
in which they carry pollen.   Their honey is nectar elabo-
rated in the crop by an unknown process; while the wax
is secreted from the sides of the abdomen and mixed with
saliva.   There is a subdivision of extra labor: thus  there
are wax-workers, masons, and nurses.   Ants  (except the
Saiiba) have but two classes of workers.  While Ants live
in hollow  trees or subterranean chambers (called formi-
carium), Honey-bees and Wasps construct hexagonal cells.
The comb of the Bee is hung vertically, that  of the Wasp
is horizontal.
  Such are the main divisions of the Invertebrates—creat-
ures which are commonly regarded with aversion, and con-
sidered our foes rather than friends.  Many of  them are
unmistakable nuisances, and it is difficult to see the pur-
pose of their creation.   Yet not a few have put us under
obligations.   Protozoans give us sponge and chalk; Radi-
ates yield us coral;  Mollusks contribute pearls; and In-
sects spin us silk.  Nearly every grand group sends repre-
sentatives to  our tables: Oysters, Sea-slugs, and  Lobsters
are staple articles with many people; Bees gather honey;
and Amazonian Indians make Ants into salad. 
VERTEBRATA.                   295
               Subkingdom Vektebrata.
  This grand division includes the most  perfect animals,
or such as have  the most varied functions and the most
numerous and complex organs.  Besides  the  unnumbered
host of extinct forms, there are about 25,000 living spe-
cies, widely differing  among themselves in shape and hab-
its, yet closely allied in the grand features of their organi-
zation, the general type being endlessly modified.
  The fundamental distinctive  character of  Vertebrates
is the  separation of the main mass of the nervous system
from  the  general cav-
ity of the  body.   A
transverse  section  of
the body  exhibits  two
cavities, or tubes—the
dorsal, containing  the
cerebro - spinal nervous
system; the ventral, in
closing the  alimentarv
canal, heart, lungs, and
a double chain of gan-
glia, or sympathetic sys-
tem.   This  ventral, or
haemal,  cavity  corre-
sponds  to   the  whole
body  of  an  Inverte-
brate ;  while  the  dor-
sal, or neural, is entire-
ly  extra.
  Vertebrates are  also
distinguished by an in-
ternal, jointed skeleton, endowed  with vitality,  and capa-
ble of growth and  repair.  During embryo-life it is rep-
resented by the notochord; but this is afterward replaced
Fio. 270.— Ideal Plans of the Subkingdoms.  V,
 transverse section of vertebrate type; v, the
 same, inverted.  M, transverse section of mol-
 luscous type; and Md, of molluscoid. A and
 Ad, transverse sections of articulate type, high
 and low.  C, longitudinal section of coelente-
 rate type; a, alimentary canal;  c, body-cavity.
 In the other figures, the alimentary canal is
 Bhaded, the heart is black, and the nervous
 cords are open rings. 
296             COMPARATIVE ZOOLOGY.
                          by a more highly developed
                          vertebral column of  cartilage
                          or bone.173  The column and
                          cranium  are never   absent;
                          other parts may be  wanting,
                          as the ribs in Frogs,  limbs in
                          Snakes,  etc.   The limbs  are
                          never  more  than four, and
                          are always articulated to  the
                          haemal side of the body, while
                          the legs of Invertebrates  are
                          developed from  the  neural
                          side.    The  muscles  moving
                          the limbs are attached to  the
                          endoskeleton.
                             The circulation of the blood
                          is complete, the arteries being
                          joined to the veins by  capil-
                          laries, so that the blood never
                          escapes into  the visceral cav-
                          ity as  in the  Invertebrates.
                          All  have a  portal vein, car-
                          rying blood  through the  liv-
                          er; all have lacteals and lym-
                          phatics.   The  blood is red,
                          and contains  both  kinds  of
                          corpuscles.174   The teeth  are
                          developed from the  dermis,
                          never from the cuticle, as in
                          Mollusks and Articulates; the
                          jaws move vertically, and are
                        a never  modified  limbs.   The
the higher vertebrates: i, heart; 2, liver and kidneys are always
lungs; 3, head and upper extremities;                ^           ^
4,spleen; 5,intestine; 6,kidney; 7, present.  The  respiratory or-
lower extremities; 8, liver.  (From           . .     .,-    ,
Dalton's "Physiology.")          gans are either gills  or lungs,
 
VERTEBRATA.
297
or both.  Vertebrates are the only animals which breathe
through the mouth.
   The nervous system has two marked divisions:  the cer-
ebro- spinal,  presiding over the functions of animal life
(sensation  and locomotion); and  the sympathetic, which
controls the  organic  functions (digestion, respiration, and
circulation).   In no case does the gullet pass through the
nervous system, as in Invertebrates, and the mouth opens
on the  side opposite to the  brain.   Probably none of the
five  senses are ever altogether absent.  The form of the
brain is modified by the relative development of the  vari-
ous  lobes.  In the lower Vertebrates, the cerebral hemi-
spheres are small — in  certain Fishes they  are  actually
smaller than  the  optic lobes—in  the higher, they nearly
or quite overlap  both olfactories and cerebellum.   The
brain may be smooth, as in most  of the cold-blooded an-
imals, or richly convoluted, as in Man.
   The skull is distinctly set apart from the spinal  column,
except  in Fishes.   It is bony in Mammals, mingled bone
and  cartilage in Birds and Reptiles, and  in Amphibians
and  Fishes mainly or wholly cartilaginous.   The human
skull contains fewer bones than the skull of most animals,
excepting Birds.  The skull of all Vertebrates is divisi-
ble into two  regions:  the cranium, or brain-case,  and the
face. The size of the cranial capacity, compared with the
area of  the face, is generally the ratio of intelligence.  In
the lower  orders, the facial part  is enormously predomi-
nant, the eye-orbits are directed outward, and the occipital
condyles are nearly on a line with the axis of the body.
In the higher orders, the face becomes subordinate to the
cranium, the  sensual to the mental, the eyes look forward,
and the condyles approach the base of the cranium. Com-
pare the "snonty" skull of the Crocodile and the almost
vertical profile of civilized  Man.   A straight line drawn
from the  middle  of  the ear to the base of the nose, and 
298
COMPARATIVE ZOOLOGY.
another from the forehead to the most prominent part of
the upper jaw, will include what is called the facial an-
gle, which roughly gives the  relation between the two re-
gions, and  therefore the  rank of the animal.176  In the
cold-blooded Vertebrates  the  brains do not fill the crani-
um ; while in Birds and  Mammals  a cast of the cranial
cavity well exhibits the general features of the cerebral
surface.
  The subkingdom  is  divided into  five great classes:
Fishes, Amphibians, Reptiles, Birds, and Mammals.  The
first three are " cold-blooded," the other two are " warm-
blooded."  Fishes and Amphibians have gills  during the
whole or a part of their lives, while  the rest never  have
gills.  Fishes  and Amphibians in  embryo have neither
amnion nor allantois, while the other  three are provided
with both.
  Fishes and  Amphibians agree in having gills, in want-
ing amnion and allantois, and in possessing nucleated red
blood-corpuscles.
  Birds and Reptiles agree  in having no gills, but  both
amnion and allantois, in the articulation of the skull  with
the spine by a single condyle, in the development  of the
skin into feathers or scales, and in circulating oval, nucle-
ated, red corpuscles.
  Mammals differ from Birds and Reptiles in having two
occipital condyles, and  their  blood-corpuscles are not nu-
cleated.176
  All Vertebrates are single and free.   Mammals and a
few Reptiles bring forth  their young alive; the rest are
oviparous.

                   Class I.—Pisces.
  Fishes are  the lowest  of  Vertebrates.  They fall far
behind  the rest in strength, intelligence, and  sensibility.
The eyes, though large, are almost immovable, bathed by 
VERTEBRATA.
299
no tears, and protected by no lids.   Dwelling in the realm
of silence, ears are little needed, and such as they have are
without  external parts, the  sound being obliged to pass
through  the cranium.   Taste and smell are blunted, and
touch  is nearly  confined to  the  lips.   Destitute  of the
means of  social intercourse  (being  almost mute),  their
chief enjoyment is to eat, and to  be eaten is the end of
their existence.
   But the  class yields to no other in the number and varie-
ty of its  forms.   It includes nearly one-half of all the ver-
tebrated species.  So great  is the  range of variation, it is
difficult  to frame a  definition which will characterize all
the finny tribes.177   It may be said, however,  that Fishes
are the  only backboned animals  having  median fins (as
dorsal  and anal) supported  by fin-rays, and whose limbs
(pectoral and ventral  fins)  do not exhibit that three-fold
division  (as thigh, leg, and foot) found in all other Verte-
brates.178
   The form of Fishes is admirably adapted to  the element
in which  they live  and move.  Indeed, Nature nowhere
presents in one  class such  elegance of  proportions with
such variety  of  form  and beauty of color.  The  head is
Fig. 272__Scales of Fishes: A, cycloid scale (Salmon); B, ctenoid scale (Perch); C,
   placoid scale (Ray); D, ganoid scales (Amblypterus)—a, upper surface; b, under
   surface, showing articulating processes.

disproportionately  large, but pointed  to meet  the  resist-
ance of the water.  The neck  is wanting, the head being
a  prolongation  of the  trunk.   The viscera are  closely
packed near the  head, and the  long, tapering trunk  is left 
300
COMPARATIVE ZOOLOGY.
free for  the  development  of muscles which are to move
the tail—the instrument of  locomotion.   The  biconcave
vertebrae, with intervening cavities filled with elastic gel-
atine, are designed  for rapid and versatile  movements.
The body is  either naked, as in the Eel, or covered with
polished, overlapping scales, as in the Perch.   Rarely, as
in the Sturgeon, it is defended by bony plates, or by mi-
nute, hard spines, as in the Shark.
   The vertical fins (dorsal, anal,  and caudal)  are peculiar
to Fishes.   The  dorsal vary in number, from  one, as in
the Herring, to three, as in the Cod;  and it may be soft,
as in the Trout, or spiny, as  in the Perch.  If the dorsals
Fig. 273.—Blue-fish (Temnodon saltator). All seas.
be cut off, the Fish reels to and fro.  The caudal may be
homocercal, as in ordinary species; or heterocercal, as in
Sharks.  In ancient heterocercal Fishes, the tail was  fre-
quently vertebrated.  The pectoral and ventral fins stand
for the fore and hind limbs of other Vertebrates.   As the
specific gravity of  the body is greater  than that of the
water, most Fishes  are provided  with  an  air - bladder,
which is an outgrowth from the oesophagus.   This is ab-
sent in such as grovel at the bottom, as the Rays, and in
those, like  the Sharks,  endowed with  compensating mus-
cular power.
  Fishes have no  prehensile  organ besides the  mouth. 
VERTEBRATA.
301
Both  jaws are alike movable.  The teeth  are  numerous,
and are generally recurved spines, as in the Pike; flat and
triangular, with serrated edges, in  the  Shark;  and tessel-
lated  in the Ray.   They feed principally on animal mat-
ter.   The digestive  tract is relatively shorter than in oth-
er Vertebrates.178  With one exception (the Amphioxus),
the blood is red, and the heart has rarely more than two
cavities, an auricle  and a ventricle, both  on the venous
side.   Ordinary Fishes have four gills, the  water escaping
by one external aperture, or " gill-slit;"  but in  the Sharks
Fig. 274.—Salmon (Salmo salar).  Both hemispheres.
there is a separate opening for each gill.  The brain con-
sists of several ganglia placed one behind the other, and
occupies but a small part of the cranial cavity.   Its aver-
age weight to the rest  of the body is as low as 1 to 3000.
The eggs of bony Fishes are naked, and multitudinous,
sometimes numbering  millions in a single spawn; those of
the Sharks are few, and protected by a horny shell.
  There are six orders of Fishes:
  1. Pharyngobranchs, represented  by a  single  species, 
302
COMPARATIVE ZOOLOGY.
the Lancelet, or Amphioxus.  This lowest known Verte-
brate is about two inches long, semi-transparent, and of
worm-like form,  and is found  in the sandy bottom of
many seas, especially the Mediterranean.173
  2. Marsipobranchs, as the eel-like Lamprey and Hag.
They have a cartilaginous skeleton and sac-like  gills, but
no scales,  limbs, or lower jaw, and only one nasal organ,
all other Vertebrates having two.
  3. Teleosts, including all  the common Fishes, having a
bony endoskeleton and  a scaly exoskeleton.180  The skull
is  extremely complicated; the upper and  lower jaws are
complete (whence the name of the order), and the gills
are comb-like or tufted.   The tail is homocercal; the other
fins are variable in number and position.   In the soft-fin-
ned Fishes, the ventrals are  absent, as  in the Eels;  or at-
tached to the abdomen, as in the Salmons, Herrings, Pikes,
and Carps; or placed under the throat,  as in the Cod, Had-
dock, and  Flounder.   In the spiny-finned  Fishes, the ven-
trals are generally under or  in front of the pectorals, and
the scales  ctenoid, as in the Perches, Mullets, and Mack-
erels.
  4. Ganoids, distinguished  by their enameled bony plates
or scales.   The endoskeleton is not completely  ossified;
the ventral fins  are placed far back; and  the tail is gen-
erally heterocercal.  The gills are like those of  the bonv
Fishes.  This was one  of the largest  orders in  old geo-
logical history.  The  few modern representatives, as the
Sturgeon,  Gar-pike, and Polypterus, are essentially  fresh-
water.
  5. Elasmobranchs, having a gristly skeleton, and a harsh
skin, called "shagreen."  The gill-openings  are uncovered;
and  the mouth is generally under  the  head.  The ventral
fins  are placed far back; the pectorals are large, in the
Rays enormously developed; and  the  tail is heterocercal.
Suuh are  the Sharks, Rays, and Chimaera.  They are all 
VERTEl'.KATA.
303
Fig. 275.—Lamprey (Petromyzon Americanus).
                     lantic.
At-
Is
Fig. 276.—Cat-fish, or Horned Pout (Pimelodus catus).
                 American rivers.
Fig. 277—Cod (Morrhua Americana).  Atlantic coast.
«*

»
a	»	c
		
		
		
ss	re	"
		
		
i*	.T	c
s	s	c
ro		
ft		c'
- •" H
Fig. 278.—Gar-pike (Lepidosteus bison). Lake Ontario.
Fig. 279.—Sturgeon (Acipmser sturio).  Atlantic coast. 
304              COMPARATIVE ZOOLOGY.
            Fig. 281.—Shark (Carcharias vulgaris). Atlantic.

marine.   The  largest  Shark  found, and  therefore the
largest Fish, measured  thirty-five feet.
  6. Dipnoi,  or Mud - fishes  (lepidosiren),  of tropical
rivers.  They form a link between the typical  Fishes and
Fig. 282.—Thornback (Raia clavata).  European seas. 
VERTEBRATA.
305
the Amphibians.  They have  an eel-like body covered
with cycloid scales; an embryonic notochord for a back-
Fio. 283.—Lepidosiren annectens; one-fourth natural size. African rivers.
bone;  long, ribbon-like pectoral  and ventral fins, set far
apart; two auricles, and one ventricle; and, besides gills,
a cellular air-bladder, which is used as a lung.

                  Class II.—Amphibia.
  These cold-blooded Vertebrates are distinguished  by
having gills  when young, and  true  lungs when  adult.
They have no fin-rays, and the  limbs, when present, have
the same divisions as  those of higher animals.   The skin
is soft, and generally  naked, and  the  skeleton is ossified.
The skull is flat, and articulates  with the spinal column
by two condyles.  There is no distinct neck ; and the ribs
are usually small or wanting.   The heart consists of two
auricles  and  one  ventricle.  All  undergo metamorphosis
upon leaving the egg, passing through the "tadpole" state.
They commence as water-breathing larvae, when they  re-
semble Fishes in their respiration, circulation, and locomo-
tion.   In the lowest forms, the  gills are retained through
life; but all  others have,  when mature, lungs  only, the
gills disappearing.  The cuticle  is frequently shed, the
mode varying with the habits of the species.181  The com-
mon Frog, the type of this class, stands intermediate be-
tween  the two extremes of the  vertebrate series; no fun-
damental part is excessively developed.
  There are four orders—the first two are tailed, the other
two tailless:
                           20 
306
COMPARATIVE ZOOLOGY.
  1. Urodelans have a naked skin, a tail, and two or four
limbs.   Some retain their gills through life, as the Prote-
                a                  us of Austria, Axolotl
Fig. 284—Head and Gills of Menobranchus. Ca-  limbs, as the  aquatic
             yuga Lake.               Newts and  land Sal-
amanders.18'  The fore limbs first make their appearance
in the tadpole.
   2. Labyrinthodonts, now  extinct,  resembled  gigantic
Salamanders, except in their  complex teeth and exoskele-
ton of bony plates.
   3. Ccecilians have neither  tail nor limbs, a snake-like
Fig. 285.—Proteus anguinua. Europe.
form, minute scales in the skin, and well-developed ribs.
They are confined to the tropics. 
VERTEBRATA.
307
Fig. 286—Red Salamander (Paeudotriton ruber).
           United States.
   4. Batrachians include all the well-known tailless Am-
 phibians, as Frogs and Toads.  They have a moist, naked
 skin,  ten  vertebrae,
 and  no  ribs.   As
 they   breathe   by
 swallowing  the air,
 they  can  be  suffo-
 cated  by   holding
 the  mouth   open.
 They have four limbs —the hinder  the  longer, and the
 first  developed.   They have four  fingers and five  toes.
                                   The  tongue  is  long,
                                   and, fixed  by its  an-
                                   terior  end, it  can be
                                   rapidly thrown out as
                                   an organ of  prehen-
                                   sion.183  The eggs  are
                                   laid in the water en-
         Fig. 287.—Frog (Rana).            i    i          ■, .
                  B               veloped   m  a  glairy
 mass; and the tadpoles resemble the Urodelans,  till both
 gills  and tail  are absorbed.  Frogs (Rana) have teeth in
 the upper jaw, and webbed feet; Toads (Bufo) are higher
 in rank, and have  neither teeth nor  fully webbed feet.
 The former have been known  to live sixteen years, and
 the latter thirty-six.

                  Class  III.—Reptilia.
  These air-breathing, cold - blooded Vertebrates are dis-
 tinguished from all Fishes and Amphibians by never hav-
 ing gills, and  from Birds  by being covered  with horny
 scales or bony plates.  The skeleton  is never cartilaginous;
 and the skull has one occipital condyle.  The  vertebrae are
ordinarily concave in front; and the  ribs are well devel-
oped.  With  few exceptions, all are carnivorous; and teeth
are  always present,  except  in the Turtles, where  a horny
 
308
COMPARATIVE ZOOLOGY.
sheath covers the jaws.  The teeth are never fastened in
sockets, except in  Crocodiles.  The jaws are usually very
wide.  The heart  has  three chambers, save in Crocodiles,
where the  ventricle is partitioned.184  But in all cases a
mixture of arterial and venous blood is  circulated.  The
lungs are large, and coarsely cellular.   The  limbs, when
present, are provided with three or more fingers as well as
toes.
  There are  four orders of living Reptiles—the first two
have  horny scales, and  two external nostrils; the others
have bony  plates combined with scales, and  one external
nostril:
  1. Ophidians, or Snakes,  are characterized by the ab-
sence of visible limbs ;185 by the great number of vertebrae,
amounting to over 400 in the great Serpents; by a  corre-
sponding number of ribs, but no sternum; and immovable
Fig. 288.—Adder, or Viper (Vipera berus).  England.
transparent eyelids.  The tongue differs from that of near-
ly all other reptiles in  being bifid and extensile.  The
mouth is very dilatable.  The skin is frequently shed, and 
VERTEBRATA.
309
always by reversing it.  Snakes  make  their way on land
or in water with equal facility.
  As a rule, the venomous  Snakes, as Vipers and Rattle-
snakes, are distinguished by a triangular head covered with
small scales; a constriction behind the head; two or more
fangs, and few teeth;  small eyes, with vertical pupil; and
Fig. 289.—a, Head of a harmless Snake (upper view); 6, heads of various venom-
                        ous Snakes.
short, thick tail.   In the harmless Snakes, the head grad-
ually blends with the neck, and is covered with plates; the
teeth are comparatively numerous in both jaws; the pupil
is round, and the tail tapering.   This rule, however, has
many exceptions.
   2. Lizards  may be likened to Snakes provided  with
four limbs, each having five digits.186  The body is cover-
ed  with horny scales.  All  have teeth, which are simple
in structure;  and the halves of  the lower jaw are firmly
united in front, while those  of Snakes are loosely  tied to-
gether by ligaments.   Nearly all have a breast-bone;  and
the eyes (save in the Gecko) are furnished with movable
lids.  In the common Lizards and Chameleon, the tongue
is extensile.  The  tail is usually long, and  naturally snaps
off  at the twelfth caudal.   The Chameleon has a prehen-
sile tail.  The Iguana is distinguished by a dewlap on the 
310              COMPARATIVE ZOOLOGY.
Fig. 290.—Lizard (Lacerta).
throat and a crest on the back.   Except some of the Mon-
itors of the Old World, all the Lizards  are terrestrial.
  3. Chelonians, or  Tortoises and Turtles, approach the
Amphibians in  some respects, but are of anomalous struct-
ure.  The skeleton is external, so as to include not only
Fig. 291.—Hawk's-bill Turtle (Eretmochelys imbricata).  Tropical Atlantic. 
VERTEBRATA.
311
all the viscera, but also the whole muscular system, which
is  attached internally, as in Articulates;  and even  the
limbs are inside, instead of outside, the thorax.  The exo-
skeleton  unites with the  endoskeleton, forming the cara-
pax, or case, in which the body is inclosed.  The exoskele-
ton consists of horny plates, known as " tortoise-shell;" in
the  soft  Tortoises
(Trionyx)   this   is
wanting.   The ver-
tebrae  of the back
are soldered togeth-
er, and the ribs are
expanded,   making   ^^  ^G^/l=^ ^^T
the  Walls  Of  the pIG< 292.—Box-tortoise (Cistudo virginea).  United
carapax.   The ven-                 states-
tral  piece  is called  the plastron, or sternum.187  All are
toothless.  There  are  always four stout legs; and the or-
der furnishes the only examples of Vertebrates lower than
Birds that  really walk, for Lizards and Crocodiles wriggle
and  drag their body along.   The eggs  are  covered with a
calcareous  shell.
   The  Sea-turtles, as the  edible Green Turtle  and the
Hawk's-bill Turtle, which furnishes the "tortoise-shell" of
commerce,  have  the limbs converted into paddles.   The
fresh-water forms, represented  by  the  Snapping Turtle
(Chelydra),  are  amphibious,  and  have palmated  feet.
Land Tortoises (Testudo) have short, clumsy limbs, fitted
for slow motion on the land; the  plastron is very broad;
and  the carapax  is  arched (while it  is flattened in the
aquatic species),  and  head, legs, and  tail can be drawn
within it.   The land and  marine  species are vegetable-
feeders ; the others, carnivorous.
   4. Crocodiles, the- highest and largest  of Reptiles, have
two  exoskeletons — one of horny  scales  (epidermic),  and
another  of bony plates (dermal).   The bones of the skull
 
312
COMPARATIVE ZOOLOGY.
are firmly united, and furnished with numerous teeth im-
planted in distinct sockets.  The lower jaw extends back
of the cranium.  The heart has four cavities, but the pul-
monary artery and aorta communicate with each other, so
that there is a mixture of venous and arterial blood.  They
have external ear-openings, closed  by a flap of  the skin,
and eyes with movable  lids;  a muscular gizzard; a long,
compressed tail; and four legs, with feet more or less web-
bed, and having five toes in front and four behind.   The
existing species are confined to tropical rivers, and are car-
nivorous.  The eggs are covered with a hard shell.
   There are three  representative forms: the Gavial of the
Fig. 293.—Alligator (A. Mississippiensis).  Southern States.
Ganges, remarkable for its long snout and uniform teeth;
the Crocodile of the Nile, whose  teeth are unequal, and
the lower canines fit into a notch in the edge of the upper
jaw, so that it is visible when the mouth is closed;  and the
Alligator of the Mississippi, whose canines, in shutting the
mouth, are concealed in a pit in the upper jaw.  The toes
of the Gavials and Crocodiles are webbed to the tip; those
of the Alligators are not more than half-webbed.
   In the mediaeval ages of  geological history, the class of
Reptiles was far more  abundantly represented than now.
Among the many forms which geologists have unearthed 
VERTEBRATA.
313
are numerous gigantic Saurians, which can not be classi-
fied with any of the  four  living orders.   Such are the
Ichthyosaurus,  Plesiosaurus,  Pterodactyle,  Megalosau-
rus, and Iguanodon.

