AGRICULTURAL AND BIOLOGICAL PUBLICATIONS 
CHARLES V. PIPER, Consulting Editor 
INVERTEBRATE 
ZOOLOGY McGRAW-HILL 
AGRICULTURAL AND 
BIOLOGICAL PUBLICATIONS 
Charles V. Piper, Consulting Editor 
Babcock and Clausen's- 
GENETICS IN RELATION TO AGRICULTURE 
Babcock and Collins'- 
GENETICS LABORATORY MANUAL 
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Shull's- 
LABORATORY DIRECTIONS IN PRINCIPLES OF 
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CHEMISTRY OF PLANT LIFE 
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BREEDING CROP PLANTS 
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FUNDAMENTALS OF FRUIT PRODUCTION 
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LABORATORY MANUAL OF FRUIT AND VEGETABLE 
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THE SOYBEAN 
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TEXTBOOK OF GENERAL PHYSIOLOGY FOR 
COLLEGES 
Van Cleave's- 
INVERTEBRATE ZOOLOGY (Frontispiece'). 
Stages in the life cycle of Plasmodium vivax, the tertian malarial parasite of man 
(After Luhe~). See legend accompanying Fig. 29 for full description. INVERTEBRATE 
ZOOLOGY 
BY 
HARLEY JONES VAN CLEAVE 
* 
Associate Professor of Zoology 
University of Illinois 
First Edition 
McGRAW-HILL BOOK COMPANY, Inc. 
NEW YORK: 370 SEVENTH AVENUE 
LONDON: 6 & 8 BOUVERIE ST., E. C. 4 
1924 Copyright, 1924, by the 
McGraw-Hill Book Company, Inc. 
PRINTED IN THE UNITED STATES OF AMERICA 
THE MAPLE PRESS COMPANY, YORK, PA. Dedicated to 
HERBERT VINCENT NEAL PREFACE 
An understanding of structure, development, and relationships 
of animals is essential as a basis for all lines of zoological study 
and investigation. In the early history of the teaching of 
zoology, a thorough grounding in these fundamental elements of 
the science was the chief objective of the introductory courses 
offered in colleges and universities. It was through an early 
training of this sort that most of the prominent general zoologists 
of the present and the preceding generations have passed. 
Whether they remained in the older fields of general zoology or 
entered as pioneers into virgin lines of investigation, this broad 
training gave to them an understanding of animals which could 
be gained by no other method. 
There is a growing practice of placing major emphasis upon 
biological principles in the fundamental courses in college 
zoology. There are many arguments in support of such training 
based upon principles for beginners in the science. However, in 
such a course students frequently have so little knowledge of 
animals that there is but limited opportunity for them to have 
either a full understanding of the principles or capacity for 
making applications of them. Under these conditions it becomes 
essential that the introductory course be supplemented by a 
systematic study of organisms. Even in the instances where the 
initial course follows essentially the old type method of instruction, 
the number of forms covered is inadequate and should be followed 
by further studies. 
Because of their relations to human anatomy and development 
and as specific material for instruction of premedical students, 
specialized courses in vertebrate zoology are very generally 
given. For students who are seeking training either as teachers 
or as investigators, there is fully as much need for specific courses 
pertaining to the invertebrates. 
There have been some admirable large treatises dealing with 
the invertebrates, and there have been several American labora- 
tory guides intended for such a course but there have been few 
books available as student textbooks. In the present work, the 
writer has endeavored to collate materials which will serve as a 
class room text and reference work at the same time. An 
ix X 
PREFACE 
introductory course in college zoology is assumed as a prerequisite 
to a course for which this book is designed as a text. 
More material has been included than could ordinarily be 
covered in a single semester. This offers the instructor greater 
opportunity of selection in organizing his work than is possible 
when only the materials for a specific course are presented. 
In zoology textbooks, it is frequently the custom to give a 
detailed description of a representative of each of the major 
groups. The writer has had a firm conviction that specific 
information of this nature is more rdadily grasped by students 
when they approach it through laboratory study of type forms 
with a mind unbiased by minute textbook descriptions. 
When a highly characteristic representative of a group is 
described in full detail in the text, the instructor has the alter- 
native of choosing some less characteristic or less easily available 
species for laboratory study or of permitting his laboratory 
instruction to become largely routine verification of statements 
set forth in the text. 
By avoiding duplication in the text of materials intended for 
laboratory approach, it has been possible to place greater emphasis 
upon biological principles and generalizations in the treatment 
of each group. 
A considerable number of my colleagues, and especially those 
in the University of Illinois, have rendered valuable assistance in 
reading and criticising portions of the manuscript and in offering 
suggestions during its preparation