                   Class IV.—Aves.
  Birds form the most clearly defined class in the whole
Animal Kingdom.  The Eagle and Hummer, the Ostrich
and Duck, widely as they seem  to be separated by size,
form, and habits, still exhibit one common type of struct-
ure.  On the whole, Birds are more closely allied to Rep-
tiles than to Mammals.   In number, they approach the
Fishes, ornithologists having determined 11,000 species.
  A Bird is an air-breathing, egg-laying, warm-blooded,
feathered Vertebrate, with two limbs (legs) for perching,
walking, or  swimming, and two limbs (wings) for flying
or swimming.   Organized  for flight, it  is  gifted with a
light skeleton, very contractile muscular  fibre, and  a re-
spiratory function of the highest development.
  The skeleton  is more compact than  those  of Reptiles
and Mammals, at the same  time that  it is lighter, and the
bones are harder and whiter.    It contains fewer bones
than usual, many parts being anchylosed together, as the
cranial, dorsal,  and sacral.    The  lumbar  vertebrae are
wanting; but the  neck is  remarkably long  (containing
from 9 to 24 vertebrae) and  flexible, enabling  the head to
be  a most perfect prehensile organ.  The  ribs generally
are jointed in the middle,  as well  as with  the  backbone
and sternum.  The last, where the muscles  of flight orig-
inate, is  highly  developed.188  The skull  articulates with
the spinal column by a single condyle, and with the low-
er jaw, not directly, as in Mammals, but through the in-
tervention of a separate bone, as in Reptiles.
  All Birds  always  have four limbs, while every other
vertebrate class shows exceptions.  The fore-limbs are fit- 
314
COMPARATIVE ZOOLOGY.
ted for flight.   They ordinarily consist  of nine separate
bones, and from the  hand, fore-arm, and humerus  are de-
veloped the  primary, secondary, and tertiary feathers  of
the wing.  The hind limbs are  formed for progression—
walking, hopping, running, paddling, and  also for perch-
ing and grasping.  The modifications are more numerous
and important than those of the bill, wing, or tail.  There
are twenty bones ordinarily, of which the tibia is the prin-
                                     cipal ;  but the  most
                                     characteristicis the tar-
                                     so - metatarsus, which
                                     is a fusion of the low-
                                     er  part of the tarsus
                                     with  the metatarsus.
                                     The  thigh is so short,
                                     the knee is never seen
                                     outside of the  plum-
                                     age ;   the  first  joint
                                     visible  is  the  heel.189
                                     Most Birds have four
   h  g    f    e d c b      a         toes  (the external  or
Fig. 294__Principal Parts of a Bird: a, primaries;  u i.,,-,  5,  ,    .   •>
  b, secondaries; c, spurious wing; d, wing-coverts;   llltie   toe  IS always
  e, tertiaries; /.throat, or jugulum; g, chin; h,  -man finer) •  manv "hnvp
  bill;  the meeting line between the two mandi-  wammgj >  many nave
  bles is the commissure; the ridge on the upper  three  the hallux  01'
  inaudible is called culmen; that of the lower,     #  '             '
  gonys; the space between the base of the upper  " big " toe, being ab-
  mandible and the eye is the lore; i, forehead; k,         , ..  ,   „   .  ,
  crown; I, scapular feathers; m, back; n, meta-  Sent; Willie tne VJstriCh
  tarsus, often called tarsus or tarso-metatarsus; o,  LQQ  Knf +tw\ a-naxrrav
  abdomen ; p, rump; q, upper tail-coverts; r, low-  nas  DUl l ™ °> »nsWei -
  er tail-coverts.                          jng  to  t^e third and
fourth. The  normal number of  phalanges, reckoning from
the hallux, is 2, 3, 4, 5.   The  toes always end in  claws.
   Birds have neither lips nor  teeth, epiglottis nor dia-
phragm.   The teeth are  wanting,  because a heavy  mas-
ticating  apparatus in the  head  would be  unsuitable for
flight.  The  beak, crop, and gizzard vary with the food.190
The sole organs of prehension are the beak and legs.  The
 
VERTEBRATA.
315
circulation is double, as in Mammals, starting from a four-
chambered heart.   Respiration is more complete than  in
other Vertebrates.   The lungs are fixed, and communicate
with air-sacs in various parts of the body, as along the ver-
tebral column, and also with the interior of  many bones,
as the humerus and femur, which are usually hollow and
marrowless.191  Both brain and cord are much larger rela-
tively than in Reptiles; the  cranium is larger in propor-
tion to  the face;  and  the parts are  not situated in one
plane, one behind  the other.   The cerebrum  is round and
smooth; and the cerebellum  single-lobed.   The ears re-
semble  those of Crocodiles; but  the  eyes are well devel-
oped, and protected by three lids.  They are placed on the
sides of the  head, and the pupil is  always  round.   The
sexes generally differ  greatly in plumage, in some cases
more widely  than  two  distinct species.  But the coloration
of either sex  of any one species is very constant.
   A. Aquatic  Birds.—Specially organized for swimming;
the body flattened, and covered with  water-proof clothing
—feathers and down; the legs short (the knees being whol-
ly withdrawn within the skin
of the body), and set far apart
and far back; the feet web-
bed, and hind toe elevated or
absent.   The legs  are always
feathered to  the heel at least.
They are the only Birds whose
neck is sometimes longer than
the legs.
   1. Pygopodes, or Divers.—
These   lowest  of  the  feath-
ered  tribe have  very short
wings  and tail, and the legs
are  placed so  far  back that
they are obliged,  when  on
Fig. 295.—Penguin (Aptenodytes Pennan-
     tii). Falkland Islands. 
316
COMPARATIVE ZOOLOGY.
Fig. 296.—Loon (Colymbus torquatus).  North America.
land, to stand nearly bolt upright.   The}7 are better fitted
for diving than for flight or even  swimming.  They be-
long to the high latitudes, living on  Fishes mainly, and are
represented by the  Penguins, Auks, Loons, and Grebes.
  2. Longipennes, or Gulls.—Distinguished by their long,
pointed wings, usually long  tail, and by  great powers of
flight.  They are all carnivorous.  Such are the Gulls and
Fig. 297.—Tern (Sterna). 
VERTEBRATA.
317
                 Fig. 298.—Cormorant (Graculus).
Terns, which frequent the sea-coast, lakes, and rivers; and
the Albatrosses and Petrels (the  largest  and smallest  of
web-footed Birds), which are oceanic.
   3. Totipalmates, or Cormorants. — Characterized by a
long   bill,  generally
hooked; wings rather
long;  and toes long,
and all four joined to-
gether by  broad webs.
Throat generally  na-
ked,   and  furnished
with a sac.  The major-
ity are large sea-birds,
and feed  on  Fishes,
Mollusks, and  Insects.
Examples are the Cor-
morants, Pelicans,  and
        '                 Fig. 299.—Wild Goose (Bernicla Canadensis).
GannetS.                           United States.
 
318             COMPARATIVE ZOOLOGY.
Fig. 300.—Wild Duck (Anas boschas).  North America.
  4. Lamellirostres,  or  Ducks, have a heavy  body, mod-
erate wings, short tail,  flattened bill, covered by  a soft
skin, with ridges along the edges.  Diet  more commonly
vegetarian than animal.   The majority inhabit fresh wa-
ter—as the Ducks, Geese, Swans, and Flamingoes.
  B. Terrestrial Birds. —• This  group exhibits  great
diversity of structure;  but all agree in  being especially
Fio. 301.—Sandpiper (Tringa hypoleuca). England.   Small and elevated. 
VERTEBRATA.
319
   5. Grallatores, or Waders. — These  are  readily  distin-
guished  by  their
long and bare legs.
Generally, also, the
toes, neck, and bill
are of  proportion-
ate length, and the
tail  short.   They
feed on small an-
imals,   and,  with
a  few  exceptions,
frequent the banks
of rivers.   In fly-
ing, their legs are
stretched  out be-
hind, while in  most
other  Birds  they
are folded  under
the body.   Such are the Rails, Cranes, Herons,  Storks,
Ibises, Stilts,  Snipes, Sandpipers, and Plovers.
                                     6. Cur sores, or Run-
                                  ners.— This small, ab-
                                  errant order includes
                                  the  Ostriches,   Casso-
                                  waries, and  Apteryx,
                                  well marked by their
                                  gigantic   size,   rudi-
                                  mentary wings, keelless
                                  breast-bone, and robust
                                  legs.  The African Os-
                                  trich has  two toes, the
                                  Cassowary three, and
                                  the Apteryx four. The
                                  barbs  of  the feathers
                   They subsist chiefly on plants,  seeds,
Fig. 302.—Heron (Ardea).
Fig. 303.—Rail, or Marsh Hen (Rallus elegans)
           United States.
are disconnected. 
320             COMPARATIVE ZOOLOGY.
and fruit, and, excepting the Rhea, or American Ostrich,
                                      belong to the Old
                                      World.
                                         7. Rasores,   or
                                      Scratchers.   As a
                                      rule, this order, so
                                      valuable to Man, is
                                      characterized by a
                                      short,  arched bill;
                                      short and concave
                                      wings, unfitted for
                                      protracted  flight;
                                      stout legs, of me-
                                      dium length; and
                                      four toes, the three
                                      in front being uni-
                                      ted by a short web,
                                      and terminating in
                                      blunt  claws.  The
                                      legs   are  usually
 feathered to the heel, sometimes (as in Grouse) to the toes.
 The feathers of the body are large and coarse.  The males
 generally  have  gay
 plumage,  and  some
 appendage   to  the
 head.    The  nostrils
 are  covered   by  a
 scale or valve. Their
 main food is grain.
 Such are the  Grouse,
 Partridges, Turkeys,
 Pheasants,  Poultry,
 and Cnrassows.   To
 these may be added
 Pigeons  and Doves, although they stand intermediate be-
Fig. 304.—African Ostrich (Struthio camelus).
Fig. 305.—Prairie-chicken (Cupidonia cupido).
         Western prairies. 
VERTEBRATA.                   321
tween the terrestrial and perching  Birds, as the Flamin-
goes  link  the  aquatic and
terrestrial. They differ from
the typical Rasores in hav-
ing  wings for  prolonged
flight, and slender legs, fit-
ted rather for an  arboreal
life, with  toes  not united,
and the hind toe on a level
with the rest.
   C. Aerial Birds.—This
highest and largest group
includes  all   those   Birds
whose  toes  are fitted  for
306.—Ring-dove (Columba palumbus).
        England.
grasping  or perching,  the Fl0
hind  toe  being on a level
with  the  rest.   The knee is free from the body, and the
leg is generally feathered  to  the heel.   The wings are
adapted for rapid or long flight; and they  hop, rather
than  walk, on  the  ground.192  They always live in pairs;
and the young are hatched helpless.
   8. Raptores, or Birds of Prey, differ  from all  other
                      Birds, except Parrots, in having a
                      strongly hooked bill and a  waxy
                      membrane (cere) at the base of the
                      upper mandible; and from Parrots,
Fig. 307.—Barn-owl (Strix flam-
  mea).  Both hemispheres.
Fig. 308. — Fish - hawk (Pandion Carolinensis).
             United States.
   21 
322             COMPARATIVE ZOOLOGY.
in having three toes in  front  and one behind.  The toes
are armed with long, strong, crooked  talons; the legs are
robust; and the wings  are of considerable size, adapted
Fig. 309__Golden Eagle (Aquila chrysaetos).  North America and Europe.
for rapid and powerful flight.   The bill is stout and sharp,
and usually toothed.   All are carnivorous.  The female is
larger than the male, except the Condor.   There are two
Fig. 310.—Foot of Parrot and Woodpecker.
 
VERTEBRATA.                 323
sections:  the Diurnal, whose eyes are on the sides  of
the head, wings point-
ed, and metatarsus and
toes covered over with
scales, as the Vultures,
Kites, Hawks, Falcons,
and Eagles; the Noc-
turnal,  whose   large
eyes  are directed for-
ward  and surrounded
by radiating feathers,
metatarsus  feathered,
and plumage soft,  as
the Owls.
   9.  Scansores,  or
Climbers.193 — These
Birds  have no  other
exclusive   peculiarity
than the pairing of the
toes, two being turned
forward and two back-
ward.  Usually it is the
outer toe which  pairs
with the hind toe;  in
the Trogons, it is the
inner one.  They are
not musical, and only
ordinary fliers.   They
feed  on   Insects   or
fruit   The majority
make nests  in the hol-
lows of old trees; but
the Cuckoos lay in the
nests  of other Birds.
In climbing, the Wood-   pI0. 311—Trogon elegans.  Central America.
 
324
COMPARATIVE ZOOLOGY.
peckers are assisted by their stiff tail, and the Parrots by
their hooked bill.  The important Scansores are the Par-
Fig. 312.—Head of a Fly-catcher (Tyrannus).
rots, Woodpeckers, Barbets, Toucans, Cuckoos, Jacamars,
and  Trogons.
  10. Insessores, or Perchers.—This order is the most nu-
merous and varied in the whole class.  It comprehends all
those tribes which live habitually among trees, excepting
Fig. 313.—Goat-sucker (Caprimulgus). 
VERTEBRATA.
325
Fio. 314.—White-throated Sparrow (Zonotrichia
        albicollis).  United States.
the Rapacious and Climbing Birds, and whose toes—three
in front, and one behind—are eminently fitted for perch-
ing only.  The legs
are slender, and sel-
dom  used for loco-
motion.
  They are divisible
into three sections:
a. The Volitores, re-
markable  for their
powers  of   flight.
The wings are long
and  pointed,   the
voice  is  incapable
of  modulation, and
the eggs are white.
Such are the Hum-
mers, having a long,
slender bill; and the
Swifts,  Goat - suck-
ers, and Kingfishers,
having  a  short  bill
and  wide  gape.194
b. Clamatores,  with
nothing  in common
but  a harsh voice.
In most, the tarsus is
enveloped in a row
of plates, which meet
behind in  a groove,
and the bill  broad,
and bent  down  ab-
ruptly  at  the tip.
The typical  representatives are the Tyrant Fly - catchers.
c. Oscines, or  Songsters,  all of  whom  have a  vocal  ap-
Fig. 315.
-Redstart (Setophaga ruticilla). United
        States.
Fig. 316.—White-eyed Vireo (Vireo Noveboraeensis).
            United States. 
326              COMPARATIVE ZOOLOGY.

paratus, though all do not sing.   The anterior face of the
tarsus is one continuous plate, or  divided transversely into
Fig. 317.—Kingfisher (Ceryle).
large scales; and the plates on the sides meet behind in a
ridge.  The toes, always three in front and one behind, arc
Fig. 318.—Swallow (Hirundo). 
VERTEBRATA.
327
on the same level.  The eggs are usually colored.   Here
belong the Ravens, Crows, Jays, Birds of Paradise, Black-
birds, Orioles, Larks, Sparrows, Tanagers, Wax-wings, Swal-
lows, Wrens, Warblers, Thrushes, etc.

                  Class V.—Mammalia.
  Mammals are distinguished from all other creatures by
any  one of  the following characters: they suckle their
young; the thorax and abdomen are separated by a per-
fect diaphragm; the  red  corpuscles of the blood have no
nucleus, and are therefore double-concave;  and either  a
part or whole of the body is hairy.196
  They are  all warm-blooded Vertebrates, breathing only
by lungs, which are suspended freely in the thoracic cavi-
ty;  the  heart  is four-chambered, and the circulation  is
double, as in  Birds;  the aorta is single, and  bends over
the left bronchial tube; the large veins are furnished with
valves; the  red corpuscles differ  from those of all other
Vertebrates in being circular (except in the Camel); the
entrance to the windpipe is always guarded by an epiglot-
tis; the cerebrum is  more highly developed than  in any
other class, containing a greater  amount of gray  matter
and (in the  higher  orders) more convolutions; the cere-
bellum has lateral lobes, a mammalian peculiarity;198 the
cranial bones  are united by  sutures, and they are fewer
than in cold-blooded Vertebrates; the skull has two oc-
cipital condyles, a feature imitated only by the Amphib-
ians; the lower jaw consists of two pieces only (often uni-
ted), and articulates  directly with the cranium; with two
exceptions (Manatee  and Hoffman's Sloth), there  are  al-
ways seven cervical  vertebrae; the dorsals, and therefore
the  ribs, vary from ten to twenty-four; the  articulating
surfaces of  the vertebrae are  generally flat; the  front
limbs are never wanting, and the  hind limbs only in a
few aquatic forms;  excepting the Whales, each digit car- 
328
COMPARATIVE ZOOLOGY.
ries a nail, claw, or hoof; the teeth (always present,  save
in certain low  tribes) are planted in sockets; the mouth
                        is  closed   by  flexible  lips;  an
                        external ear is rarely  absent ;197
                        the   eyes   are  always   present,
                        though rudimentary in some  bur-
                        rowing animals;  they are vivipa-
                        rous;  and, finally, and perhaps
                        above all, while in  all  other an-
                        imals  the  embryo  is  developed
                        from the nourishment laid up in
                        the  egg itself, in  Mammals it
                        draws  its  support, almost  from
                        the  beginning, directly  from the
                        parent, and, after birth, it is sus-
                        tained  for  a time by the milk se-
                        creted  by  the  mammary  glands.
                        From  the  first, therefore, till it
                        can  care   for  itself,  the  young
                        Mammal  is in  vital connection
                        with the parent.198
Fig. 319.—Longitudinal Section
 of Human Body (theoretical):
 a, cerebro-spinal nervous sys-
 tem ; b, cavity of nose; c, cav-
 ity of mouth; d, alimentary
 canal; «, chain of sympathet-
 ic ganglia; /, heart; g, dia-
 phragm.
Fig. 320__Transverse Section of Human Body
  (theoretical): a, cerebro - spinal nervous axis
  contained in neural tube; e, chain of sympa-
  thetic ganglia ; d, alimentary canal; /, heart;
  h, hfemal tube.
  1. Monotrernes.—This order  is created  to  include two
singular forms, the  Duck-mole  (Ornithorhynchus) and
Spiny Ant-eater (Echidna), both confined to the Australian
continent.   The former has  a covering of fur, a bill like 
VERTEBRATA.
329
that of a  Duck, and webbed feet.  The latter is covered
with spines, has a long toothless snout, like the Ant-eater's,
and the feet are not webbed.  Both burrow, and feed upon
Fig. 321.—Ornithorhynchus.
Insects.  The brain is smooth in the Ornithorhynchus, and
folded in the Echidna.  In both, the cerebral hemispheres
are loosely united by transverse fibres, and do not cover
the cerebellum and olfactory lobes.199
  2. Marsupials are  distinguished by the fact that  the
young, always  born  premature, are transferred by  the
mother to a pouch on the abdomen, where they are at-
tached to the  nipples, and the milk is forced into their
mouths by  special muscles.300   They have "marsupial
bones" projecting from  the  pelvis,  which may serve to
support the pouch; but as the  Monotremes have the same
bones, but no pouch, they doubtless have some other func-
tion.   These bones are peculiar to animals having no pla-
centa, namely, to Monotremes and Marsupials.  The brains
of Marsupials resemble those  of the Monotremes, except
that the  cerebrum of the Kangaroo covers the olfactory
lobes.  All have the four kinds of teeth,  and all are cov- 
330
COMPARATIVE ZOOLOGY.
ered with  fur, never with spines or scales.   Except  the
Opossums of America, all are restricted to Australia and
          Fig. 322__Virginian Opossum (Didelphys Virginiana).
adjacent islands.   The Wombat, Kangaroo, and Phalanger
are herbivorous;  the Bandicoot, Opossum, Hylacinus, and
Dasyurus are chiefly carnivorous.
  3. Edentates.—This strange order contains very diverse
forms, as the leaf-eating Sloths and the insectivorous Ant-
eaters and Armadillos of South America, and the Pango-
lin and Orycteropus of the Old World.  The gigantic fos-
                                  sils, Megatherium and
                                  Glyptodon, belong  to
                                  this group. The Sloths
                                  and Ant-eaters are cov-
                                  ered with coarse hair;
                                  the  Armadillos  and
                                  Pangolins, with an ar-
mor of plates  or scales.  The Ant-eaters  and Pangolins
are strictly edentate, or toothless; the rest have molars,
wanting, however, enamel and roots.   In general, it may
be said that the order includes all quadrupeds having sep-
Fig. 323.—Skull of the Great Ant-eater (Myrme-
 cophaga jubata): 15, nasal; 11, frontal; 7, pa-
 rietal ; 3, superoccipital;  2, occipital condyles;
 28, tympanic; 73, lachrymal; 32, lower mandi-
 ble.  Teeth wanting. 
                      VERTEBRATA.                   331

arate, clawed toes and no incisors.   The Sloths are arbo-
real ; the others burrow.   The brain is generally smooth;
Fig. 3'24.—Armadillo (Dasypus).
but that of  the  Ant-eater is convoluted, and has a large
corpus  callosum;  but in all, the  cerebellum  and part of
the olfactories are exposed.801
  4. Rodents, or Gnawers, are characterized by two long,
curved  incisors  in each jaw, enameled in front, and per-
petually growing; they are specially formed for nibbling.
Separated from them  by  a wide space (for canines  are
wanting), are  the  flat molars, admirably fitted for  grind-
15           n
Fig. 325.__Skull of a Rodent (Capybara): 22, premaxillary; 21, maxillary; 26, mo-
   lar; 27, squamosal; 73, lachrymal; 15, nasal; 11, frontal; 4, occipital processes,
   unusually developed; i, incisors; a, angle of lower jaw. 
332
COMPARATIVE ZOOLOGY.
ing.   The lower jaw has longitudinal condyles, which
work  freely backward and forward in longitudinal fur-
Fig. 326.—Incisor Teeth of the Hare.
rows.  Nearly all have clavicles ;  and the toes are clawed.
The cerebrum is nearly or quite smooth, and covers but a
small part of the cerebellum.  All are vegetarian.
  About two-thirds of all known Mammals are  Rodents.
They range from the equator to the poles, over every con-
tinent, over mountains and plains, deserts and woods.  The
Fig. 327.—Beaver (Castor Canadensis). North America.
more important representatives are the Porcupines, Capy-
baras, Guinea-pigs, Hares, Mice, Rats, Squirrels, and Bea-
vers.   The Capybara  and Beaver are  the giants of  the
race. 
VERTEBRATA.
333
insect - eating  animals,
smallest  of Mammals.
  5. Insectivores are diminutive
some,  as  the Shrew,  being  the
They have small, smooth  brains,
which, as  in the preceding orders,
leave  uncovered the cerebellum
and  olfactory lobes.  The molar
teeth bristle  with sharp, pointed
cusps, and are associated with ca-
nines and incisors.   They  have a FlG> 82S—Shrew Mouse <*"*>■
long muzzle, short legs,  and clavicles.  The feet are form-
ed for walking or grasping, and are plantigrade, five-toed,
and  clawed.   The  Shrew,  Hedgehog, and  Mole  are  ex-
amples.
  6. Cheiropters, or Bats,  repeat  the chief characters of
the Insectivores; but some (as the Flying-fox) are  fruit-
eaters, and have corresponding modifications of the teeth.
Fio. 329.—Bat (Vespertilio).
They are distinguished  by their very long  fore - limbs,
which are adapted for flight, the fingers being immense-
ly lengthened, and united by a membranous web.   The 
334              COMPARATIVE ZOOLOGY.
toes, and one or two of the fingers, are armed with hook-
ed nails.  The clavicles are remarkably long, and the ster-
num is of great strength;  but the whole skeleton is ex-
tremely light, though not filled with air, as in Birds.  The
Fig. 330__Skeleton of a Bat.
eyes are small, the ears large, and the  sense of touch is
very acute.  The favorite  attitude of  a  Bat  when  at rest
is  that of  suspension by the claws, with head downward.
They are all nocturnal.
   7. Cetaceans, or Whales, have the form and life of Fish-
es, yet they possess a higher organization than the preced-
ing orders.   They  have  a broad brain, with  many and
deep  foldings; the  foramen magnum of the skull is en-
tirely  posterior;  the  whole  head  is  disproportionately
large, and  the jaws greatly prolonged.   The  body  is cov-
ered with a thick, smooth skin, with a layer of fat (" blub-
Fig. 331.—Outline of the Sperm-whale (Physeter): a, blow-hole; 6, the case contain-
   ing spermaceti; c, junk; d, bunch of the neck—between it and the corner of the
   mouth is the eye; h, hump; i, ridge;  A;, the small; /, tail, or flukes.  Between
   the dotted lines are the spiral strips of blubber.  Maximum length, sixty feet.
   South Atlantic. 
VERTEBRATA.                  335
ber") underneath; there are no clavicles; the hind limbs
are wanting, and  the front pair changed to paddles; the
tail expands into a powerful, horizontal fin; neck and ears
are apparently wanting; the eyes small, with only two lids;
the nostrils ("blow-holes")—double in the Whale, single
in the Porpoise—are on the top of the head.  All are car-
nivorous, and  essentially marine, a  few Dolphins only be-
ing found in the great  rivers.   In  the Whalebone Whales,
the teeth are  absorbed, and disappear before birth, and
their  place is  supplied  by horny "baleen" plates.  "The
Fig. 332.—Greenland Whale (Balmna mysticetus). North Atlantic. 
336
COMPARATIVE ZOOLOGY.
Whale feeds by putting this gigantic strainer into opera-
tion, as it swims through the  shoals of minute  Mollusks,
Crustaceans, and Fishes, which are constantly found at the
surface of the sea.  Opening its capacious mouth, and al-
lowing the sea-water, with its multitudinous tenants, to fill
the oral cavity, the Whale shuts the lower jaw upon the
Fig. 333.—Troop of Dolphins, with Manatee in the distance.
baleen plates, and, straining  out the water through them,
swallows the prey stranded upon its vast tongue."  In all
other Cetaceans teeth are developed, especially in Dol-
phins and Porpoises;  but the Sperm Whale has them only
in the lower jaw, and the Narwhal can show but a single
tusk.   The Dolphins are the  only Mammals having no or-
gan of smell.
   8. Sirenians resemble the Cetaceans in shape, but  are
closely allied to the hoofed animals in organization.  They
have  the limbs of the Whales, and are aquatic;  but they
are herbivorous, and  frequent great rivers  and estuaries. 
VERTEBRATA.                  337
They have two sets of teeth, the Cetaceans never having
but one.  They have a narrow brain ; bristles scantily cov-
ering the body; and nostrils placed on the snout, which is
large and fleshy.  Such are the Manatee and Dugong.
  9. Proboscidians.—This race of giants, now nearly ex-
tinct, is characterized by two upper incisors in the form of
tusks, mainly composed  of dentine (ivory).  In the extinct
Dinotherium the tusks projected from the lower jaw; and
in the Mastodon, from both jaws.  Canines are wanting.
The molars are few and large, with transverse ridges (Ele-
phant) or tubercles (Mastodon).   The cerebrum  is large
and convoluted, but does not cover the cerebellum.  The
skull is enormous,  the size arising in  great measure from
the development of  air-cavities  between the inner and
outer plates.  The nose is prolonged into a flexible trunk,
which is a strong and delicate organ of prehension. There
are four massive  limbs, each with  five toes incased  in
broad, shallow hoofs, and also with a thick, tegumentary
pad.  The knee is below and free from the body, as in
Monkeys and Men.  Clavicles are wanting.  The body of
the Elephant is nearly  naked; but the Mammoth, an ex-
tinct species, had  a covering  of long woolly  hair.  Ele-
phants live in large herds, and subsist on foliage and grass.
There are but  two living species: the Asiatic, with long
head, concave forehead, small ears, and  short tusks; and
the African, with round head,  convex forehead, large ears,
and long tusks.203
  10. Ungulates, or Hoofed Quadrupeds.—This  large or-
der, comprehending many animals most useful to Man, is
distinguished by four well-developed limbs, each furnished
with not more than four complete toes, and each toe in-
cased in a hoof.   The leg, therefore, has no prehensile
power; it is only for support and locomotion.  Clavicles
are wanting; and  the radius and ulna are so united as to
prevent rotation.   There are always two sets of teeth, i. e.,
                          22 
338
COMPARATIVE ZOOLOGY.
milk-teeth are succeeded by a permanent set.  The grind-
ers  have broad crowns.   As a rule, all are herbivorous.
The brain  is  always convoluted, but the  cerebellum is
largely uncovered.
  Ungulates are divided into the odd and even toed.   a.
The Odd-toed, as the three-toed Rhinoceros and Tapir,203
and  the one-toed Horse.204  The  first is distinguished by
its very thick skin, the absence of canines, and one or two
horns on the nose.  The tapir has the four kinds of teeth,
and a short proboscis.   The dental formula of the Horse
is—
           .3-3    1-1     3 — 3     3 — 3    An
           % 3—3> G I"' P171 3—3'  m 3^ = 40'
The canines  are often wanting in the mare.  The Horse
walks on the third finger and toe.  The metacarpals and
metatarsals are greatly elongated, so that the wrist and
heel are raised to the middle of the leg.  b. The Even-toed
Ungulates — Hog,  Hippopotamus, and Ruminants — have
Fig. 334__Indian Rhinoceros (R. unicornis). 
VERTEBRATA.
339
two or four toes.105  The Hog and Hippopotamus have the
four kinds of teeth, and, in the wild state, are vegetarian.
The  Ruminants have two toes on each foot, enveloped in
hoofs which face each other by a flat side, so that they ap-
pear to be a single hoof split or " cloven."  Usually there
are also two supplementary hoofs behind, but they do not
ordinarily touch the ground.  All chew the cud, and have
a complicated stomach.  They have incisors in the lower
jaw  only, and  these  are  apparently eight; but the two
outer ones are canines.208   The molars are flat  typical
grinders.   The dental formula of the Ox is—
           .o—0   0—0     3—3    3—3   on
           * jZv «£=!, JPm 3—3, m—s = 32.
  With few exceptions, as  the Camel, all Ruminants have
horns, which are always in pairs.  Those of the Deer are
solid, bony, and deciduous; those of the Giraffe and An-
Fig. 335.—Stag, or Red Deer (Cervus elaphua). Europe. 
340
COMPARATIVE ZOOLOGY.
Fig. 336—Raccoon (Procyon lotor). United States.
telope are  solid,  horny, and permanent;  in the  Goat,
Sheep, and Ox they are hollow, horny, and permanent.
  11. Carnivores, or Beasts of Prey, may be recognized
by  their four long, curved, acute,  canine teeth, the gap
                                    between the incisors
                                    and canines in  the
                                    upper jaw for  the
                                    reception of the low-
                                    er  canine,  and mo-
                                    lars graduating from
                                    a  tuberculate  to a
                                    trenchant form   in
                                    proportion  as  the
                                    diet deviates from a
                                    miscellaneous  kind
                                    to  one  strictly   of
                                    flesh.  The incisors,
                                    with rare exceptions,
                                    number  six in each
                                    jaw.  The teeth  are
                                    lodged   in  distinct
                                    sockets, and covered
                                    with enamel.  There
                                    are always  two sets.
                                    The  skull  is  com-
                                    paratively small, the
                                    jaws are shorter and
                                    deeper than in Un-
                                    gulates,  and  there
                                    are numerous  bony
                                    ridges on the inside
                                    and outside of  the
                                    cranium — the high
occipital crest being specially characteristic. The cerebral
hemispheres are joined by a large corpus callosum, but  the
                 ft-
         .i tr-r.....p^Z(„jN
Fig. 337.—Wolf (Lupus occidentalis)
United States.
ftwsa^-
Fig. 338__Ermine-weasel (Putoriua Noveborocensis)
            United States. 
VERTEBRATA.
341
  Carnivores are divided  according to the modifications
of the limbs:  a. Pinnigrades, having short feet expanded
into webbed paddles for swimming, the hinder ones being
bound in with the skin of the tail.  Such are the Seals,
Walrus, and Eared Seals, or Sea-lions,  b. Plantigrades, in
which the whole,  or nearly the whole, of the hind foot
forms a sole, and rests on the ground.  The claws are not
Fig. 340.—Southern Sea-lion (Otaria jubata).  Antarctic Ocean.
retractile;  the ears are small, and tail short.  Bears, Bad-
gers, and  Raccoons are well-known examples,   c. Digiti- 
342
COMPARATIVE ZOOLOGY.
grades keep the heel raised above the ground, walking on
the tips of the toes.  The majority have long tails.   Such
are the Weasels, Otters, Civets, Hyenas, Foxes, Jackals,
Wolves, Dogs, Cats, Panthers, Leopards, Tigers, and Li-
ons.  The last five differ from all others in having retract-
ile claws, and the radius rotating freely on the ulna.  The
Cats have thirty teeth; the Dogs, forty-two, or twelve more
molars.  In the former, the tongue is prickly; in the lat-
ter, smooth.
  12. Primates, the head of the kingdom, are character-
ized by the possession of two hands and a pair of feet, the
thigh free from  the body, and all the  digits  furnished
with nails, the first on the foot enlarged to  a " great toe."
Throughout the order, the  hand is eminently or  wholly
prehensile, and the foot, however prehensile it may be, is
always locomotive.208  The clavicles are perfect.  The eyes
are situated in a  complete bony cavity, and look forward.
                                 There  are two  sets  of
                                 teeth,  all  enameled;
                                 and the  incisors, num-
                                 bering  four  in  each
                                 jaw.  They are divided
                                 into Lemurs, Monkeys
                                 and Apes, and Man.
                                   a. Lemurs, or  " Mad-
                                 agascar Cats," are cov-
                                 ered with soft fur, have
                                 usually  a  long  tail,
                                 pointed  ears, fox-like
muzzle, and curved nostrils.  They walk  on all fours, and
the thumb and  great  toe are generally opposable  to the
digits.  The second toe has a long, pointed claw  instead
of a nail.  The cerebrum is relatively small, and flattened,
and does not cover the cerebellum and olfactory lobes.209
  b. The Monkeys of tropical America have, generally, a
Fig. 341.—Lemur (L. ruber). Madagascar. 
VERTEBRATA.
343
long, prehensile tail ;210  the  nostrils are placed far apart,
so that the nose is wide and flat; the thumbs and great
toes  are fitted for grasping, but are not opposable to  the
other digits; and they have four molars  more than  the
Apes or Man—that is, thirty-six teeth in all.  In the Apes
of the  Old World the tail is never prehensile, and is some-
times wanting; the nostrils are close together; both thumbs
and  great toes are opposable;  and the teeth, though num-
bering the same as Man's,  are uneven (the incisors being
   Fio. 342.—White-throated Sapajou (Cebua hypoleucua).  Central America.
prominent, and the canines large), and the series is inter-
rupted by a gap  on one  side  or  other  of the canines.
Their average size is much  greater than that of the Le-
murs  or  Monkeys, and they are not so strictly arboreal.
In both Monkeys and Apes, the cerebrum covers the cere-
bellum.211  While  in the Lemurs and Monkeys the skull
is rounded and smooth, that  of the Apes, especially those
coming nearest to Man —the anthropoid, or long-armed, 
344
COMPARATIVE ZOOLOGY.
Apes,  as Gorilla, Chimpanzee, Orang, and Gibbon — is
characterized by  strong crests.   Lemurs  and Monkeys
Fig. 343.—Skull of Orang-utan (Simia   Fig. 344.—Skull of Chimpanzee (Troglo-
          satyrus).                       dytes niger).
shorter than the arms.  In all the Primates but Man, the
body is clothed with hair, which is generally longest  on
the back.   Several Monkeys  and Apes have a beard, as
the Howler and Orang.
Fig. 345.—Female Orang-utan (from photograph).  Borneo. 
VERTEBRATA.
345
  The Orang is  the least human  of all the anthropoid
Apes as regards the skeleton, but comes nearest to Man in
the form of the brain.  The Chimpanzee approaches Man
most closely in the character of its cranium and teeth, and
the proportional size of the  arms.   The Gorilla is most
Fig. 346__Skeletons of Man, Chimpanzee, and Orang.
Man-like in bulk (sometimes reaching the height of five
feet six inches), in the proportions of the leg to the body
and of the foot to the hand, in the size of the heel, the
form  of the pelvis  and shoulder-blade, and  volume of
brain.212
  c. Man differs from the Apes in being an erect biped.
In him, the vertebrate type, which began in the horizontal
Fish, finally became vertical.  No other animal habitually
stands erect; in no other are the fore-limbs used exclusive-
ly for head-purposes, and the hind pair solely for locomo-
tion.
  Man alone can stand, walk, run, jump, climb, swim, ride,
drive, sit, or lie on his back for any length of time.
  His limbs are naturally parallel to the axis of his body,
not perpendicular.  They have a near equality of length, 
346            COMPARATIVE ZOOLOGY.

but the arms are always  somewhat  shorter than the legs.
In all the great Apes the arms reach below the knee, and
the legs of the Chimpanzee and Gorilla  are relatively
shorter than Man's.
  Man only has a finished hand, most perfect as an organ
of touch, and most versatile.  Both hand and foot are rel-
atively shorter than in the Apes.   The foot is plantigrade;
the leg bears vertically upon it; the  heel and great toe are
longer than in other Primates; and the great toe is not
o                             b
Fig. 347.—Foot (a) and Hand (b) of the Gorilla.
opposable, but is used only as a fulcrum in locomotion.
The Gorilla has  both an  inferior hand and inferior foot.
The  hand  is clumsier, and with a  shorter thumb than
Man's; and the  foot is prehensile, and is not applied  flat
to the ground.213
   The scapular and pelvic bones are extremely broad, and
the neck of the femur remarkably long.  Man is also sin-
gular in the double curve of the spine: the Baboon comes
nearest to Man in this respect.
   The human skull has a smooth, rounded outline, eleva-
ted in front, and devoid  of  crests.  The cranium greatly
predominates over the face, being four to one ;214 and no
other animal (except the Siamang Gibbon) has a chin.
   Man stands alone in the peculiarity of his dentition : his 
VERTEBRATA.
347
teeth are vertical, of nearly uniform height, and close to-
gether.   In every other animal the incisors and canines
are more or less inclined, the  canines project, and there
are vacant spaces.215
  Man has a longer lobule to his ear than  any Ape, and
no muzzle.  The bridge of  his nose is decidedly convex;
in the Apes generally it is flat.
  Man has  been called  the only naked  terrestrial  Mam-
mal.  His hair  is most abundant on the scalp; never on
the back, as in the Apes.
  Man has  a more -pliable constitution than the Apes, as
Fig. 348.—Australian Savage.
shown by  his  world - wide  distribution.   The  animals
nearest him soon perish when removed from their native
places.
  Though Man is excelled by some animals in the acute-
ness of some senses, there is no other animal in which all
the senses  are capable of  equal  development.   He only
has the power of expressing his thoughts by articulate
speech, and the power of forming abstract ideas.
  Man differs from the Apes in the absolute size of brain, 
348            COMPARATIVE ZOOLOGY.
and  in the greater complexity and less symmetrical dis-
position of its convolutions.   The cerebrum is larger  in
proportion  to the  cerebellum (being as 8£ to 1), and the
former not only covers  the latter, but projects beyond  it.
The brain of the Gorilla scarcely amounts to one-third  in
Fig. 349—Skull of European.         Fig. 350.—Skull of Negro.
volume or one-half in weight of that of Man.  Yet, so far
as cerebral structure goes, Man differs less from the Apes
than they do from the Monkeys and  Lemurs.  The great
gulf between Man and the brute is not physical, but psy-
chical.218 
CHAPTER  XXII.
                            SYSTEMATIC ARRANGEMENT  OF  REPRESENTATIVE FORMS.
Subkingdom  Protozoa.—An  artificial  group, with  no common tvpe;  animals simple, minute,
    aquatic.
        Class   I. Monera.—Entirely homogeneous in structure: as  Bathybius.
        Class  II. Gregarinida.—Consisting of one cell, and parasitic :  Gregarina.
        Class III. Rhizopoda—Having the power of throwing out parts of the body for prehension.
             Order 1. Amujbea.—Naked: Amoeba, Actinophrys.
                                                 ^ „  { single: Lagena.
             Order 2. FonAMHirraA.-With calcareous shell; j compound: Nummulite&
             Order 3. Radiolaria.—With siliceous shell: Polycystina.
        Class IV. Infusoria.—Having mouth and cilia.
             Order 1. Flagellata.—With long, lash-like filaments:  Monad.
                                                     ( fixed: Vorticella.
             Order 2. Ciliata.-Covered with vibratile cilia; j free: ParmMeiumm
        Class  V.  Spongida.—Amcebiform bodies united into a composite mass, generally secreting a common skeleton.
             Order 1. Myxospongia.—Gelatinous; no skeleton : Halisarca.
                                  __, „,     , ,      ( horny:  Spongia.
             Order 2. Fibbospongia.—With fibrous skeleton ; <  ...      „  ,  .„
                                                   ' ( siliceous: Euplectella.
             Order 3. Cai.oispongia.—Skeleton calcareous, not fibrous: Sycon. 
Subkingdom  Cgxenterata.—Radiated animals, with  distinct  body-cavity,  tentacles,  and nettling    £=
    thread-cells.                                                                                                         °
         Class   I.  Hydrozoa—Single digestive cavity, with which the mouth communicates freely.
             Order 1. Hydroida__Fixed by adherent disk; \    b     V ra-
                                                   { compound: Serttilaria.
             Order 2. Siphonophora.—Free and oceanic, swimming by crested air-sac: Physalia.
             Order 3. Luoernarida.—Cup-shaped, with marginal tufts of tentacles, and eight or more radiating canals; attached ; single: Lucer-
                       naria.                                                                                               O
             Order 4. Disoophora.—Free and oceanic; disk-shaped, with marginal fringe of tentacles and "veil";  four canals; single: Aurelia.     §
         Class  II. Anthozoa—Double digestive cavity, with radiating septa.                                                     ^
                                                                                                                           W
                                                        r" soft-bodied ; single; slightly locomotive: Actinia.                         >■
             Order 1. AoTiNARiA.-Parts in some multiple of six; J                   .      f sclerodermic  rough, calcareous coral: Mad-    g
                                                        j composite;  fixed;  secreting <    repore.                                -<
                                                        L                         I sclerobasic, smooth, horny coral: Isis.
             Order 2. Aloyonaria__Parts in some multiple of four; composite; fixed ;\     \    '                                       O
                                                                          ( secreting furrowed sclerobasic coral: Corallium. ■         O
         Class III. Ctenophora.—Soft-bodied ;  transparent;  free, moving by  eight rows of tiny paddles ; two tentacles ; digestive    O
                      cavity with anal outlet:  Pleurobrachia.                                                                   h<

Subkingdom Echinodermata.—Radiated animals, with distinct alimentary canal and well-developed
    nervous system;  body-walls secreting calcareous plates; parts in multiple  of five.
        Class  I. Crinoidea— Body cup-shaped ; fixed by jointed stalk ;  mouth uppermost: Pentacrinus.
                                                                        ( moving by long slender arms: Ophiura.
        Class II.  Asteroidea.—Body star-shaped;  free; mouth underneath; { moving  by suckers under the hollow lobes of the
                                                                        t    body: Asterias. 
        Class III. Echinoidea — Body inclosed in a spinous shell;  free, mov-("spherical: Cidaris.
                      ing by suckers ; mouth  underneath, with five teeth ;    \flat: Chjpcaster.
        Class IV. Holothuroidea—Body cylindrical and soft; free, moving by suckers ; mouth forward, surrounded by tentacles :
                      Penfada.

Subkingdom  Mollusca.—Soft-bodied, unjointed animals, with muscular  skin (" mantle"), generally
    protected by  a calcareous shell; nervous system scattered.

                                § 1.  Destitute of masticatory apparatus, feeding by ciliary action.
        Claps   I. Polyzoa.—Minute; composite ; fixed ; mouth with tentacles : Flustra.
                                                                                         fsingle and fixed :  Ascidia.
        Class  II. Tunicata-Inclosed in a leathery, contractile bag, with two openings ; marine;|gingle Qr compoun(j. free: Salpa.

        Class III. Brachiopoda—With equilateral but unequivalved bivalve fwith hinge-line : Tcrebratula.
                      shell; no gills ; two ciliated arms ; fixed ; marine ;     \with no hinge-line : Lingula.
                                                                          {no pallial sinus, siphons, or foot; one musculrr im-
                                                                             pression ; unequivan eel .  t/sn ea.
                                                                          pallial sinus, siphons, and foot;  two muscular i.n-
                                                                             pressions ; equivalved : Venus.
                                          §  2.  With distinct head and rasping tongue.
        Class   V. Gasteropoda—Generally with univalve, unchambered  shell;  two-chambered heart; three nervous ganglia; two
                      tentacles.
                                             * Swimming by wing-like appendages.
                                                  „  ( with shell: Hualea.
             Order 1.  PrKBOPODA—Free and oceanic; small; -j without ghe,,.  ^ 
Subkingdom  Mollusca—Continued.                                                                              £g

                                                * * Crawling by ventral disk.
             Order 2. OpiSTHOBRANoniATA.—Naked, with external gills toward the rear of the body;  marine: Doris.
                                     „.„ .  „      ...  .  „  ( aperture entire:  Natica.
             Order 3. Prosobranchiata__Gills in front; with shell ;<            , ,       .
                                                           ( aperture notched: Strombus.
                                             ["naked:  Umax.
             Order 4. Pulmonata.—Air-breathing; -l  . h fa  n, j terrestrial: HeJi*.
                                             i         ' 1 fresh-water: Limncea.                                                 „

        Class VI.  Cephalopoda—Symmetrical body;  with arms around the mouth; walk and swim ;  marine; three-chambered    g
                      heart.                                                                                                tj
             Oder 1. Tetrabranchiata.—Having four gills, many tentacles, and an external, chambered shell: Nautilus.                       F
                                                  • it   ( eight arms: Octopus.                                               H
             Order 2. Dibranohiata.-Two gills ; naked; ink-bag; -j ^ ams. ^.^                                                   ^
                                                                                                                           W
Subkingdom. Articulata.—Symmetrical, jointed animals ;  skeleton external;  nervous cord ventral;    g

    tubular heart dorsal.                                                                                               ^
                                                                                                                           o
                                                      § 1. Aquatic.                                                          CP

        Class   I.  Annelida.—Soft  body; segments similar; no jointed legs.

             Order 1. Annttloida.—Parasitic: Taenia.
             Order 2. Abranohiata__Breathe by skin: Lumbricus.
             Order 3. Branohiata.—Breathe by gills: Nereis.

        Class  II.  Crustacea.—Having gills and more than  eight jointed legs ;  twenty-one segments, with head and thorax united;
                      four antennae.

             Order 1. Cirripedia.—Fixed; shelly covering; feathery arms: Balanus. 
                 Order 2. Entomostraca.—Horny carapax; joints of abdomen and thorax more or less than fourteen; no abdominal legs: Limulus.
                 Order 3. Tetkadeoapods.—Carapax of mingled lime and chitine ; fourteen-footed;  small: Oniscus.
                 Order 4. Decapods.—Carapax of mingled lime and chitine ; ten-footed; large: Astacus.

                                                                                                                                           >
                                                            § 2.  Air-breathing.                                                             f^

            Class III.  Arachnida—Eight thoracic legs ; cephalo-thorax ; antennae modified.                                               ^
                                                                                                                                           6
                 Order 1. Aoaeina.—Minute;  no marked articulations ; tracheal respiration ; suctorial: Acarus.                                     W
                 Order 2. Pedipalpi.—Long, jointed abdomen ;  pulmonary sacs ; lobster-like claws: Scorpio.                                        ?*
                 Order 3. Abaneina.—Soft, short, unjointed abdomen ; spinnerets ; pulmonary sacs: Epeira.                                        54
                                                                                                                                           H
            Class IV.  Myriapoda.—Segments similar; worm-like ; chitinous skin ; tracheae ; two antennae.                                2

                 Order 1. Cuilognatha.—Cylindrical; four legs  to each joint: lulus.                                                             W
££               Order 2. Chilopoda.—Flattened; two legs to each joint:  Scolopendra,                                                            t-tf
                                                                                                                                           W
            Class  V.  Insecta—Head, thorax, and abdomen distinct; chitinous crust; six thoracic legs; winged; two antennas; tracheae.     22

                 Order 1. Neueoptera.—Slender abdomen ; four equal, large, transparent wings ;  biters: Libellula.                                  h3
                 Order 2. Orthopteea.—Four wings, front pair slightly thickened, narrow and overlapping, hind pair transparent, broad, and folded;     f^
                             biters:  Gryllus.
                                                                                                                         H
                                                                                                                         M
Order 3. HEMiPTERA.-Suctorial; legs slender; \ 7inSle8S:  Cimex-                                                              ^
                                          ( four wings : Cicada.
Order 4. Coleopteea.—Four wings, front pair horny, uniting by straight edge ; biters: Scarabmus.                                  q
Order 5. Dipteea.—Two transparent wings ;  slender legs ; suctorial: Musca.                                                     Jd
                                                                                           f antennae feathery: Attacus.    g;
Order 6. Lepidoptera—Four large, scaly wings; legs not locomotive; spiral proboscis for suction ; J  antennae  6Pmdle - shaped:
                                                                                           ]    Sphinx.
                                                                                           L antennae knobbed: Papilio.    W
Order 7. Hymenopteea.—Four transparent wings ;  mouth fitted for both biting and suction ; social: Apis.                           os 
Subkingdom  Yeetebeata. —Internal, jointed  skeleton;  nervous  cord  dorsal  and  separated  from   g

    body-cavity;  circulation complete ;  limbs not more than four.

                                                       § 1.  Cold-blooded.

         Class   I. Pisces.—Permanent gills ;  embryo without amnion and allantois ; heart with two cavities.
              Order 1. Marsipobranohii.—Body naked; eel-like;  limbless; skeleton cartilaginous ; mouth circular: Petromyzon.
                                                                                  \ soft-finned: Salmo.
              Order 2. Teleostei.—Having scales, fins, and bony skeleton ; tail homocercal; j gpiliy.finne(j . perea.                             O
              Order 3. Ganoidei.—With enameled plates or scales ; skeleton partly cartilaginous ; tail heterocercal; gills free: Acipenser.          O
              Order 4. Elasmobranohii.—Shagreen skin ;  cartilaginous skeleton ; tail heterocercal; gills fixed and uncovered: Squalus.            ^
              Order 5. Dipnoi.—No vertebral centra ; filiform fins ; heart with three cavities: Lepidosiren.                                     >

         Class  II. Amphibia-Temporary or permanent gills, but true lungs when adult; amnion and allantois wanting ; develop   >
                       through tadpole state ; two occipital condyles ; heart of three cavities ; skin soft.                              ^

              Order 1. Ophiomorpha__Snake-like ; no limbs : Ccecilia.
                                m ., ■.  ( gills permanent: Proteus.                                                                   O
              Order 2. Urodela.—Tailed; \  .„ f          .„ ,     .                                                                  o
                                       ( gills temporary: Salamandra.                                                               ^
                               „, .„     .„ .          ( no teeth: Bufo.                                                             O
              Order 3. Anoura.-Tailless ; gills temporary; j nppej. teeth Qnly. Rm^                                                     ^

         Class III.  Reptilia-Having lungs and scales ; one occipital condyle ; heart with three cavities ;  embryo with amnion and
                       allantois.
              Order 1. Ophidia.— Body elongated, scaly, and limbless ; numerous vertebrae j head with scales ; venomous: Crotalus.
                         and ribs;  no eyelids ; lower jaw loosely united in front;        I head with plates ; harmless:  Coluber.
                                                                                ( no limbs: Anguis.
              Order 2.  Laoertilia—Scaly; lower jaw firmly united in front;  eyelids;    -j fom. limbg. Lacerta_
                                                                                i natatory; Chelone.
              Orders.  Chelonia.—Horny and bony carapax; no teeth; eyelids; four legs ;•] amphibious: Chelydra.
                                                                                { terrestrial: Testudo. 
     Order 4. Cbocodima.—Covered with scales and bony plates; teeth in distinct fl0Dg SD°Ut: Gavmlis-
                sockets; heart with four cavities; evelids and earlids ;          I lower caniue9 flttiug int0 a notch:  Crocodil™-
                                                                            y. lower canines fitting into a pit: Alligator.

                                                § 2. Warm-blooded.

Class  IV. Aves.—Feathered; four limbs, hind pair for progression on land or water, front for flight; no teeth ; three eye-
               lids ;  one occipital condyle ;  heart with four cavities ;  lungs.

                                            * Hind toe wanting or elevated.
     Order 1. Natatores.—Swimmers; webbed toes; legs short:  Anser.
     Order 2. Grallatores.—Waders ; long necks, legs, and toes:  Ardea.
     Order 3. Cursores.—Runners ; short wings;  strong legs: Struthio.
     Order 4. Rasoreb.-Scratchers; strong, blunt claws; three toes front, one behind; arched bill:  Gallus.

                                          * * Hind toe on a level with the rest.
     Order 5. Soansores__Climbers; two toes in front and two behind: Picua.
     Order 6. Raptores.—Birds of prey; sharp, curved beak and talons; strong legs ; three toes front, one behind: Falco.
     Order 1. Insessoees.—Perchers; short, sleuder legs ; three toes front, one behind: Tardus.

Class   V.  Mammalia.—Suckle their young ;  red  corpuscles double-concave ; heart with four cavities ; lungs ; diaphragm ;
               body hairy;  two occipital condyles.

                                                   * Implacental.
     Order 1. Monotremata__Duck-billed ; webbed feet: Ornithorhynchus.
     Order 2. Maesupialia__With pouch for immature young:  Diadelphys.

                                               * * Placental; with fins.
     Order 3. Cetaoea.—Hind limbs wanting, front pair for swimming ; nostrils ou top fteeth 7antin£: Balcena.
                „,...„  .       .                                               < teeth in lower jaw: Physeter.
                of the head;  carnivorous;                                               .  ,
                                                                                L teeth in both jaws: Delphinua,
     Order 4. Sieenia.—Herbivorous cetaceans; nostrils at the end of the snout; molars in both jaws: Manatus. 
Subkingdom  Yeetebeata— Continued.
                                       *** Placental; with hoofs.
                                                ( even-toed; fourfold stomach:  Bos.
Order 5. TjNGULATA.-Vegetanan;  large, flat molars; j odd.toed. single stomach: Equus.

                                       **** Placental; with claws.
         „           ( Toothless: Myrmecophaga.
Order 6. Edentata.— ■{ T  .        ..     _   ,
                     I Incisors wanting: Bradypus.
      7. Rodentia.—Canines wanting ; incisors highly developed: Mits.
      8. Inseotivora.—Molars with sharp points: Scalops.
      9. Cheiroptera.—Fore-limbs webbed for flight:  Vespertilio.
      10. Peoboscidea.—Mammoth quadrupeds, with tusks and proboscis:  Elephas.
                                                                       c pinnigrade: Phoca.
Order 11. Caenivora__Flesh-eaters; canines well developed; molars trenchant; j plantigrade:  Ursus.
                                                                       I digitigrade: Felis.
Order
Order
Order
Order
Order 12. Primates.—Four incisors in each jaw; great toe with  hands on four limbs; teeth uneven; j nostrils apart: Cebus.
            fl„f„„5i.                                     ">                                  I nostrils close: Simia.
nostrils twisted: Lemur.
flat nail;
[ hands on fore-limbs only; teeth even; erect: Homo.
CO
n
o

>
>
H
h-1
<

N
o
o
o
o
Hi 
THE DISTRIBUTION OF ANIMALS.         357
                  CHAPTER XXIII.
             THE DISTRIBUTION OF ANIMALS.
   Life is everywhere.   In the air above, the earth  be-
neath, and the waters under the earth, we are surrounded
with life.  Nature lives: every pore is bursting with life;
every  death is only a  new birth,  every grave a cradle.
The air swarms with Birds, Insects, and invisible animal-
cules.   The waters are peopled with innumerable forms,
from the Protozoan, millions of which would not weigh a
grain, to the Whale, so  large that it seems an island as it
sleeps upon the waves.   The bed of the sea is alive with
Crabs, Shells, Polyps, Star-fishes, and Foraminifera.  Life
everywhere—on the earth, in the earth, crawling, creep-
ing, burrowing, boring, leaping, running.
   Nor does the vast procession end here.   The earth we
tread is largely formed of the debris of life.  The quarry
of limestone, the  flints which  struck the fire  of the old
Revolutionary muskets, are the remains of countless skele-
tons.   The major  part of the Alps, the Rocky Mountains,
and the chalk cliffs of England are the monumental rel-
ics of by-gone generations.   From the ruins of this living
architecture we build our Parthenons and Pyramids, our
St. Peters and Louvres.  So generation follows generation.
But we have not yet exhausted the  survey.  Life cradles
within life.   The bodies of animals  are little worlds hav-
ing their own fauna and flora.  In the fluids and tissues,
in the eye, liver, stomach, brain, and muscles, parasites are
found; and  these  parasites often have their parasites liv-
ing on them. 
358
COMPARATIVE ZOOLOGY.
      "Great fleas have little fleas and smaller fleas to bite 'em;
       And these again have other fleas, and so ad infinitum."
  Thus the ocean of life  is inexhaustible.   It spreads in
every direction, into time  past and present, flowing every-
where, eagerly surging into every nook and corner of cre-
ation.  On the mountain-top, in the abysses  of the Atlan-
tic, in the deepest  crevice of the earth's crust,  we find
traces  of animal life.  Nature  is prodigal of space, but
economical in filling it.an
  Animals are distributed over the globe  according to
definite laws, and with remarkable regularity.
  Each of the three great provinces, Earth, Air, and Wa-
ter, as also e\ery continent, contains representatives of  all
the classes; but the various classes are unequally repre-
sented. Every great climatal region contains some species
not found elsewhere, to the exclusion of some other forms.
Every grand  division of  the globe, whether of land or
sea, each zone of climate  and altitude,  has its own fauna.
And, in spite  of the many causes tending  to disperse ani-
mals beyond their  natural  limits, each country  preserves
its peculiar zoological physiognomy.
  The space occupied by the different groups of animals
is  inversely as the  size of the  individuals.   Compare the
Coral and Elephant.
  Fauna now occupying  a separate area  is closely allied
to the fauna which existed in geologic times.  Thus, Aus-
tralia has always been the home of Marsupials, and South
America of Edentates.
  It is a general rule that  groups of distinct species are
circumscribed within definite,  and  often narrow, limits.
Man is the only cosmopolitan ;  yet even he comprises sev-
eral marked races, whose distribution corresponds with the
great zoological regions.  The natives  of Australia are as
grotesque as the animals.   Certain brutes  likewise have a
great range: thus, the Puma ranges  from Canada to Pata- 
            THE DISTRIBUTION OF ANIMALS.        359

gonia; the Musk-rat, from the Arctic Ocean to Florida;
the Ermine, from Behring's Straits to the Himalayas; and
the Hippopotamus, from the Nile and Niger to the Orange
River.219
  Frequently species of  the  same genus, living side by
side, are widely different, while  there is a close  resem-
blance between forms which are antipodes.  The Mud-eel
of South Carolina and Axolotl of Mexico have their con-
necting links in Japan and Austria.  The American Tapir
has its mate in Sumatra; the Llama is related to the Cam-
el, and the Opossum to the Kangaroo.
  The chief causes  modifying distribution  are tempera-
ture, topography, ocean and wind currents,  humidity and
light.  To these  may  be  added the fact that animals are
ever intruding on each other's spheres of existence.  High
mountain-ranges, wide deserts, and  cold currents in the
ocean  are impassable  barriers to the migration of most
species.   Thus, river-fish on opposite  sides  of  the Andes
differ widely, and the cold Peruvian current prevents the
growth of coral  at  the Galapagos Islands.   So a broad
river, like the Amazons, or a deep, narrow channel in the
sea, is an effectual barrier to  some tribes.  Thus,  Borneo
belongs to the Indian region, while Celebes, though but a
few miles distant, is Australian in its life.   The faunae of
North America, on the east coast, west coast, and the open
plains between, are very different.
   Animals  dwelling at high elevations resemble those of
colder latitudes.   The same species of Insects are found
on Mount Washington, and in Labrador and Greenland.
   The  range does not depend upon the powers of loco-
motion.   The Oyster extends from  Halifax to  Charles-
ton, and  the Snapping-turtle from Canada to the equa-
tor ; while many Quadrupeds and Birds have narrow hab-
itats.
   The distribution of any group  is qualified by the nature 
360
COMPARATIVE ZOOLOGY.
of the food.  Carnivores have a wider range than herbi-
vores.
  Life diminishes as we depart from the equator north or
south, and likewise as we descend or ascend from the level
of the sea.
  The zones of geography have been divided by zoologists
into narrower provinces.   Five vertical regions in the sea
have been recognized:  the Littoral, extending between
tide-marks; the Laminarian, from  low water to 15 fath-
oms ;  the  Coralline, from 15 to 20 fathoms; the deep-sea
Coral, from 50 to 100 fathoms; and the Bathybian, from
100 fathoms  down.  Every marine species has its  own
limits of depth.  It would be quite as difficult, said Agas-
siz, for  a  Fish or  a Mollusk to cross  from the coast of
Europe  to the coast of America as for a Reindeer to pass
from the  arctic to the antarctic regions across  the torrid
zone.  Marine animals congregate mainly along the coasts
of continents and  on soundings.   The meeting-place of
two maritime currents  of  different temperatures, as on
the Banks  of Newfoundland,  favors the development of
a great diversity of Fishes.
  Every great province of the ocean contains some repre-
sentatives  of all the subkingdoms.   Deep-sea life is diver-
sified, though comparatively sparse.  Examples of all the
five invertebrate divisions were found in the Bay of Bis-
cay, at the depth of 2435 fathoms.219
  Distribution in the sea is influenced by the temperature
and composition of  the water, and  the character of the
bottom.   The depth acts indirectly by modifying the tem-
perature.   Northern animals approach nearer to the  equa-
tor in the sea than on the  land, on account of cold cur-
rents.  The heavy  aquatic Mammals, as Whales, Walruses,
Seals, and Porpoises, are mainly polar.
  The land consists of  the following  somewhat distinct
areas:  the Neotropic, comprising South America, Mexi- 
THE DISTRIBUTION OF ANIMALS.         361
co, and West Indies; the Neoarctic, including the rest of
America; the Palaearctic, composed of the eastern conti-
nent north of the Tropic of Cancer; the Africano-Indian,
or Africa south of Sahara, Southern Asia, and the western
half of the Malay Archipelago ;  and the Australian, or the
eastern half of the Malay Islands and Australia.
  Life in the polar regions is characterized by great uni-
formity,  the species  being few in number, though  the
number of individuals is immense.  The same  animals in-
habit the arctic portions of the three continents; while the
antarctic ends of the continents, Australia, Cape of Good
Hope,  and  Cape Horn exhibit  strong contrasts.   Those
three  continental  peninsulas are, zoologically,  separate
worlds.  In fact, the  whole southern hemisphere is pecul-
iar.  Its fauna is  antique.  Australia possesses a strange
mixture of the old and new.  South America, with newer
Mammals, has older  Reptiles; while Africa has a rich
vertebrate  life, with a striking uniformity in  its distribu-
tion.220
  In the tropics, diversity is the law.   Life is more varied
and  crowded than elsewhere, and attains its highest devel-
opment.
  The New-world fauna is old-fashioned, and inferior in
rank and size, compared with those of the eastern conti-
nents.
  As a rule, the more isolated  a region, the greater the
variety.  Oceanic  islands have comparatively few species,
but a large proportion of endemic or peculiar forms.  Ba-
trachians are generally absent,  and there are no indige-
nous terrestrial Mammals.   The productions are related to
those of  the nearest continent.  When an island, as Brit-
ain, is separated from the main-land by a shallow channel,
the mammalian life is the same  on both sides.
  Protozoans, Ccelenterates, and Echinoderms are limited
to the waters, and nearly all are marine.   Sponges are 
362
COMPARATIVE ZOOLOGY.
mostly obtained from the Grecian Archipelago and Baha-
mas.  Corals abound throughout the Indian Ocean and
Polynesia,  east coast of Africa, Red Sea  and  Persian
Gulf, West  Indies  and around  Florida.  True Crinoids
are found only in the Caribbean Sea and on the coast of
Norway.   The other Echinoderms abound in  almost ev-
ery sea, the Star-fishes chiefly along the shore, the  Sea-
urchins in the Laminarian zone, and the Sea-slugs around
coral-reefs.
  Mollusks have a world-wide distribution over land and
sea.  The land forms are restricted by climate and food,
the marine by shallows or depths, by cold currents, by
a sandy,  gravelly, or mud  bottom.   Living Brachiopods,
though few in number, occur  in  tropical, temperate, and
arctic seas, and  from  the  shore  to  the  greatest depths.
The rest of the Bivalves are  also found on every coast
and in every climate, as well as in rivers and lakes, but do
not flourish at the depth of much  more than 200 fathoms.
The fresh-water Mussels are more numerous in the United
States  than in Europe, and west  of the Alleghanies than
east.   The sea-shells along the Pacific coast of America
are unlike those of  the Atlantic, and are arranged in five
distinct groups—Aleutian, Californian, Panamic, Peruvian,
and Magellanic.   On  the  Atlantic coast, Cape Cod and
Cape Hatteras separate distinct provinces.  Of land-snails,
Helix has an almost universal range, but is characteristic
of North America, as Bulimus is of South America, and
Achatina of Africa.  The  Old World and America have
no species in common, except a few in the extreme north.
  The limits of Insects are  determined by temperature
and vegetation, by  oceans  and mountains.  There is an
insect-fauna  for  each continent, and zone, and altitude.
The Insects near  the snow-line on the sides of mountains
in the temperate region are similar to those in polar lands.
The Insects on our Pacific slope resemble those of Europe, 
THE DISTRIBUTION OF ANIMALS.        363
while those near the Atlantic coast are more like those of
Asia.  Not half a dozen Insects live in the sea.
  The distribution of Fishes is bounded by narrower lim-
its than that of other animals.  A few tribes may be called
cosmopolitan, as the Sharks and Herrings; but the species
are local.  Size does not appear to  bear any relation to
latitude.  The  marine forms are  three times as numerous
as the fresh-water.  The migratory Fishes of the northern
hemisphere pass to a  more southern  region in the spring,
while Birds migrate in the autumn.
  Living Reptiles form but a fragment of the immense
number  which prevailed  in the  Middle Ages of Geol-
ogy.  Being less under  the influence of Man, they have
not been forced from their original  habitats.  None are
arctic.   America is the  most  favored spot for  Frogs and
Salamanders, and India  for Snakes.   Australia  has  no
Batrachians, and two-thirds of its Snakes are  venomous.
In the United  States, only 22 out of 176 are  venomous.
Frogs, Snakes, and  Lizards occur at elevations  of  over
15,000 feet.   Crocodiles, and most  Lizards and  Turtles,
are tropical.
  Swimming Birds, which constitute about one-fourteenth
of the entire class, form one-half of the whole number in
Greenland.  As we approach  the tropics, the variety and
number of land Birds increase.  Those of the torrid zone
are  noted for their brilliant plumage, and the  temperate
forms for their more  sober hues,  but  sweeter voices.   In-
dia  and  South  America are the  richest regions.  Birds
with rudimentary  wings, as Penguins and Ostriches, pre-
vail  in  the southern  hemisphere.  Hummers,  Tanagers,
Orioles, and Toucans  are restricted  to the New  World.
Parrots  are found  in  every continent, except Europe; and
Woodpeckers occur everywhere, save in Australia.
  The vast majority  of Mammals are terrestrial;  but Ce-
taceans  and Seals  take to the sea, Otters and Beavers de- 
364
COMPARATIVE ZOOLOGY.
light in lakes and rivers, and Moles are subterranean.  As
of Birds, the aquatic species abound in the polar regions.
Marsupials inhabit two widely separated areas—America
and 'Australia.   In the latter continent, they constitute
three-fourths of the fauna; while Edentates, Ruminants,
Fia. 351.—Zones of Animal Life.
Horses, Elephants, Hogs, Squirrels, Moles,  Carnivores,
Monkeys,  and Apes are wanting.  Excepting a few spe-
cies in South Africa and  South Asia, Edentates are con-
fined  to tropical South America.  The equine  family is
indigenous to South and East Africa and Southern Asia.
In North America, Rodents form about one-half the num- 
            THE DISTRIBUTION OF ANIMALS.         365

ber of Mammals;  they are entirely wanting in Madagas-
car.   Ruminants are sparingly  represented in America.
Carnivores flourish in every zone  and continent.   The
prehensile - tailed Monkeys are strictly South American;
while the anthropoid Apes belong to the west coast of
Africa, and  to Borneo and Sumatra.  Both Monkeys and
Apes are  most abundant  near the equator; in fact, their
range is limited by the distribution of palms. 
 
NOTES.
  1 The complete and elaborate natural history of a single species or limited
group is called  a Monograph, as Darwin's " Monograph of the Cirripedia."
A Memoir is not so formal or exhaustive, giving mainly original investiga-
tions of a special subject, as Owen's "Memoir on the Gorilla."
  3 Before  the time of Linnaeus, the Lady-bug, e. g., was called "the Cocci-
nella with red coleopters having seven black spots."  He called it Coccinella
septem-punctala.
  3 Mandino (1315) and Berenger (1518), of Bologna, and Vesalius,  of Brus-
sels (1550), were the first anatomists.  Circulation  of the  blood discovered
by  Harvey, 1616.  The lacteals  discovered by Asellius, 102:3, and the lym-
phatics by Rudbek, 1650.  Willis made the first minute anatomy of the brain
and nerves, 1664.  The red  blood-corpuscles were  discovered by Leeuwen-
hoek and Malpighi, 1675.  Infusoria first observed  by  Leeuwenhoek, 1675;
the name given by Miiller, 1786.  Swammerdam was the  founder  of Ento-
mology, 1675.   Comparative anatomy was first cultivated  by Perrault, Pec-
quet, Duverney, and Me"ry, of the Academy of Paris, the latter part of the
seventeenth century.  Malpighi, the founder of structural  anatomy, was the
first to demonstrate the structure of the lungs and skin,  1690.  About the
same time, Ray and Willoughby first classified Fishes on structural grounds.
Foraminifers were seen by Beccarius one hundred  and fifty  years ago; but
their true structure was not demonstrated till 1835, by Dujardin.  Peyssonel
published the first elaborate treatise on Corals, 1727.  Haller  was the first to
distinguish between contractility and sensibility, 1757. White blood-corpus-
cles discovered by Hewson in 1775.  Spallanzani was the first  to demonstrate
the true nature of the digestive process, 1780.  Cuvier and  Geoffroy, in 1797,
proposed the first natural classification of animals.   Before that, all Inverte-
brates were divided into Insects and Worms.  Lamarck was the first to study
Mollusks, 1800;  before him, attention was confined to  the shell.  He sepa-
rated Spiders from Insects in 1812.   The law of correlation enunciated by
Cuvier, 1826.  Von Baer was the founder of Embryology, establishing the
doctrine omnia ex ovo, 1827;  but the first researches  in  Reproduction  were
made by Fabricius about 1600, and by Harvey in 1651.   Wolff, early in this
century, was the pioneer in observing the phenomena of Development.  Sars
first observed alternate generation, 1833.   Dume>il is considered the father
of Herpetology, and Owen of Odontology.  Schleiden  and  Schwann  pub-
lished their celebrated researches  in cell-structure, 1841;  but Bichat, who
died 1802, was the founder  of Histology.  Protoplasm was discovered by
Dujardin. 
368
NOTES.
  * This twofold division is arbitrary.  No essential distinction, founded on
the nature of the elements concerned, or the laws of their combination, can
be made;  and so many so-called organic substances, as urea, ammonia, alco-
hol, tartaric and oxalic acids, have been prepared by inorganic methods, that
the boundary-line is daily becoming fainter, and may in time vanish alto-
gether.   It should be added, however, that "those organic compounds
which have been  artificially formed are invariably products of decomposition,
or, in other words, the excretions or secretions of organized bodies; and are
far less  complex in their constitution than organized structures."—Grego-
ry's  Organic  Chemistry.   " Chemical synthesis has  in reality reproduced
only matters unfitted for life; that is to say, mineral matters."—M. Dumas.
We would here utter our protest against the introduction of any more terms
like inorganic, invertebrate, acephalous, etc., which express no qualities.
  5 Even the works of nearly all animals proceed in circles or segments of
circles.
  6 London Quarterly Review, January, 1869, p. 142.   It is true of any great
primary group of animals, as of a tree, that it is much more easy to define
the summit than the base.
  7 De Bary on "Myxomycetae;" Darwin on "Carnivorous Plants."
  8 This, of course, is  not  universally true.  If we regard a tree as an asso-
ciation  of phytons, or plants, instead of an individual, then each leaf and
petal when developed is perfect and abiding, like the separate Polyps of a
compound Coral.  Some consider every organ a distinct  individual  exist-
ence ; in this view, an animal, like a tree, is a compact community.
  9 It should  he  noted that plants  evolve carbonic acid  only when in a
state  of decomposition or exhaustive process, not during normal, vigorous
growth. Both animals and plants in decay consume oxygen.  " There is
every reason to believe that carbonic acid is continually given off from  the
interior  of plants, while oxygen  is absorbed."—Carpenter.  It is interest-
ing to compare the temporary  respiratory organ of plants, the cotyledon,
with the gills of  a tadpole:  both disappearing when the evolution of  the
permanent apparatus renders them unnecessary.
  10 There  are certain phenomena, even among the higher plants, connected
with the habits of climbing plants and with the functions of fertilization,
which it is very difficult to explain without admitting some low  form of a
general  harmonizing and regulating fanction, comparable to such an obscure
manifestation  of reflex nervous action as we have in Sponges and in oth-
er animals  in  which a distinct  nervous system is absent.—Prof. Wtville
Thomson's Introductory Lecture at Edinburgh.
  11 If nature had endowed us with microscopic powers of vision, and  the
integuments of plants had been rendered perfectly transparent to our eyes,
the vegetable world would present a very different aspect from the apparent
immobility and repose in which it is now manifested to our senses.—Hum-
boldt's Cosmos, L, 341.
  12 See Gray's "Structural Botany," p. 350; Rolleston's "Forms of Animal
Life," p. 143.
  13 We may safely say that there is no plant which may not  serve as food
for some animal. 
NOTES.
369
  14 Life has been called the vital force, and it has been suggested that it
may be found to belong to the same category as the convertible forces, heat
and light.  Life seems, however, to be more a property of matter in a certain
state of combination than a force.  It does no work, in the ordinary sense.
—Prof. Wyville Thomson.  The recent experiments of Robert Hamilton
tend to prove the existence, in every highly  developed organism, of two
lives:  a life resident in  every atom of the structure, however complex, and
another life for which we fail to find an expression. But the latter is the
life which keeps together the structure as  a whole:  it is the life that selects
the nutrition best suited to its individual self; it is the life that has to do
with the continuation of the species; lastly, it is the life to  which the mo-
lecular lives, which make up the structure, are subordinated; and when this
nameless life departs, these myriad lives, no longer co-operating, start on an
independent course.
  15 There was a time in our history when a single membrane discharged all
the functions of life—digesting, respiring, secreting.  The separation of a
heart,  lung, stomach, liver, etc., for special duty, was an after-consideration.
  16 The vegetable cell has usually two  concentric coverings:  cell-wall and
primordial utricle.   In  animal cells the former is wanting,  the membrane
representing the utricle.  As  a general fact, animal cells are smaller than
vegetable cells.
  17 Cells are not the sources  of life, as once thought, but are the products
of protoplasm.   " They are no more the producers of vital phenomena than
the shells scattered in orderly lines along the sea-beach are the instruments
by  which the gravitation-force  of the  moon acts  upon the  ocean.  Like
these,  the cells  mark only where the vital  tides have been and how they
have acted."—Prof. Huxley.
  18 The white fibres are inelastic, and from ^J^  to 55^00 of an  incn in
diameter.  They are best seen in the tendons.   The yellow fibres are elastic,
curled at the ends, very long, and from 5I^5o to sihJS of an inch in diam-
eter.   They are  shown in the hinge-ligameut of an Oyster.  Connective tis-
sue appears areolar, i. e., shows interspaces, only under the microscope.
  19 Certain bones, as those of the face and forehead, are preceded by mem-
branes instead of cartilage.
  30 In the heart, the muscular fibres are striated, yet involuntary; but the
sarcolemma is wanting.
  31 Other names are medullary sheath and white substance of Schwann.
  83 We may, however,  infer that the animal  functions are  not absolutely
essential to the vegetative, from the facts that plants digest without mus-
cles or nerves, and that  nutrition takes place in the embryo long before the
nerves have been developed.
  33 This is not  strictly  true, for the Elm and Oak, the Trout and Alligator,
do reach a maximum size.
  ** The suctorial Insects always subsist  upon one and the same kind of
food.                                                         .
  « Scorpions and Spiders properly feed  upon the juices of their victims
after lacerating them with their elaws;  b-ut fragments of Insects have  been
found in their stomachs. 
370
NOTES.
  26 At one end of the Tape-worm is a minute pore, by some considered the
mouth, with a circlet of spines and four suckers.
  27 The real tongue forms the floor of the mouth, and is found as a distinct
part in a few Insects, as the Crickets.
  28 In a few Fishes, it is circular or oval.
  29 The mouth of the Whale is exceptional, the walls not being dilatable.
The act of sucking is characteristic of all young Mammals, hence the need
o flips.
  30 The Ant-eater has two callous  ridges in the mouth, against which  the
insects are crushed by the action of the tongue.
  31 Strictly speaking, the  baleen plates do not represent teeth; for in  the
embryo of the Whale we find minute calcareous teeth in both jaws, which
never cut  the gum.   The whalebone is probably a peculiar development of
hair in the palate, and under the microscope it is seen to be made up of
fibres which are hollow tubes.
  32 The "tusks" of the  Narwhal  and Elephant are prolonged incisors;
those of the Walrus and Wild Boar are  canines.
  33 " I was one day talking with Professor Owen in the Hunterian Museum,
when  a gentleman approached, with a request to be informed respecting the
nature of  a curious  fossil which had been dug up  by one of his workmen.
As he drew the fossil from a small bag, and was about to hand it for exam-
ination, Owen quietly remarked, ' That is the third molar of the under jaw
of an extinct species of rhinoceros.' "—Lewes's Studies in Animal Life.
  34 This  gap or interspace, so  characteristic  of the inferior Mammals, is
called diastema.  It is wanting in the extinct Anoplotherium, and is hardly
perceptible in one of the Lemurs.
  35 In the Spermaceti-whale, the teeth are fixed to  the gum.
  36 The Iguana among Reptiles, and Fishes with pavement-teeth, approach
the Mammals in this respect.
  37 This  movement  is called peristaltic or  vermicular, and characterizes all
the succeeding movements  of the alimentary canal.
  38 Fishes and Amphibians have no saliva, but a  short gullet.  Birds  are
aided  by a sudden upward jerk of the head.
  39 Fishes and Reptiles have no pharynx  proper, the nostrils and glottis
opening into the mouth.
  40 This  movement of the pharynx and oesophagus is wholly involuntary.
Liquids are swallowed in exactly the same way as solids.
  41 The few  animals in which  the digestive  cavity is wanting are called
agastric, and agree in having a very simple structure, and in being parasitic.
Such are some Entozoa (as Tape-worm), and unicellular Protozoa (as Grega-
rina).   They absorb  the juices, already prepared, by the physical process of
endosmose.  There are other minute organisms which seem to be able to
extract the necessary elements,  CHON, from the medium in which they
live.
  42 Moreover, as a Sponge is an aggregation of animals, these canals are for
a community, not for a single individual.  According to Alexander Agassiz,
the Ctenophorae have a true  alimentary canal, passing through the body-
cavity. 
NOTES.
371
  43 " Nothing is more curious and entertaining than to watch the neatness
and accuracy with which this process is performed.  One may see the reject-
ed bits of food passing rapidly along the lines upon which these pedicellariae
occur in greatest number, as if they were so many little roads for the con-
veying away of the refuse matters; nor do the forks cease from their labor
till the surface of the animal is completely clean and free from any foreign
substance."—Agassiz's Sea-side Studies.
  44 In the larva of the Bee, the anal orifice is wanting.
  45 The length of the canal in Insects is not so indicative of the habits as in
Mammals.  Thus, it is nearly as long and more complicated in the carnivo-
rous Beetles than in the honey-sipping Butterflies.
  46 The object of this is unknown.  It does not occur in the Oyster.
  47 In the Nautilus, this is preceded by a capacious crop.
  48 In the Shark, this is impossible, owing to a great number of fringes in
the gullet hanging down toward the stomach.
  49 At the beginning of the large intestine in the Lizards (and in  many Ver-
tebrates  above them, especially the vegetarian orders), there is a blind 6ac,
called caecum.  The worm-like appendage to the caecum is  almost peculiar
to Man and the Apes.
  50 The Crocodile is said to swallow stones sometimes, like  Birds, to aiu
the gastric mill.
  51 In the crop of the common Fowl, vegetable food  is detained sixteen
hours, or twice as long as animal food.  The Dormouse, among  Mammals,
has an approach to a crop.
  62 In Mollusks, the gizzard,  when present, is situated between the crop
and the true stomach ; in Birds, it comes after the stomach.
  63 The fourth stomach of Ruminants is the largest so long as the animal
sucks.
  64 The Tape-worm has no digestive apparatus, and " flesh which is decom-
posed by decay into a semi-fluid mass is absorbed by the sponge-like bodies
of certain animals which live in stagnant pools" (Clark); but these are not
real exceptions to the rule.  In both  cases, transmutation  goes  before ab-
sorption.
  55 As starch is a vegetable product, we would look for the most abundant
saliva in those mammals that feed on herbs and grain; and  such is the fact.
Moreover,  as sugar  is heat-producing, in  cold-blooded Reptiles, Fishes,
Mollusks, and other like carnivores, a fluid to convert 6tarch into  sugar
would be out of place.
  66 These  substances  are only dissolved and chemically modified (being
converted into what are termed peptones), not "organized" or "vitalized."
  57 It is probable that the digestive part of the alimentary canal in all ani-
mals manifests a similar mechanical movement. It is most remarkable in
the gizzard of a fowl, which corresponds to  the pyloric end of the human
stomach.  This muscular organ, supplying the want of a masticatory appa-
ratus in the head, is powerful enough to pulverize, not only grain, but even
pieces of glass and metal.  This is  done by two hard muscles moving ob-
liquely upon each other, aided by gravel purposely swallowed by the bird.
The grinding may be heard by means of the stethoscope. 
372                           NOTES.
  58 Chyle is opaque in carnivores;  more or less  transparent in all other
Vertebrates, as in Birds, since the food does not contain fatty matter.
  69 In Fishes, the villi are few  or wanting.  In Man, they  number about
10,000 to the square inch.
  60 Except, probably,  the brain, spinal-marrow, bones, tendons, ligaments,
epidermis, etc.
  61 The lacteals  also carry lymph when the intestine is empty and they
have nothing else to do.
  62 The phenomenon produced by these properties conjointly, capillary at-
traction and diffusion, is called endosmosis.
  63 The blood is  colorless also in the muscular part of all Fishes.  That of
Birds is of the deepest red.   The coloring matter of the red blood in Worms
is not in the corpuscles, but in the plasma.
  64 Coagulation in the living body  is mainly prevented by being kept in
constant motion.   It may be artificially arrested by common salt.  Arterial
blood coagulates more  rapidly than venous.  The disposition of the red cor-
puscles in chains, or rouleaux, does not occur within the blood-vessels.  The
cause has not been discovered.
  65 The corpuscles of Invertebrates are usually colorless, even when the
olood is tinged.  It should be  observed that those animals whose blood
resembles chyle or lymph have no lacteals or lymphatics.
  66 Except during foetal life.  The corpuscles of the Camel  are non-nucle-
ated, as in other  Mammals.—If the transparent fluid from a boil be exam-
ined with a microscope, it will be seen to be almost composed of colorless
corpuscles, showing their U6e in repairing injuries.
  67 There are no valves in the  veins of Fishes, Reptiles, and Whales, and
few in Birds.
  68 Capillaries are wanting in the epidermis, nails, hair, teeth, and carti-
lages.  Hence, the epidermis, for example, when  worn out  by use, is not
removed by the blood,  like  other tissues, but is shed.
  69 A part of the blood, however, in going from the capillaries to the heart,
is turned aside and made to pass  through the liver  and kidneys for purifica-
tion.  This is called the portal circulation, and exists in all Vertebrates, ex-
cept that in Birds and Mammals it is  confined to the liver.
  70 Two in the higher Mammals, three in the lower  Mammals, Birds, and
Reptiles.  They are called venae cavos.
  " Tricuspid  in Mammals, triangular in Birds.
  72 The pulse of a Hen is 140; of a Cat, 110 to 120; of a Dog, 90 to 100; and
of an Ox, 25 to 42.
  73 The bivalve Braehiopods, having no gills, breathe by delicate arms about
the mouth, or by the " mantle."
  74 The  air-bladder, found  in most Fishes, is another rudiment of a lung,
although it is used, not for respiration, but for altering the specific gravity
of the Fish.  In the Gar-pike of our  Northern lakes, it very closely resem-
bles a lung, having a cellular structure, a tracheal tube, and a glottis.  The
gills represent lungs only in function ;  they are totally distinct parts of the
organism.  (See Huxley, " Anatomy of Vertebrated Animals," p. 92.)
  7S Well seen in Tadpoles, or young  of the Frog. 
NOTES.
373
  76 In the human lungs, they number 600,000,000, each about tJq of an inch
in diameter, with an aggregate area of 132 square feet.  The thickness of the
membrane between the blood and the air is 2^0 of an inch.  The lungs of
Carnivores are more highly developed  than  those  of Herbivores.  In the
Manatee, they are not confined to the thorax, but  extend  down nearly to
the tail.
  77 Crocodiles are the only Reptiles whose nostrils open in the throat be-
hind the palate, instead of directly into the mouth-cavity.  This enables the
Crocodile to drown its vietim without drowning itself;  for, by keeping its
snout above water, it can breathe while its mouth is wide open.
  78 A rudimentary diaphragm is seen in the Crocodile and Ostrich.
  79 The poison-glands of venomous Serpents and the silk-vessels of Cater-
pillars are considered to  be modified salivary glands.  Birds, Snakes, and
Cartilaginous Fishes have no urinary bladder.
  80 Since the weight of a full-grown animal remains nearly uniform, it  must
lose as much as it receives; that is, the excretions, including the solid re-
siduum ejected from the intestinal  canal, equal the food and drink.
  81 Other names for derm are, cutis, corium, enderon, and true skin; and for
epidermis, cuticle, ecderon,  and scarf-skin.  The derm is often so intimately
blended with the muscles, that its  existence as a distinct layer is not easily
made out. Even in, Infusoria, we find the tunic double, an outside cuticula
lined by a soft cortical layer; and in Jelly-fishes, naturalists distinguish an
ectoderm and endoderm.
  82 See Fig. 146.  Papillae are scarcely visible in the skin of Reptiles and
Birds.
  83 The animal basis of this structure is chitine, a peculiar  substance found
in the hard parts of all the articulated animals.
  84 The large claws within the old crust are soft, and hence are able  to be
drawn through the small joints.
  85 The shell is always an epidermal structure, even when apparently inter-
nal.  The horny "pen" of the Squid, the "bone" of the Cuttle-fish, and the
calcareous spot on the back of the Slug, are only concealed under a fold of
the mantle.  So the shell  of the common Unio, or Fresh-water Clam, is cov-
ered with a brownish or greenish membrane, which is  the outer layer of the
epidermis.  Where the mantle covers the lips of a shell, as in  most of the
large sea-snails, or where  its folds cover the whole exterior, as  in the pol-
ished Cowry, the epidermis is wanting, or covered up by an  additional layer.
  86 The pearls of commerce, found in the mantle of some Mollusks, are simi-
lar in structure to the shell;  but what is the innermost layer in the shell, is
placed on the outside in the pearl, and is much finer and more compact.  The
pearl is formed around some nucleus, as an organic particle,  or grain of sand.
  87 When the centrum is concave on both sides, as in  Fishes, it is said to be
amphicoelous; when concave in front and convex behind, as  in Crocodiles, it
is called proccelous ; when concave behind and  convex in front, as in the  neck-
vertebra; of the Ox, it is opisthocoelous.   In the last two cases, the vertebrae
unite by ball-and-socket joints.
  88 A few have but one pair, the Whale and Siren wanting the hind  pair;
while some have none at all, as the Snakes and lowest Fishes.  In land ani- 
374
NOTES.
mals, the posterior limbs are generally most developed;  in aquatic animals,
the anterior.  Dr. Wyman contends that the limbs are tegumentary organs,
and attached to the vertebral column in the same sense that the teeth are
attached to the jaws.
  89 The muscles of some Invertebrates, as Spiders, are yellow.
  90 The muscles of the heart and gullet are striped.  In the lowest animals,
these distinctions of voluntary and involuntary, striated and smooth, solid
and hollow, muscles can seldom be made.
  91 The skeleton of the Carrion-crow, for example, weighs, when dry, only
23 grains.
  92 The Dragon-fly can outstrip the swallow, nay, it can do in the air more
than any bird—it can fly backward and sidelong, to right or left, as well as
forward, and alter its course on the instant without turning.  It makes 28
beats per second with its wings; while the Bee makes 190, and the House-
fly 330.  The swiftest Race-horse can double the rate of the Salmon.  So
that Insect, Bird, Quadruped, and Fish would be the order according to ve-
locity of movement.
  93 These suckers (pulvilli) have a delicate fringe of hairs, each hair being a
minute tube containing a viscid fluid by  which the Fly adheres.
  94 The cilia of Infusoria appear to act independently of any nervous power.
  95 More precisely, the term brain, or brains, applies only to the cerebrum,
while the total contents of the cranium are called encephalon.
  96 The exact functions of the cerebrum are not yet clearly understood.
If we remove it from Fishes, or even Birds, their voluntary movements are "
little affected; while the Amphioxus, the lowest of Fishes, has no brain at
all, but  its life is regulated by the spinal cord.   Such  mutilated animals,
however, make no intelligent efforts.   The substance of the cerebrum, as
also the cerebellum, is insensible, and may be cut away without pain to the
animal;  and when both are thus removed, the animal still retains sensation.
  97 Parts destitute of blood-vessels, as hair, teeth, nails, cartilage, etc., are
not sensitive.  The impressibility of the nerves is proportioned to the activ-
ity of circulation.  According to the recent investigations of Dr. Bowditch,
the channels of motor and sensitive impressions lie in the lateral, and not in
the anterior and posterior, columns of the spinal cord.
  98 "Tentacles" and "horns" are more or less retractile, while antennae
are not,  but all are hollow.  Antennae alone are jointed.
  99 In Man, the soft palate and tonsils also have the power of tasting.
  100 No organ of hearing has been discovered with certainty in the Radiates
and Spiders.
  101 It is wanting in the aquatic mammals.  Crocodiles have the first rep-
resentative of an outside ear in the form  of two folds of skin.
  102 This, like the definition of smell and hearing, is loose language.  There
is no such thing as sound till the vibrations strike the tympanum, nor even
then, for it is  the work of the brain, not of the auditory nerve.  Sound is
the sensation of the wave-movement of  the air, and hearing is that sensa-
tion.  So without eyes the world would be wrapped in darkness;  light is
nothing.
 'I03 In  Invertebrates and aquatic Vertebrates, the crystalline lens is globu- 
NOTES.
375
lar;  or, in other words, it is round in short-sighted animals, and flattish in
the long-sighted.   The lens of the Invertebrate is not exactly the same as
the lens of the Vertebrate eye, though it performs the same function;  it is
really a part of the cornea.
  104 The Ant has 50 in each eye, the House-fly 8000, the Dragon-fly 12,500.
  105 The pigment, therefore, while apparently in front of the retina,  is  real-
ly behind it, as in Vertebrates.  The layer beneath the cornea, serving as an
"iris," is wanting in nocturnal insects, since they need every ray of light.
The optic nerve alone is insensible to the strongest light.
  106 It should be noticed that this corresponds with another peculiar fact
already mentioned, that either hemisphere of the brain controls the muscles
on the opposite side of the body.  In Invertebrates the motor apparatus is
governed on its own side.
  107 Sharks have eyelids, while Snakes have none.  The third eyelid  (called
nictitating membrane) is rudimentary in Mammals.
  108 An infant would doubtless learn to walk if brought up by a wild beast,
since it was made to walk.  Just  as an infusorium moves  its cilia, not be-
cause it has any object, but because it can move them.  New-born puppies,
deprived of brains, have suckled; and decapitated centipedes run rapidly.
Such physical instincts exist without mind, and may be termed  "blind im-
pulses."
  109 We say  "apparently," because it may be a fixed habit, first learned by
experience, transmitted from generation to generation.  A duckling may go
to the water, and a hound may follow game  in some sense, as Sir John  Her-
schel takes to astronomy, inheriting a taste  from his father.  Breeders  take
advantage of this power of inheritance.
  no "Thus,  while the human organism may be likened to a keyed  instru-
ment, from which any music it is capable of  producing can be called forth
at the  will of the performer, we may compare a Bee, or any other Insect,
to a barrel-organ, which  plays with the greatest exactness a certain num-
ber of tunes that  are set upon it,  but can do  nothing else."—Carpenter's
Mental Physiology, p. 61.  This constancy may be largely  due  to the  uni-
formity of conditions under which Insects live.
  111 We may say, as a rule, that the proportion of instinct and intelligence
in an animal corresponds to the relative development of the spinal cord and
cerebrum.  As a  rule, also, the addition of the power to  reason  comes in
with the addition of a cerebrum,  and is proportioned to its development.
Between the  lowest Vertebrate and Man, therefore, we observe successive
types of intelligence.  Intelligence, however, is not according to the size of
the brain  (else Whales and  Elephants  would be wisest), but rather  to the
amount of gray matter in it.   A honey-comb  and  an Oriole's nest are  con-
structed with more care and art than the hut of the savage.  It is true, this
is no test  of the capability of the  animal in any other direction; but when
they are fashioned to suit  circumstances, there is proof of intelligence in
one direction. Physiologists now hold that the cerebrum is not essential to
consciousness.
  112 Air-breathing Vertebrates, as the Manatee and Sea-lion, are exceptions.
There are, doubtless, many sounds we can not hear.  The noise  of a  Spider 
376
NOTES.
may be terrific to a Fly;  and while Flies hear one another, they take no
notice of the human voice.
  1,3 An exception to the general rule that the smaller animals have more
acute voices.
  114 It is wanting in a few, as the Storks.
  115 The Nightingale and Crow have vocal organs similarly  constructed,
yet one sings, and the other croaks.
  116 The three methods are substantially alike; for an egg is only a sepa-
rated bud.   In the lower  organisms, the parent is completely broken up
into new individuals; in the higher, new individuals form but  an infinitesi-
mal part (germ) of the parent.  Under any form, reproduction is a process
of disintegration.   Some Protozoa do not appear to produce by eggs.
  1,7 These cells are detached portions, or buds, of the parental organisms.
Generally, these two kinds of cells are produced by separate sexes;  but in a
few cases, as the Snail, they originate in the same individual.  Such an ani-
mal, in whom the two sexes are combined, is called an hermaphrodite.
  1,8 If an egg be violently shaken, this connection is broken; and this is
the secret of making an egg stand on end without breaking it, as Columbus
is said to have done.
  119 The eggs of Mammals are of nearly uniform  size;  those  of Birds, In-
sects, and most other animals are  proportioned to  the  size of the  adult.
Thus, the egg  of the ^Epyornis, the great extinct bird  of Madagascar, has
the capacity of 50,000 Humming-birds' eggs.
  120 As a general rule, when both sexes are of gay and conspicuous colors,
the nest is such as  to conceal the sitting Bird; while, whenever there is a
striking contrast of colors, the male being gay and the female dull, the  nest
is open.  Such as form no nest are many of the Waders, Swimmers, Scratch-
es, and Goatsuckers.
  121 As the  Crocodile, by its gizzard and its rude nest, looks forward, so the
pouched Kangaroo looks backward,  to the true ornithic type.
  122 This rudiment lies transversely to the long axis of the egg;  and as the
chick develops, it turns upon its side, so that the forepart of the head usual-
ly faces the narrow end of the egg.
  123 The blood comes into being before the blood-vessels, and veins before
arteries; i. e., the very first motion  is toward the heart.  The blood is first
yellowish.   The red corpuscles are supposed to be derived from the nuclei
of the white corpuscles : the origin of the latter is undetermined.
  124 Exactly as the blood in the capillaries of the skin is aerated by the ex-
ternal atmosphere.
  125 Thus, the hollow wing-bone was first  solid, next a marrow-bone, and
finally a thin-walled air-cell.  The solid bones of the Penguin are examples
of arrested development.
  136 The thigh-bone of the child consists of five distinct parts; in the adult,
they are united into one.
  127 Muscle is mainly fibrine, while nerve is chiefly albumen.
  128 This generalization must not be confounded with the old statement,
which is not true, that the higher  animals  pass through all the phases of
the lower life.  See Spencer's "Principles of Biology,"  i.,  143; Clark's
" Mind in Nature." 159. 
NOTES.
377
   129 For this reason, Mammals are called viviparous; but, strictly speaking,
 they are as oviparous as Birds.  The process of reproduction is the same,
 whether the egg is  hatched within the parent or without.  The eggs of
 Birds contain whatever is wanted for the development of the embryo, ex-
 cept heat,  which must come from without.  Mammals, having no food-
 yolk, obtain their nutrition  from the blood of the parent, and after birth
 from milk.   Most of the sharks are viviparous.
   130 The larvae  of Butterflies and Moths are  called caterpillars; those of
 Beetles, grubs; those of Flies, maggots; those of Mosquitoes, wigglers.—The
 terms larva, pupa, and imago are  relative only; for, while the grub and cat-
 erpillar are quite different from the pupa, the bee-state is reached by a very
 gradual change  of form, so  that it is  difficult to say where the pupa ends
 and the imago begins.  In fact,  a large number  of Insects reach maturity
 through an indefinite number of slight changes.  The Humble-bee  molts at
 least ten times before arriving at the winged state.
   131 Every tissue of the caterpillar disappears before the  development of
 the  new tissues of the  imago is commenced.  The organs do not change
 from one into the other; but the new set is developed out of formless mat-
 ter.—The pupa  of the Moth is protected by a silken  cocoon, the spinning
 of which was the last act of the larva; that of the Butterfly is  simply in-
 closed in the dried skin of the larva, which is called chrysalis because of its
 golden spots. The pupa of the Honey-bee is called nymph; it is kept in a
 wax-cell lined with  silk, spun by the nursing-bee, not by the larva.  The
 time required to pass from  the  egg to the imago varies greatly:  the  Bee
 consumes less than twenty days, while the Cicada requires seventeen years.
   132 Compare the amount of food required in proportion tp the bulk of the
 body, and also with the amount of work done, in youth,  manhood, and old
 age.
   133 Excepting, perhaps, that the new tail of a Lizard is cartilaginous.
   134 The patella, or knee-pan, has no representative in the fore-limb, and,
 strictly, it  belongs  to  the muscular system rather than to  the skeleton.
 Some anatomists contend that the great toe is homologous with the little
 finger, instead of the thumb.
   13i It is doubtful whether the dorsal tube of Insects and the heart of Mam-
 mals are homologous, as the circulatory organs of Invertebrates may prove
 to be homologous with the lymphatic system of Vertebrates.  The jaw-
 bones and  limb-bones of Vertebrates are homologous, according  to some
 naturalists.
   136 Polarity begins, as we  have already seen, in the very first change of
:m egg.
  137 The structure  of the highest plants is more complex than is that of
 the lowest  animals;  but, for all  that, powers are possessed by jelly-fishes
of which oaks and cedars are devoid.—Miyart.
  138 It is, however,  true that the number of eggs laid is proportioned to
the risk in development.
  139 According to Mr. Darwin, the characters which naturalists consider as
showing  true affinity between any two or more  species are those which
have been inherited from a common parent, and, in so far, all true classiti- 
378
NOTES.
cation is genealogical;  i. e., it is not a mere grouping of like with like, but
it includes like descent, the cause of similarity.  In the  existing state of
science, a perfect classification is impossible, for it involves a perfect knowl-
edge of all animal structure and life's history.  As it is, it is  only a provis-
ional attempt to express the real order of nature, and it comes as near to it
as our laws do in  explaining phenomena.   It  simply states  what we now
know about comparative anatomy and physiology.  As science grows, its
language will become more precise and its classification more natural.
  140 The term type is also used to signify that form which presents all the
characters of the group most completely.  Each genus has its typical spe-
cies, each order its typical genus, etc.  The word  is also applied to the
specimen on which a new species is founded.  A persistent type is one which
has continued with very little change through a great range of time. The
family of Oysters has existed through many geological ages.
  141 The Coelenterata and Echinodermata together make up the Sadiata,
the old  subkingdom of Cuvier.  Echinoderma is probably more correct than
Echinodermata ; but we retain the old orthography.
  142 Strictly speaking, no  individual is independent.  Such is the division
of labor in a hive, that a single  Bee, removed from the community, will
soon die, for its life is bound up with the whole.  In a philosophical sense,
Man is a composite being, every organ being an individual, though not an
independent, existence.  An individual repeats the type of its kingdom, sub-
kingdom, class, order, family, genus, and species, through its whole line of
descent.
  143 The Millepore coral, so abundant in the West Indian Sea, is the work
of Hydroids.  The surface is nearly smooth, with minute punctures.  Ge-
genbauer, Haeckel, and others hold that the Acalephs have no body-cavity
at all, the internal system of canals being homologous with the intestinal
cavity of other animals.
  144 Among the exceptions are Tubipora, which have eight tentacles and
no septa, and the extinct Cyathophylla, whose septa are eight or more.
  145 The longest septa (called primary) are the oldest: the shorter, second-
ary ones, are developed afterward.  As a rule, sclerodermic  corals are cal-
careous, and a section  is star-like; the sclerobasic are  horny and solid.
The latter are considered higher in rank.
  146 Some Star-fishes (Solaster) have twelve rays.  In all Echinoderms, prob-
ably, sea-water is freely admitted into the body-cavity around the viscera.
The canals likewise contain water, which enters through a porous tubercle,
the madreporiform plate, or "dorsal wart," best seen on the back of the Star-
fish and Sea-urchin.—By some, Echinoderms are regarded as Worms.
  147 The shell is not strictly external,  like  the crust of a Lobster, but is
coated with the soft substance of the animal.
  148 Six hundred pieces have been counted  in the shell  alone, and twelve
hundred spines. The feet number about three thousand.  They can be pro-
truded  beyond the longest spines.
   149 fhe most important genera are Terebratula, Rhynchonella, Discina, Lin-
quia, Orthis, Spirifer, and Productus.  The first four  have representatives in
existing seas.  Professor  Morse, indorsed by Kowalensky,  maintains  the 
NOTES.
379
affinity of Brachiopods to the Worms.  Davidson, however, retains them in
their old position, as a class independent of, but related to, the Mollusca.
By many, the Tunicates are joined to the Brachiopods; by others, they are
called Worms in disguise.
  150 There are some exceptions :  the Oyster is unequivalved, and the Pecten.
equilateral.
  151 The chief impressions left on the shell are those made by the muscles
—the dark spots called "eyes" by oyster-men; the pallial line made by the
margin of the mantle; and the bend in the pallial line, called pallial sinus,
which exists in those shells having retractile siphons, as the Clam.
  152 The Pearl Oyster has a small, anterior muscle in the umbo, and the
Spondylus has teeth.
  153 The muscular impressions in Tridacne are blended into one.  It is  con-
jectured that this is the case in the Oyster.
  154 The Clam is the highest of Lamellibranchs, and the Oyster one of the
lowest.   The Venus arenaria, or " Soft Clam," has its mouth always open a
little; while V. mercenaria, or "Hard Clam," keeps its mouth closed.
  155 The Slug has no  shell to speak of, and the Chiton is covered with eight
pieces.  It  may be remembered, as a rule, that all univalve  shells in and
around the United States  are  Gasteropods, and that  all bivalves in our
rivers and lakes, and along our sea-coasts (save a few Brachiopods), are
Lamellibranchs.
  166 That is, if viewed with the aperture facing the observer.  Shells twist-
ed in the opposite direction are called " left-handed."
  167 Such as Dentalium, which may be  a Pteropod, and the swimming Nu-
cleobranch, as Carinaria.
  168 Instead of a strong breathing tube with a valve, answering for a force-
pump and propeller, as in the Cuttle-fish, it has only an open gutter made
by a fold in the mantle, like  the siphons of the Gasteropods.  The back
chambers are filled with nitrogen gas.
  169 The common Poulpe has two thousand suckers, each a wonderful little
air-pump, under the control of the animal's will.
  160 Hence the theory of Spencer, favored by Wallace and Clark, that Artic-
ulates are compound animals, each segment representing an individual.  The
dorsal tube is probably homologous with the right ventricle.
  161 The order is one of relation rather than of rank.   The classes can not
be arranged serially.  The Myriapods have a worm-like multiplication of
parts, degrading them, and their nervous system is simpler than that of Cat-
erpillars ; yet their heads show a close relationship to Insects.  The Arach-
nids  include some lower forms than Myriapods; on the other hand, for their
wonderful instincts, Owen places them above the Insects.   They are closely
allied to Crustaceans, and stand more nearly between Crustaceans and In-
sects than between Myriapods and Insects.  The higher Articulates begin
life as worm-like embryos.  None of the air-breathers have two pairs of
antennae, while the aquatic classes may have.—Articulates  with jointed ap-
pendages articulated to the body are called Arthropoda.
  163 The joints  of the Tape-worm are not true segments, only successive
growths  containing ova.  The true  animal is the  so-called " head."  The 
380
NOTES.
intestinal Worms are, by some eminent naturalists, separated under the dis-
tinct name of Helminthozoa.
  163 Apparent exceptions: Some lower forms have no branchiae, but respire
by the skin, usually of the legs, but this is substantially a gill; certain Crabs,
also, live on dry land, but they manage to keep their gills wet.
  164 The student should remember that this threefold division is not equiv-
alent to the like division of a vertebrate body.
  165 Each ring (called somite) is divisible into two arcs, a dorsal and ventral,
and each arc consists of four pieces.
  166 Sight and hearing are the only senses discovered in this class.
  167 The four pairs of legs in Arachnids answer to the two pairs of maxillae,
the great claws, and the first pair of legs of the Lobster.
  168 Compare the single thread of the Silk-worm and other caterpillars.
  169 The common  Spider, Epeira, which constructs with almost geometric-
al precision its net of spirals and radiating  threads, will finish one  in forty
minutes, and just as regularly if confined in  a perfectly dark  place.
  170 These parts do  not correspond to the  parts so named in human anat-
omy.
  171 The pupa-case is generally ornamented with golden  spots; hence the
common name chrysalis.
  172 More properly,  at least in the Bee, the lip is not converted into a suc-
torial tube, but  into an extensible tongue, with  which  the liquid food is
lapped up.
  173 All Vertebrates have a notochord, but  not all have a vertebral column,
as  the Amphioxus.  This eccentric creature, without skeleton, limbs, brain,
heart, lymphatics,  or red blood, we leave out of account.  It is not fairly a
member of the subkingdom, but rather a link between the Mollusks and
Fishes.  In aquatic animals the posterior limbs are the ones aborted or re-
duced, if any; in land  animals the fore-limbs are usually sacrificed.   The
vertebrae correspond with and are dependent on the nervous centres.  This
is shown by the fact that the tail, which is reproduced by Lizards in case of
loss, is a single bone, because although bone may be reproduced, the spinal
cord can not be.
  174 The smallest  corpuscles are found in Ruminants; the  largest in Am-
phibians with permanent gills.  The average size in Birds is double that of
Man's, and about  equal to that of the Elephant.  Those of Monkeys are a
trifle smaller than the human.  In the embryo they are larger than in the
adult.  Camels only among Mammals  have  oval disks.
  175 Oblong skulls, whose diameter from the frontal to the occipital greatly
exceeds the transverse diameter, are called dolichocephalic; and such are usu-
ally prognathous, i. e., have projecting  jaws, as the negro's.   Round skulls,
whose extreme length does not exceed the extreme breadth by a greater
proportion than 100 to 80, are brachycephalic; and such are generally orthog-
nathous, or straight-jawed.
  176 The classes are  variously grouped into the Hcematocrya, or Cold-blood-
ed, and the Scematotherma, or Warm-blooded;  into the  Branchiata and
Abranchiata; into  the  Allantoidea and Anallantoidea; and into Ichthyopsida
(Fishes  and Amphibians), Sauropsida  (Reptiles and Birds),  and Mammalia. 
NOTES.
381
According to Owen, the only  character which absolutely  distinguishes
Fishes and Reptiles is whether or not there is an open  passage from the
nostrils to the mouth.
  177 Some Ichthyologists, as  Agassiz, Haeckel, Cope, and Gill, divide the
Vertebrates below the  Amphibians into three or four distinct classes.   See
"Smithsonian Misc. Coll.," vol. xi.
  178 It would be safe to say that any living Vertebrate with side fins sup-
ported by fin rays is a Fish; but the extinct Amphibian Ichthyosaui-us also
had them.
  179 The capacity for growing as long as life lasts, which some Fishes are
said to possess, may be explained by the facts that their bodies are, firstly,
of very nearly the same specific gravity as the water in which they live, and,
secondly, of a temperature which is but a very little higher than that which
they are there exposed to.  Thus the force which in other animals is ex-
pended in the way of opposition to that of gravity and  in the way of pro-
ducing heat is available for sustaining continuous growth.—Rolleston.
  180 Scales with  smooth, circular outline  are called cycloid;  those  with
notched or spiny margins are ctenoid.   A few Teleosts are without scales or
osseous skeleton.   The ventral fins are often wanting, and the pectoral oc-
casionally.  There are about 9000 species of bony fishes.
  181 Amphibians with a moist skin are also remarkable for their cutaneous
respiration.  They will live many days after the lungs are removed.  Their
vertebrae vary  in form:  in the lowest  they  are biconcave, like those of
Fishes;  in Salamanders they are opisthoccelian: in the Frogs and Toads
they are usually proccelian.
  182 Salamanders are  often taken for Lizards, but differ in having gills in
early life and a naked skin.  The Proteus and Siren  resemble a tadpole ar-
rested in its development.
  183 The Surinam Toad has no tongue.
  184 The Reptilian heart may be likened to a persistent foetal heart of the
higher animals.
  185 The posterior pair of limbs is sometimes represented by a pair of small
bones; and there is one Ophidian which shows traces of external limbs.
  186  There are  some  notable exceptions.   The Slow-worm is  legless, and
the Chameleon has a soft skin.
  187 According to Owen; but Huxley insists that the plastron belongs to
the exoskeleton.
  iss Knees always bend forward, and heels always bend backward.
  189 It is a peculiarity of all Birds, though not confined to them, that the
generation products and the refuse of digestion are  all discharged through
one common outlet.
  190 Existing Birds have been  divided  into two primary groups, accord-
ing to the  development of  the breast-bone: (1)  Eatitce, or Runners, as
the  Ostrich, without a keel;  and  (2) Carinalce,  or  Fliers, comprising
the grand majority of Birds, having a prominent keel.  The fossil Ar-
chceopteryx,  a lizard-like Bird,  is placed in a separate  division,  Saururce.
Birds  have  also  been divided  according to their degree of development
at birth into (1) Hesthogenous, as Fowls, Ostriches, Plovers, Snipes,  Rails, 
382
NOTES.
Divers, and Ducks, whose chick is hatched completely clothed, has perfect
senses, runs about, and feeds itself.  When full grown, it uses its feet rather
than wings, flying with a rapid, labored stroke, and taking the first opportu-
nity to settle on land or water,  not on trees; the male is polygamous and
pugnacious;  the female makes little  or  no nest;  and neither sex sings.
This group is of the best use to man, and approaches more nearly to Mam-
mals, the habitual use of the legs and preference for land or water degrad-
ing it as a Bird and raising it  in the list of animals; (2) Gymnogenous,  as
Gulls, Pelicans, Birds of Prey,  Herons, Sparrows, Woodpeckers, and  Pig-
eons, whose chick comes helpless^ blind, and naked; it can neither walk nor
feed itself, but gapes for food;  the adult is monogamous, and builds elabo-
rate nests in trees and perches; many sing; all are  habitual fliers.  These
are birds par excellence, gifted with higher intelligence than the others, and
are never domesticated for food.
  191 We can not claim that this airy skeleton is necessary for  flight.  The
bones of the Bat are free from air, yet it is able to keep longer on the wing
than the Sparrow.  The common Fowl has a hollow humerus;  while some
Birds of long flight, as the Snipe and Curlew, have airless bones.
  192 Hopping is characteristic  of and confined to the Perchers;  but many
of them, as the Meadow-lark, Blackbird, and Crow, walk.
  193 This  order, founded on the disposition of the toes, is purely artificial.
But it is better to retain it until ornithologists agree upon some natural ar-
rangement.
  194 In the Swifts and Goatsuckers, the hind toe is versatile, being turned
sideways, or even forward;  while the third and fourth toes of the Kingfish-
er are united, and its wings are short.
  195 The Whales are hairy during foetal life only.
  196 The brain of Mammals differs also from that of lower Vertebrates  in
that the lobes of the cerebrum  are connected by a band called corpus callo-
sum, and the lobes of the cerebellum by the pons Varolii.
  197 As in the Whale, Porpoise, Seal, and Mole.  Teeth are wanting in the
Whalebone Whales, Ant-eaters, Manis, and Echidna.
  198. An acceptable classification of Mammals is still a desideratum. Owen's
subclasses, founded on the structure of the brain, and De Blainville's (adopt-
ed by Huxley and Gill), founded on the nature of the reproductive organs,
are unsuitable for a text-book.  It is sufficient to state here that the two
lowest orders are implacental, and their young are born in a very imperfect
condition;  while all the rest are placental, whose embryos are more com-
pletely formed before birth, being connected with the blood of the mother
by means of the placenta, a development of the allantois.
  199 The Monotremes resemble  Birds and Reptiles in having but one outlet
for the rectum, genital, and urinary organs. They resemble Marsupials  in
having marsupial bones, but have no pouch.  They differ from all other
Mammals in having no distinct nipples.
  200 The pouch is wanting in some Opossums and the Dasyurus.
  201 The  Edentates are allied  to the Reptiles, especially the Tortoises:
compare the carapax of the Armadillo, the broad ribs, toothless mouth, and
gizzard-like stomach of the Ant-eater, and the great size of the blood-cor-
puscles in the Sloth. 
NOTES.
383
  303 For the best account of the Elephant, see Tennant's " Ceylon."
  203 The forefeet of the Tapir have four toes, but one does not touch the
ground.
  204 The extinct Horse (Hipparion) had three toes, two small hoofs  dan-
gling behind.  The foot of the Horse is of wonderful structure.  The bones
are constructed and placed with a view to speed, lightness, and strength,
and bound together by ligaments of marvelous tenacity.  There are elastic
pads aud cartilages  to prevent jarring; and all the parts are covered  by a
living membrane which is exquisitely sensitive, and endows the foot with
the sense of touch, without which the animal could not be sure-footed.  The
hoof itself is a world of wonders, being made of parallel fibres, each a tube
composed of thousands of minute cells, the tubular form giving strength.
There are three parts, "wall," "sole," and "frog"—the triangular, elastic
piece in the middle,  which acts as a cushion to prevent concussion and also
slipping.
  305 The American  Peccary has three toes on the hind foot.
  306 The Camel and Llama are exceptional, having two upper incisors and
canines, are not strictly cloven-footed, aud are hornless.
  807 The Hyena alone of the Carnivores has only four toes on all the limbs,
and the Dog has four hind toes.—The Lion is the king of beasts in majesty,
but not in strength.  Five men can easily hold down a Lion, while it re-
quires nine to control a Tiger.
  208 The old term Quadrumana is rejected because  it misleads, for Apes, as
well as Men, have two feet and two hands.  There is as much anatomical
difference between the feet and hands of an Ape as between  the feet and
hands of Man.  Owen, however, with Cuvier, considers the Apes truly " four-
handed."
  209 The eye-orbits of the Lemurs are open behind.  The Flying Lemur
(Galeopithecus) is considered an Insectivore.
  210 The little Marmosets are not typical Monkeys, having a non-prehensile
tail and only 32 teeth.
  211 It fails to cover in the Howling Monkey and Siamang Gibbon; but in
the Squirrel Monkey it more than covers, overlapping more than in Man.
As to the convolutions, there is every grade from the almost smooth brain
of the Marmoset to  that of the Chimpanzee or  Orang, which falls but little
below Man's.
  212 The tailed Apes of the Old World have longer legs than arms, and gen-
erally have "cheek-pouches," which serve as  pockets  for  the  temporary
stowage of food.
  213 In the human  infant, the sole naturally turns inward; and the arms
of the embryo are longer than the legs.
  214 The Aye-aye, the lowest of the Lemurs, is remarkable for the large pro-
portion  of the cranium to the face.
  315 This feature was  shared by the extinct Anoplotherium, and now to some
extent by one of the Lemurs (Tarsius).
  316 We have treated Man zoologically only. His place in Nature is a wider
question than his position in Zoology; but it involves metaphysical and psy-
chological considerations, which do not belong here. 
384
NOTES.
  217 See Lewes's charming " Studies in Animal Life."  Doubtless an exam-
ination of all the strata of the earth's crust would disclose forms immensely
outnumbering all those at present known.  And even had we every fossil,
we would have but a fraction of the whole, for many deposits have been so
altered by heat that all traces have been wiped out.  Animal life is much
more diversified now than it was in the old geologic ages;  for several new
types have come into existence, and few have dropped out.
  218 Among the types characteristic of America are the Gar-pike, Snapping-
turtle, Hummers, Sloths, and Musk-rat. Many of our most common animals
are importations from the Old World, and therefore are not reckoned with
the American fauna; such as the Horse, Ox, Dog and Sheep, Rats and Mice,
Honey-bee, House-fly, Weevil, Currant-worm,  Meal-worm,  Cheese-maggot,
Cockroach, Croton-bug,  Carpet-moth and Fur-moth.—Distribution is com-
plicated by the voluntary migration of some animals, as well as by Man's
intervention.   Besides Birds, the Bison and Seals, some Rats, certain Fishes,
as Salmon and Herring, and Locusts and Dragon-flies among Insects, are
migratory.
  219 When the cable between France and Algiers was taken up from a depth
of eighteen hundred fathoms, there came with it an Oyster, Cockle-shells,
Annelid tubes, Polyzoa,  and Sea-fans.  Ooze brought up from the Atlantic
plateau (two thousand fathoms) consisted of ninety-seven per cent, of Fora-
minifers.
  220 Only around the shores of the Arctic Sea are the same animals and
plants  found through every meridian; and in passing southward, along the
three principal lines of land, specific identities  give way to mere identity
of genera; these  are replaced by family  resemblances, and at last  even the
families become in a measure distinct, not only on the great continents, but
on the islands, till every little rock in the ocean has its peculiar inhabitants. 
THE  NATURALIST'S  LIBRARY.
  The following works  of reference, accessible to the American student,
are recommended:
 Agabsiz, Methods of Study in Natural
  History.
 Carpenter, Comparative Physiology.
 Marshall, Outlines of Physiology.
 Huxley, Lessons in Elementary Physiol-
  ogy.
 Mivaht, Lessons in Elementary Anatomy.
 Aoassiz and Gould, Principles of Zool-
  ogy.
 Rolleston, Forms of Animal Life.
 Lewes, Studies of Animal Life.
 Jones, General Outline of the Organiza-
  tion of the Animal Kingdom.
 Clark, Mind in Nature.
 Huxley  and Martin, Elementary Biol-
  ogy.
 Owen, Comparative Anatomy of Inverte-
  brates and Vertebrates.
 Siebold, Anatomy of Invertebrates.
 Huxley, Anatomy of Vertebrated  Ani-
  mals.
 Huxley and Hawkins, Atlas of Compara-
  tive Osteology.
 Flower, Osteology of Mammalia.
 Chauveau, Comparative  Anatomy of Do-
  mesticated Animals.
 Gray, Anatomy, Descriptive and Surgic-
  al.
 Foster  and Balfour, Elements of Em-
  bryology.
 Packard, Life Histories of Animals.
 Strioker, Handbook of Human and Com-
  parative Histology.
 Lankester, Half-hours with the Micro-
  scope.
 Owen, Paleontology.
 Knight,  English  Cyclopaedia  (Natural
  History).
Van der Hoeven, Handbook of Zoology.
Cuvieb, Animal Kingdom.
Wood, Illustrated Natural History.
Milne-Edwards, Manual of Zoology.
Nicholson, Manual of Zoology.
Tenney, Elements of Zoology.
Morse, First Book  of Zoology.
Jones, Animal Creation.
Agabsiz, Sea-side Studies in Natural His-
  tory.
Taylor, Half-hours at the Sea-side.
Greene, Manuals of Sponges and Coelen-
  terata.
Dana, Corals and Coral Islands.
Verrill and Smith, Invertebrates of Vine-
  yard Sound.
Gould and Binney, Invertebrata of Mas-
  sachusetts.
Woodward, Manual of Mollusca.
Packard, Guide to the Study of Insects.
Duncan, Transformations of Insects.
Stores, Fishes and Reptiles of Massachu-
  setts.
Db Kay, Natural History of New Tork.
Coues, Key to North American Birds.
Jordan, Manual of the Vertebrates,  etc.,
  of Northern United States.
Baied,  Brewer, and Ridgway,  Birds of
  North America.
Baird, Mammals of North America.
Allen,  Mammalia of Massachusetts.
Scammon, Marine Mammals of North Pa-
  cific.
Brace, Manual of Ethnology.
Darwin, Animals and Plants under Do-
  mestication.
Wallace, Geographical Distribution of
  Animals.
Murray,  Geographical  Distribution of
  Mammals.
25 
386
THE NATURALIST'S LIBRARY.
  Of serial  publications, the student should have access to  the American
Naturalist, American Journal of Science,  Popular Science Monthly, Smithsonian
Contributions and Miscellaneous Collections, Bulletins and Proceedings of the
various societies, Popular Science Review, Intellectual Observer, and Annals and
Magazine of Natural History.*
  * Perfect specimens are invaluable in the study of Zoology.  A few hundred choice
objects are the best illustrations to accompany a text-book, or course of lectures.  In
fact, they are indispensable to  the clear comprehension of the forms of life.  Many
specimens are easily obtained; but many others come from distant lands or seas,
and must be carefully prepared for preservation and use.  It is well for teachers and
students to know that there is  one establishment in America where it is possible to
secure those type-collections so important for educational purposes.  Professor H. A.
Ward, of Rochester, New York,  has the largest facilities in the country for furnishing
skeletons and special preparations for schools and museums.  The leading museums
in America are indebted to him for some of their choicest material. Every effort is
made to  supply  select and perfect specimens. ^lis collection is especially rich in
Invertebrates, and his osteological preparations are remarkable for the elegance of
their mounting. 
INDEX.
Abranchiate Worms, 272,352.   ■
Absorbent System, 93.
Acaleph, alternate generation of, 207.
   "    structure of, 238, 378.
Acarina, 277,353.
Acarus, 353.
Acipenser, 303, 354.
Acorn-shells, 274.
Actinaria, 350.
Actinia, anatomy of, 74.
   "    described, 241, 350.
   "    development of, 200.
Actinophrys, 349.
Adder, 308.
Adipose Tissue, 36.
Molis, 262.
Agastric Animals, 370.
Air-bladder of Fishes, 116, 372.
Air-breathers, 111, 113.
Albatross, 317.
Alcyonaria, 350.
Alcyonium, 350.
Alimentary Canal, 73.
Allantoidea, 380.
Allantois, 198.
Alligator, 312, 355.
Alternate Generation, 206, 239.
Ambulacra, 129, 252.
Ammonite, 266.
Amnion, 197.
Amoeba feeding, 50, 74.
   "   locomotion of, 155.
   "   structure of,  232, 349.
American Types, 384.
Amphibians, described, 305, 354,381.
      "      eggs of, 193.
Amphiccelous, 373.
Amphioxus, 50, 302, 374, 380.
Anallantoidea, 3S0.
Analogy, 211.
Anchylosis, 142.
Anguis, 354.
Animalcules, mouth of, 54.
Animals and Plants, 21, 368.
Annelids, 270, 352.
Annuloida, 271, 352.
Anoura, 354
Anser, 355.
Ant-eater, 330, 370.
Antennae, 174, 374.
Anthozoa, 241, 350.
Ants, 293.
Aorta, 103.
Ape, 151, 343, 383.
Apis, 353.
Aplysia, 262.
Apteryx, 319.
Arachnid, 277, 353.
Araneina, 278, 353.
Archetype, 137.
Ardea, 319, 355.
Areolar Tissue, 35.
Argonauta, 268.
Armadillo, 133, 330.
Arteries, 103.
Arthropoda, 379.
Articulates, circulation in, 105.
     "      described, 226, 269, 352.
     "      digestion in, 77,91.
Ascidian, circulation  in,  106.
    "     described, 256, 350.
    "     mouth of, 55.
Aetacus, 273, 352.
Asterias, 251, 350.
Asteroidea, 55,  75, 91, 111, 129, 159, 250,
  350.
Astraea, 243.
Atavism, 210.
Attacus, 291, 353.
Auk, 316.
Aurelia, 239, 350.
Aves, 313,355. 
388
INDEX.
Axolotl, 306.
Aye-aye, 383.

Babirusa-hog, 68.
Baboon, 346.
Balaena, 147, 355.
Balanus, 274, 352.
Bandicoot, 330.
Barbet, 324.
Barnacle, 57, 275.
Basket-fish, 251.
Bathybius, 349.
Batrachians, 307.
Bats, 187, 333.
Beaver, 332.
Bed-bug, 287.
Bee, anatomy of, 114.
  "  described, 293.
  "  eggs of, 192.
  "  eye of, 178.
  "  instinct of, 183.
  "  mouth of, 58.
  "  muscles of, 154.
Beetles, described, 282,287.
   "    eyes of, 179.
   "    mouth of, 58.
   "    prehension of, 53.
Belemnite, 269.
Beroe, 242.
Birds,  anatomy of, 84.
   "   brain of, 315.
   "   classification of, 355, 381.
   "   described, 313.
   "   digestive system of, 83.
   "   distribution of, 363.
   "   flight of, 157.
   "   lungs of, 115,315.
   "   mouth of, 61.
   "   prehension  of, 49, 53.
   "   respiration  in, 116.
   "  sense of smell, 175.
   "   skeleton of, 136,148,313.
   "  vocal apparatus,  186.
Bird of Paradise, 327.
Bivalve Shells, 131,258.
Blackbird, 327.
Blastema, 33.
Blastoderm, 194
Blood, circulation  of, 102.
   "   color of, 97, 372.
   "   constitution of, 98.
   "   current, 110, 372.
   "   development of,  376.
   "   office of, 101.
   "   vessels, 102.
Blubber, 334.
Blue-fish, 300.
Boa constrictor, skull of, 72.
Bones, 36,136,145.
Bos, 150, 356.
Brachiopods, 256, 351, 378.
Brachycephalic Skull, 380.
Bradypus, 356.
Brain-case, 141.
   "  development of, 199.
   "  functions of, 170.
   "  size of, 348.
   "  structure of, 166,382.
Branchiate Vertebrates, 380.
     "     Worms, 272.
Bronchial Tubes, 118.
Bryozoa, 255.
Bubble-shell, 262.
Buccinum, 260.
Budding, 188.
Bufo, 307, 354.
Bugs, described, 285.
  "   mouth of, 59.
Bulimus, 262.
Bulla, 262.
Butterfly, described, 290.
    "    mouth of, 58.

Caddis-fly, 284.
Caecilians, 306, 354.
Caecum, 371.
Calcispongia, 349.
Camel, 383.
Cameo-shell, 266.
Canaliculi, 38.
Capillaries, 103.
Capybara, 331.
Carapax, 133.
Cardium, 259.
Carinatae, 381.
Carnivores, 340, 356.
Cartilaginous Tissue, 36.
Cassis, 264.
Cassowary, 319.
Castor, 332.
Cat, brain of, 167,171.
Cat-tribe, 342.
Caterpillar, anatomy of, 77.
     "     circulation in, 104.
     "     head of, 292.
     "     nervous system of, 165.
Cebus, 356.
Cells, 31,98, 369.
Centipede, 52,114,159,166, 281.
Cephalization, 218.
Cephalopods, anatomy of, 81.
      "      described, 266,352.
Cephalo-thorax, 130,272.
Cerebellum, 169.
Cerebrum, 168,374
Cestum, 242. 
INDEX.
389
Cetacea, 334,355.
Chameleon, 309.
Cheiropters, 333, 356.
Chelonians, 310,354.
Chelydra, 354.
Chilognatha, 280, 353.
Chilopoda, 280, 353.
Chimaera, 302.
Chimpanzee, 69,151, 345.
Chitine, 373.
Chiton, 263.
Chorion, 198.
Chrysalis, 377, 380.
Chyle, 92, 372.
Chyme, 92.
Cicada, 286, 353.
Cicatricula, 190.
Cidaris, 253,351.
Cilia, 50, 374
Ciliata, 349.
Cimex, 353.
Cirripeds, 274, 352.
Clam, described, 260, 379.
   "   digestive system, 79.
   "   locomotion of, 158.
   "   respiration in, 112.
Clamatores, 325.
Class, 227.
Classification, 223,349, 378.
Claws, 134.
Clio, 55, 351.
Cloaca, 84.
Clypeaster, 351.
Coagulation, 372.
Cochineal, 287.
Cockle-shell, 259.
Cockroaches, 285.
Cod-fish, 167, 303.
Ccelenterata, 225, 237, 349.
Cold-blooded Animals, 119,354.
Coleopters, 287, 353.
 Coluber, 354.
 Condor, 322.
 Cone-shell, 264.
Connective Tissue, 34,369.
 Coral, 128, 242.
 Corallium, 247, 352.
Corium, 373.
 Cormorant, 317.
 Corpuscles, 97,  372, 380.
Correlation of Growth, 212.
 Corydalis, 285.
 Cowry, 264.
 Crab, described, 275.
   "   legs of, 159.
   "   mouth of, 56.
   "   skeleton  of, 130.
 Crane, 319.
Cranium, 139,141.
Cray-fish, 273.
Cricket, 285.
Crinoid, 248, 350.
Crocodile, described, 147, 311, 355, 373.
    "     digestive system, 81,83, 371.
    "     exoskeleton of, 133.
    "     heart of, 107.
    "     locomotion of, 160.
    "     mouth of, 64
Crotalus, 354.
Crow, 327.
Crustaceans, described, 272, 352.
     "      digestive system, 76.
     "      skeleton of, 130.
Ctenophora, 242, 350, 370.
Cuckoo,  323.
Curassow, 320.
Cursores, 319, 355.
Cuticle, 373.
Cutis, 373.
Cuttle-bone, 373.
Cuttle-fish, brain and eyes of, 176,179.
     "     circulation in, 106.
     "     described, 268.
     "     digestive system, 79.
     "     mouth of, 56.
     "     prehension of, 51.
     "     skeleton of, 132,135, 373.
Cypraea, 266.

Daddy-long-legs, 290.
Dasypus, 331.
Dasyurus, 330.
Decapods, 274, 353.
Deep-sea Forms, 359, 384
Deer, 339.
Deglutition, 71.
Delphinus, 355.
Dental Formula, 69.
   "   Tissue, 3S.
Dermis, 126, 373.
Development, 193,199, 376.
Diadelphys, 330, 355.
Diaphragm, 85,119, 373.
Diastema, 370.
Dibranchs, 267, 352.
Differentiation, 31.
Digestion, 90.
Digitigrades, 161, 342.
Dipnoi,  304, 354.
 Dipters, 288, 353.
Discophora, 240, 350.
 Distribution, 357,384
 Divers,  315.
 Dog, 342.
   "  brain of, 167.
   "  digestion in, 123. 
390
INDEX.
Dog, skull of, 140.
Dolichocephalic Skull, 380.
Dolphin, 335, 355.
Doris, 262,352.
Dove, 320.
Dragon-fly, 284, 374.
Duck,  318.
Duck-mole, 328.
Dugong, 108, 337.
Duodenum, 89.

Eagle, 322.
Ear, 176.
Ear-shell, 264.
Earth-worm, 272.
      "      circulation in, 105.
      "      feeding, 52.
      "      locomotion of, 159.
      "      nervous system, 166.
      "      respiration in, 112.
Ecderon, 373.
Echidna, 328.
Echinoderms, 226, 247, 350, 378.
Echinus, 91,104,129, 252, 350, 378.
Ectoderm, 237, 373.
Edentates, 330, 356, 382.
Eggs, generation by, 189, 376.
  "   kinds of, 191, 376.
  "   structure of, 189.
Elasmobranchs, 302, 354.
Elephant, 337, 356.
    "     brain of, 167,168.
    "     skeleton of, 150.
    "     teeth of, 70.
    "     voice of, 187.
Elytra of Beetles, 287.
Embryology, 12,193.
Embryos, likeness of, 201.
Encephalon, 172,374.
Enderon, 373.
Endoderm, 237, 373.
Endoskeleton, 126,128,135.
Endosmosis,  372.
Entomostracans, 274,353.
Epeira, 353.
Epidermis, 34,126,128, 373.
Epiglottis, 118.
Epithelium, 33.
Equus, 142,149, 356.
Euplectella, 237, 349.
Excretion, 119.
Exoskeleton, 125.
Eyes, compound, 178,375.
  "  simple, 178.
  "  structure of, 180.

Facial Angle, 298.
Palcou, 323, 355.
Family, 227.
Fangs, 67.
Feathers, 135.
Felis, 137, 356.
Fibrospongia, 349.
Fishes, anatomy of, 82.
   "   brains of, 168, 301.
   "   circulation in, 106,109.
   "   described, 298, 381.
   "   digestive system, 79.
   "   distribution of, 363.
   "   gills of, 113, 301.
   "   heart of, 107.
   "   locomotion of, 156, 300.
   "   mouth and teeth, 60, 66.
   "   muscles of, 154.
   "   prehension of, 53.
   "   scales and skeleton, 133,1*6,299.
Fish-hawk, 321.
Flagellata, 235, 349.
Flamingo, 158, 318.
Flea, 290.
Flight, 157.
Flustra, 351.
Fly, described, 288.
 "  feet  of, 160.
 "  mouth of, 59.
Fly-catcher, 324
Flying-fox, 333.
Follicles, 121.
Food, kinds of, 47.
  "   necessity of, 45.
  "   prehension of, 49.
Foraminifers, 50,128, 233, 349.
Forms of Animals, 215.
Fox, 341.
Frog, blood-cells of, 98.
  "   brain of, 170.
  "   described, 307.
  "   metamorphosis of, 205.
  "   prehension of, 53.
  "   skeleton of, 118.
Fungia, 243.
Fusus, 264, 266.

Gall-bladder, 122.
Gall-flies, 293.
Gallus, 355.
Ganglia,  nervous, 163.
Gannet, 317.
Ganoids, 302, 354
Gar-pike, 302.
Gasteropods, 260, 351.
Gastric Follicles, 121.
   "   Teeth, 63.
Gavial, 312,355.
Genus, 227.
Germinal Dot, 191. 
INDEX.
391
Gibbon, 344.
Gills, 112,113.
Giraffe, 339.
Gizzard, 63, 83, 371.
Glands, 121.
Glottis, 117.
Glyptodon, 330.
Gnawers, 331.
Goatsucker, 324.
Goose, 318.
Gorgonia, 247.
Gorilla, 345.
Grallatores, 319, 355.
Grasshopper, 285.
Grebe, 316.
Gregarinida, 231, 349.
Grouse,  320.
Growth  and Repair, 207, 381.
Grubs, 377.
Gryllus, 353.
Guinea-pig, 332.
Gulls, 316.
Gymnogeuous Birds, 382.

H.EMATOORYA, 380.
Haematotherma, 380.
Hag-fish, 302.
Hair, 134
Hair-worm, 271.
Haliotis, 264.
Halisarca, 349.
Hand, 346.
Hare, 332.
Harvest-men, 278.
 Haversian Canals, 37.
 Hawk, 323.
 Hearing, 175.
 Heart of Fishes, 107.
    "    Man, 108,195.
    "    Oyster, 105.
 Heat, animal, 119.
 Hedgehog, 333.
 Helix, 262, 352.
 Hemipters, 285, 353.
 Hen's Egg, development of, 194
     «'      parts of, 190.
 Hermaphrodite, 376.
 Heron, 319.
 Hesthogenous Birds, 381.
 Heterocercal Tail, 156.
 Hippopotamus, 338.
 Histology, 12.
 History of Zoology, 14, 367.
 Hog, 338.
 Holothurians, 253,351.
 Homo, 345, 356.
 Homocercal Tail, 156.
 Homology, 211.
Hoofs compared, 162.
Horns, 134.
Horse, brain of, 163.
   "   described, 13, 338.
   "   foot of, 134,161, 383.
   "   skull and skeleton, 142,149.
   "   stomach of, 86.
Horseshoe Crab, 52.
Hummer, 325.
Hyalea, 351.
Hydra, 75,189, 237, 350.
Hydroida, 350.
Hydrozoa, 237, 350.
Hyena, 383.
Hylacinus, 330.
Hymenopters, 293,353.

Ibis, 319.
Ichneumon, 293.
Ichthyopsida, 380.
Ichthyosaurus, 313.
Iguana, 309.
Individual, 227.
Infusoria, 50,188, 234, 349.
Insectivores, 333, 350.
Insects, circulation in, 104, 283.
    "    classification of, 284,353.
    "    described, 281, 353.
    "    development of, 200.
    "    digestive system in, 78.
    "    distribution of, 362.
    "    eyes of, 178.
    "    legs of, 160,163, 282.
    "    metamorphosis of, 203.
    "    mouth of, 53, 57, 282.
    "    nervous system, 283.
    "    noise of, 185.
         respiration in, 113,114, 283.
    "    skeleton of, 130, 282.
    "    wings of, 157, 282.
 Insessores, 324, 355.
 Inspiration, modes of, 118.
 Instinct, 181, 375.
 Intelligence, 183,375.
 Intestinal Canal, 89.
 Invertebrates, 154, 294.
 Isis, 350.
 lulus, 280, 353.

 Jaoamar, 324.
 Jaws, 53.
 Jay, 327.
 Jelly-fish, described, 238.
     "     digestive system, 54, 75.

 Kangaroo, 330.
 Kidney, 123.
 King-crab, 274. 
392
INDEX.
Kingfisher, 325.
Kite, 323.

Labium and Labrum, 58.
Labyrinthodonts, 306.
Lacerta, 310,354.
Lacertilia, 354.
Lacteals, 94
Lacunae, 38.
Lagena, 349.
Lamellibranchs, 112,257,351.
Lamellirostres, 318.
Lamprey, 302.
Lancelet, 302.
Land-snails, 263.
Lark, 327.
Larynx, 187.
Leech, 272.
   "   locomotion of, 158.
   "   mouth of, 49, 56, 63.
Legs of Animals, 155,199,213.
Lemur, 342, 356.
Lepidopters, 290, 353.
Lepidosiren, 304, 354.
Libellula, 284, 354.
Life, distribution of, 357.
  "  duration of, 219.
  "  nature of, 28, 369.
  "  phenomena of, 29,43.
  "  struggle for, 219.
Lightning-bug, 288.
Ligula, 58.
Likeness and Variation, 209.
Limax, 262, 352.
Limbs of Vertebrates, 144, 374
Limnaea, 263, 352.
Limpet, 263.
Limulus, 274, 353.
Lingula, 351.
Lion, 87,137, 383.
Liver, 121.
Lizards, 160,170,309.
Lobster, 272.
    "    circulation in, 105.
    "    eggs of, 192.
    '•    gills of, 113.
    "    locomotion of, 155.
    "    mouth of, 56.
    "    muscles of, 154.
    "    prehension of, 52.
    "    skeleton of, 130.
Locomotion, 155.
Locust, 285.
Loligo, 269.
Longipennes, 316.
Loon, 316.
Louse, 287.
Lucernaria, 240, 350.
Lumbricus, 272, 352.
Lungs, 115,123, 373.
Lymphatics, 94

Madrepore, 244, 350.
Madreporiform Plate, 378.
Maggots, 377.
Mammals, anatomy of, 86.
     "    brain of, 382.
     "    circulation in, 109.
     "    described, 327,355.
     "    distribution of, 364.
     "    locomotion of, 161.
     "    mouth and teeth of, 61, 67.
     "    respiration in, 116.
     "    skull of, 141.
     "    vocal apparatus, 186.
Mammoth, 337.
Man, 85,171, 213, 345, 383.
Manatee, 327,337,355.
Mandibles, 58,143.
Mantis, 53.
Mantle, 126.
Marsipobranchs, 302, 354.
Marsupials, 329, 355.
Mastodon, 337.
May-fly, 284.
Meandrina, 243.
Medulla oblongata, 169.
Medusa, 75, 238.
Megatherium, 330.
Melania, 266.
Membrane Bones, 198, 369.
Menobranchus, 306.
Mesentery, 82, 94.
Metamorphosis, 203, 377.
Millepede, 56,114.
Millepore, 378.
Mimicry, 210, 376.
Minerals and Organisms, 19.
Mites, 277.
Mole, 333.
Molluscoidea, 255.
Mollusks, anatomy of, 80.
     "    circulation in, 105.
     "    described, 226, 254, 351.
     "    digestion in, 91.
     "    distribution of, 362.
     "    locomotion of, 158.
     "    mouths of, 55.
     "    nervous system of, 165.
     "    respiration in, 112.
     "    shells of, 131.
 Monad, 234, 349.
 Monera, 349.
 Monkeys, 342.
 Monotreme, 328,355,382.
| Mosquito, 49, 204, 289. 
INDEX.
393
Moth, 291.
Moulting, 128,130, 205.
Mouse, 332.
Mouths of Animals, 54
Mud-eel, 306.
Mud-fish, 304.
Murex, 266.
Mus, 356.
Musca, 290, 353.
Muscle, 152.
Muscular Tissue, 38,152,199.
Mussels, 259.
Myriapods, 76, 280, 353.
Myrmecophaga, 330, 356.
Mytilus, 259.
Myxospongia, 349.

Nails, 134
Natatores, 315, 355.
Natica, 266, 352.
Natural Selection, 220.
Nautilus, 267, 352.
Nereis, 272, 352.
Nerve-cells, 163.
Nerves, 163,172.
Nervous Tissue, 40,164,199.
Neurilemma, 40.
Neuropters, 284, 353.
Newt, 306.
Nictitating Membrane, 375.
Notochord, 195.
Nucleolus, 191.
Number of Animals, 214.
Nummulite, 349.
Nutrition, 44.
Nymph, 377.

Ocelli, 178.
Octopus, 268, 352.
(Esophagus, 85.
Olfactory Nerves, 175.
Olive-shell, 266.
Oniscus, 353.
Ophidians, 308, 354.
Ophiomorpha, 354.
Ophiura, 251, 350.
Opisthobranchs, 261, 352.
Opisthocoelous, 373.
Opossum, 330.
Orang-utan, 171, 344.
Order, 227.
Organization, 30.
Organ-pipe Coral, 242.
Organs, 41.
Oriole, 327.
Ornithorhynchus, 328, 355.
Orthoceras, 266.
Orthopters, 285, 353.
Orycteropus, 330.
Oscines, 325.
Osseous Tissue, 36.
Ossification, 36,19S.
Ostrea, 259, 351.
Ostrich, 319.
Otoliths, 175.
Ovipositor, 283.
Owls, 323.
Ox, 150, 340.
Oyster,  circulation in, 105.
   "    described, 259.
   "    development of, 200.
   "    digestion in, 79.
   "    mouth of, 54.
   "    muscles of, 154.
   "    prehension of, 50.
   "    respiration in, 112.
 .  "    shell of, 131, 208.

Palate, 62,  85.
Pallial Sinus, 379.
Palpi, 58.
Paludina, 265.
Pancreas, 121.
Pangolin, 330.
Paper Nautilus, 268.
Papillae, 62,126,174, 373.
Papilio, 353.
Paramecium, 234, 349.
Parrot,  322,  324.
Partridge, 320.
Patella, 263, 377.
Pavement-teeth, 66.
Pearl-oyster, 258, 373, 379.
Peccary, 383.
Pectoral Arch, 144.
Pedicellariae, 76, 371.
Pedipalpi, 277,  353.
Pelican, 317.
Penguin, 315.
Pennatula, 247.
Pen of the Squid, 373.
Pentacta, 351.
Pentacrinus, 249, 350.
Perch, 146,170, 354.
Perchers, 324.
Periosteum, 136.
Peristaltic Movement, 370.
Periwinkle, 266.
Petrel, 317.
Petromyzon, 303, 354.
Phalanger, 330.
Pharyngobranchs, 301.
Pharynx, 72, 84, 370.
Pheasant, 320.
Phoea,  356.
Physalia, 239, 350. 
394
INDEX.
Physeter, 334, 355.
Picus, 355.
Pigeon, 320.
Pinnigrades, 161, 341.
Pisces, 354.
Placenta, 198.
Placental Animals, 355,382.
Plauorbis, 263.
Plantigrades, 161, 341.
Plant-lice, 287.
Plastron, 311.
Platypus, 328.
Pleurobrachia, 242, 350.
Plover, 319.
Poison-fang, 52, 67, 373.
Polycistines, 128, 234, 349.
Polyps, 50,54, 75.
Polyzoa, 255, 351.
Pond-snails, 263.
Porcupine, 332.
Porites, 244.
Porpoise, 87, 335.
Portal Circulation, 372.
Portuguese Man-of-war, 239.
Poulpe, 268, 379.
Prairie-chicken, 320.
Primates, 342, 356.
Primitive Streak, 195.
Proboscideans, 337, 356.
Proboscis of Butterfly, 58.
      "      Elephant, 62,150.
Procoelous, 373.
Prognathous Skull, 380.,
Prosobranchs, 263, 352.
Proteus, 306, 354.
Protozoa, 200, 231, 349.
Pseudopodia, 50, 232.
Pteropods, 55, 261.
Pulmonates, 262, 352.
Pygopodes, 315.

quadrumana, 383.

Raccoon, 340.
Radiates, 237.
Radiolarians, 233, 349.
Rails, 319.
Rana, 307, 354.
Range of Animals, 358.
Rank of Animals, 216.
Raptores, 321, 355.
Rasores, 320, 355.
Rat, 332.
Ratitae, 381.
Rattlesnake's Fangs, 67.
Raven, 327-
Ray, 302.
Razor-shell, 260.
Redstart, 325.
Reproduction, 18S, 208, 376.
Reptiles, circulation in, 107,109,308.
    "    corpuscles of, 99.
    "    described, 307.
    "    digestion in, 81.
    "    distribution of, 363.
    "    lungs of, 116.
    "    mouth of, 60.
    "    prehension of, 53.
    "    scales of, 134.
    "    teeth of, 66,
    "    voiceless, 186.
Respiration, 111.
Rete mucosum, 127.
Retiua, 180.
Rhea, 320.
Rhinoceros, 338.
Rhizopods, 232, 349.
Rodents, 331, 356.
Rotifers, 63, 272.
Rudimentary Parts, 202.
Rumiuants, 87, 339.

Salamander, 306, 354, 381.
Salivary Glands, 121, 373.
Salmon, 301, 354.
Salpians, 256, 351.
Sand-flea, 274.
Sandpiper, 318.
Sarcolemma, 39,199.
Saurians, 313.
Sauropsida, 380.
Saururse, 381.
Scales of Butterflies, 290.
    "    Fishes  and  Reptiles, 193,
  381.
Scallop-shell, 259.
Scalops, 356.
Scansores, 323, 355.
Scapular Arch, 144.
Scarabaeus, 353.
Scarf-skin, 373.
Sclerobasic Coral, 128, 247.
Sclerodermic Coral, 128, 243.
Scolopendra, 281, 353.
Scorpion, described, 277, 353.
    "     digestion in, 78.
    "     mouth of, 52, 59.
    "     spiracles of, 115.
Sea-anemone, 200, 241.
Sea-blubber, 238.
Sea-butterfly, 260.
Sea-fan, 247.
Sea-hare, 262.
Seal, 341.
Sea-lemon, 262.
Sea-lily, 248. 
INDEX.
Sea-lion, 341.
Sea-slug, 253.
Sea-urchin, circulation in, 104.
     "     described, 252.
     "     digestion in, 75,91.
     "     mouth of, 55.
     "     respiration in, 111.
     "     shell of, 129, 378.
     "     spines of, 129, 378.
     "     teeth of, 63.
Sea-worms, 272.
Secretion, 120.
Self-division, 188.
Senses, 173.
Sepia, 268,352.
Serpents, skulls of, 73.
Serpula, 272.
Sertularians, 238, 350.
Seta?, 271.
Setophaga, 325.
Shark, described, 302.
   "   eggs of, 191.
   "   skeleton of, 135.
Shells of Crustaceans, 130,272.
    "    Mollusks, 131, 257, 260.
    "    Sea-urchins, 129,378,
Shrew, 333.
Shrimp, 274.
Sight, 177.
Silk-worm, 293.
Simia, 356.
Siphonophora, 350,
Siphuncle, 207.
Siredon,306.
Siren, 306.
Sirenians, 336, 355.
Size of Animals, 214.
Skeleton, 125.
Skin, 125,126.
Skull, 142,380.
Sloth, 330.
Slug, 262.
Smell, 175.
Snail, circulation in, 105.
  "   described, 260.
  "   digestion in, 79.
  "   mouth and teeth of, 56, 64.
  "   prehension of, 51.
  "   shell of,  132.
  "   tentacles of, 178.
Snakes, described, 308, 354.
   "    locomotion of, 155, 159.
   "    prehension of, 53.
Snapping-bugs, 287.
Snipe, 319.
Somite, 380.
Songsters,  325.
Sorex, 333.
Sow-bug, 274.
Sparrow, 325, 327.
Species, 227.
Sperm-whale, 334.
Sphinx-moth, 291, 353.
Spider, described, 308, 354.
   "   digestion in, 78.
   "   legs of, 159.
   "   mouth of, 59.
   "   prehension of, 52.
   "   spiracles of, 115.
Spinal Column, 143.
   "   Cord, 166,169,172.
Spinnerets of Caterpillar, 292.
      "      Spider, 280.
Spiracles, 113.
Sponge,  49, 128, 235, 349, 370.
Squalus, 354.
Squid, 155, 269.
Squirrel, 332.
Star-fish, anatomy of, 159.
    "    circulation in, 104.
    "    described, 250.
    "    digestion in, 91.
    "    feeding, 51.
    "    locomotion of, 158.
    "    nervous system, 165.
    "    respiration in, 111.
Stilt, 319.
Stomachs,  83, 86, 88.
Stork, 319.
Stridulation, 185.
Strombus,  265,352.
Struthio, 355.
Sturgeon, 53, 302.
Subkingdom, 227.
Sun-fish, 251.
Survival of the Fittest, 220.
Suture, 145.
Swallow, 326.
Swan, 318.
Swift, 325.
Sycon, 349.
Symmetry, 215.
Synovia, 145.

Taenia, 271, 352,
Tanagers, 327.
Tapetum, 181.
Tape-worm, 49, 271, 379.
Tapir, 213, 338, 383.
Taste, 174.
Teeth, 38, 62, 65, 66, 68,70.
Teleosts, 302, 354
Temperature of Animals, 119.
Tendons, 154.
Tentacles,  50.
Terebra, 264 
396

Terebratula, 257, 351.
Termites, 285.
Tern, 316.
Testudo, 354.
Tetrabranchs, 267, 352.
Tetradecapods, 274, 352.
Thorax, 118, 281.
Thornback, 304.
Thousand-legged Worm, 280.
Throat of Mammals, 72.
Thrush, 327.
Thyroid Cartilage, 186.
Ticks, 277.
Tissues, 32.
Toad, 307.
Tongue, 53, 54, 61, 62.
Top-shell, 265.
Tortoise-shell, 311.
Totipalmates, 317.
Toucan, 324.
Touch, 173.
Tracheae, 113.
Trichina, 271.
Trilobite, 274.
Triton, 266.
Tritonia, 262.
Trochus, 266.
Trogon, 323.
Tubipora, 242, 245.
Tunicates, 126, 256, 351, 379.
Turbo, 265.
Turdus, 355.
Turkey, 170, 320.
Turritella, 266.
Turtle, 134,148,160,192, 310.
Tusks, 370.
Types, 225, 378.
Tyrant Fly-catcher, 325.

Ungulates, 161, 337, 356.
Unio, 259.
Univalves, 131, 260.
Urodelans, 306, 354.
Ursus, 356.

Variation, 209.
Variety, 227.
Veins, 94,102,103.
Vena cava, 103,372.
Venus, 260, 351.
Venus-basket, 237.
Vertebrae, 138,143,199.
Vertebrates, 226,295.
     "      circulation in, 108, 296.
      "      classification of, 298.
     "      development of, 201.
INDEX.

     Vertebrates, digestion in, 91.
           "      mouths of, 60.
           "      nervous system of, 297.
           "      prehension of, 53.
     Vespertilio, 333, 356.
     Villi, 89, 94, 372.
     Vireo, 325.
     Vitelline Membrane, 190.
     Viviparous, 377.
     Voices of Animals, 185.
     Volitores, 325.
     Volute, 264
     Vorticella, 349.
     Vulture, 148,323.

     Walking-stick, 285.
     Warblers, 327.
     Warm-blooded Animals, 119, 355.
     Wasps, 293.
     Water-boatmen, 286.
     Water-breathers, 111.
     Water-fleas, 274.
     Wax-wing, 327.
     Weasel, 340.
     Weevil, 287.
     Whale, baleen of, 64,134,370.
        "   brain of, 167,168.
        "   described, 334.
        "   feeding, 50,336.
        "   locomotion of, 156.
        "   skeleton of, 147.
     Whelk, 55, 260, 265.
     White Ants,  285.
     Wigglers, 377.
     Windpipe, 117.
     Wings of Bats, 158.
         "    Birds, 157.
         "    Insects, 157.
     Wolf, 340.
     Wombat, 330.
     Woodpecker, 322, 324.
     Worms, described, 270.
        "    digestive system, 76.
        "    locomotion of, 155.
        "    mouth of, 56.
        "    respiration of, 112.
     Wren, 327.

     Yolk, 189,194.

     Zonotrichia, 325.
     Zoological analysis, 228.
          "     barriers, 359.
          "     provinces, 360.
     Zoology defined, 11.
         "    history of, 14, 367.
THE
END. 
SHAKESPEARE.
WITH  NOTES   BY WM. J. ROLFE,  A.M.,
         Formerly Head Master of the High-School, Cambridge, Mass.

             ILLUSTRATED WITH WOODCUTS.

THE MERCHANT OF  VENICE.  i6mo, Cloth, 90 cts.
THE  TEMPEST.   i6mo, Cloth, 90 cents.
HENRY THE EIGHTH  i6mo, Cloth, 90 cents.
JUIIUS CjESAR.   i6mo, Cloth, 90 cents.
RICHARD THE SECOND.   i6mo, Cloth, 90 cents.
           From Prof. F. J. Child, of Harvard University.
  After using the book with an evening class in Shakespeare, Prof. Child
writes as follows :
  I read your " Merchant of Venice " with my class, and found it in
every respect an excellent edition.   I do not agree with my friend White
in the opinion that Shakespeare requires but few notes—that is, if he is
to be thoroughly understood.  Doubtless he may be enjoyed, and many
a hard place slid over.   Your notes give all the help a young student
requires, and yet the reader for pleasure will easily get  at just what he
wants.   You have indeed been conscientiously concise.

From L. R. WlLLlSTON, A.M., Head Master of the High-School, Cam-
                          bridge, Mass.
  Mr. Rolfe's edition of " The Merchant of Venice " is an excellent one
for school  or general  use.  The notes contain all the explanations and
references  needful for  a critical study of the language, as well as for un-
derstanding the thought of the play.  The extracts from Schlegel, Mrs.
Jameson, and others, in the Introduction, helping to a better appreciation
of the characters of the play, are a peculiar recommendation of this edition.

  From Rev. A. P. Peabody, D.D., Professor in Harvard University.
  I regard your own work on this play as of the highest merit, while you
have turned the labors of others to the best possible account.  I want to
have the higher classes of our schools introduced to Shakespeare chief
of all, and  then to other standard English authors; but  this can not be
done to advantage, unless under a teacher of equally rare gifts and abund-
ant leisure, or through editions specially prepared for such use.  I trust
that you will have the requisite encouragement to proceed with a work
so happily begun.
  Your " Merchant of Venice " seems to me by no means limited, in its
adaptation, to school use.  All who have not access to a somewhat ex-
tended  Shakespearian apparatus need such editions as this; and there
are many not unintelligent adult readers of Shakespeare who  lose half
the pleasure and  profit of reading him for lack of precisely such aid as
you supply. 
2
Rolfes Shakespeare.
    From Prof. J. Dorman Steele, Free Academy, Elmira, N. Y.
  The copy of the " Tempest" is at hand, and very carefully examined.
We shall  use it in the Spring Term.   Adoption  in our  school is, of
course, the highest commendation I can give.   The " Merchant of Ven-
ice" is now  in use and gives unqualified satisfaction.  Prior to this,
Shakespeare's plays were failing to interest the pupils, because of the
difficulty found in understanding and appreciating the text.   Your beau-
tiful and comprehensive edition is very helpful indeed, and  it has quick-
ened the enthusiasm of the pupils.

From W. C. Collar, A.M., Master of the Roxbury Latin School, Boston.
  Please accept my thanks for a copy of your  " Merchant of Venice."  I
have made a trial of it with my first class, and find it  admirably adapted
for use in  the school-room.  I think no one who was not an experienced
teacher and a careful student of Shakespeare could have anticipated and
supplied so well the needs of the learner ; and, if I may judge from my
own case, instructors will find the  copious references contained in the
notes very helpful in the  preparation of their  lessons.  Give us a few
more plays edited on the same plan, and there will no longer be any ex-
cuse for excluding Shakespeare from our classical and high schools.
 From S. M. Capron, A.M., Master of the  High-School, Hartford, Conn.
  In my judgment, you have produced, in " The Merchant of Venice,"
the best and most sensible edition of one  of Shakespeare's plays which
has yet appeared for school use.   The publishers have done every thing
for  you in respect to the form and general appearance of the book; and
your notes are not only critical, but sufficiently brief and pointed, and, so
far  as I have examined them, they seem to cover the very points in the
text which particularly  need elucidation.
  Go on as you have begun.
  This work has been done so well that it could hardly have been done
better.  It shows throughout, knowledge, tr.ste, discriminating judgment,
and, what is rarer and of yet higher value, a sympathetic appreciation of
the poet's moods and purposes. * *  * Mr. Rolfe's notes are numerous
but brief, and are generally well adapted to their purpose, which is that
of explanation, instruction, and suggestion  without discussion.  The pe-
culiarities of Shakespeare's style—which, rarely obscure, is often involved,
and in which the main thought is sometimes  suspended, and even for a
moment lost sight of amid the crowd  of others that itself has called up—
are pointed out and elucidated ; his allusions are explained; his singular
use of words, of moods and tenses and cases is remarked upon ;  and the
archaic and transitional phraseology which is  found in many passages of
his plays (positively many, but comparatively very few), are made the oc-
casion of instructive but unpedantic comment—A7: Y. Times.
      Published by  HARPER & BROTHERS, New York.

U^" Sent by mail, postage prepaid, to any part of the  United States, on receipt
                            of the price. 
OLIVER    GOLDSMITH.
SELECT POEMS OF OLIVER GOLDSMITH.   Edited,
    with Notes, by William J. Rolfe, A.M., formerly Head
    Master of  the High  School,  Cambridge, Mass.  Illus-
    trated.   i6mo, Cloth, 90 cents.    {Uniform with Rolfe's
    Edition  of  Shakespeare's  Merchant  of  Venice,  Tempest,
    Henry VIII., Richard II, and Julius  Ccesar.)
   The carefully  arranged editions of "The  Merchant of Venice" and
other of Shakespeare's plays prepared by Mr. William J. Rolfe for the
use of students will be remembered with pleasure by many readers, and
they will welcome another volume of a similar character from the same
source, in the form of the " Select Poems of Oliver Goldsmith," edited
with notes fuller than those of any other known edition, many  of them
original with the  editor.—Boston Transcript.
  Mr. Rolfe is doing very useful work in the preparation  of  compact
hand-books for study in  English literature.   His own personal culture,
and his long experience as a teacher, give him good knowledge of what is
wanted in this way.— The Congregationalist, Boston.
  Mr. Rolfe has prefixed to the Poems selections illustrative of Gold-
smith's character as a man and grade as a poet, from sketches by Ma-
caulay, Thackeray, George Colman, Thomas Campbell, John Forster, and
Washington Irving.  He has also appended, at the end of the volume, a
body of scholarly notes explaining and illustrating the poems, and dealing
with the times in which they  were written as well as the incidents and
circumstances attending their composition.— Christian Intelligencer, N.Y'.
  The notes  are  just and discriminating in tone, and supply all that is
necessary either for understanding the thought of the several poems, or
for a critical study of the language.  The use of such books in the school-
room can not but contribute largely toward putting the study of English
literature upon a  sound basis; and many an adult reader would find in
the present volume an excellent opportunity for becoming critically ac-
quainted with one of the greatest of last century's yoets.—Appletons'
Journal, N. Y.___________

      Published by HARPER & BROTHERS, New York.

J^™ Sent by mail, postage prepaid, to any part of the United States or Canada, on
                       receipt of the price. 
THE    STUDENT'S    SERIES.
With Maps and Woodcuts.  12mo, Cloth, $2 00 per Vol.
THE STUDENT'S CLASSICAL  DICTION-
   ARY.

ANCIENT HISTORY OF  THE EAST.

SMITH'S HISTORY OF GREECE.

COX'S GENERAL HISTORY OF GREECE.

LIDDELL'S HISTORY OF ROME.

MERIVALE'S  GENERAL  HISTORY OF
   ROME.

GIBBON'S DECLINE AND FALL OF THE
   ROMAN EMPIRE.
HISTORY OF FRANCE.

HUME'S HISTORY OF ENGLAND.

STRICKLAND'S QUEENS OF ENGLAND.

HALLAM'S CONSTITUTIONAL HISTORY
  OF ENGLAND.

HALLAM'S MIDDLE AGES.

OLD TESTAMENT HISTORY.

NEW  TESTAMENT HISTORY.

LYELL'S  GEOLOGY.
THE STUDENT'S  SMALLER  HISTORIES.

              With Woodcuts.  16mo, Cloth, $1 00 per Vol.
SCRIPTURE HISTORY.                       ROME.

GREECE.                     |              ENGLAND.

             ANCIENT HISTORY OF THE EAST.
        Published  by HARPER  &  BROTHERS, New York.

Either of the above volumee will  be tent by mail, postage free, on receipt of the price. 
 
 
 
i
 
 
•J^t^S'

                     XjMrWAr
               t***&Mnr.vt rwitf
   -.  -----•z/vf-.tivin'/rivr ■,v;:?;>.a(/(,
r*w&agui.Me-,\vn-iee'::,- r. r •■ >\; e /vxJ"
: fr:.«ffizfettetf m^v «vr^-.v: ■• ry v< cimam
                   (W tvr.S'.s ?WS.t.»Vtl

i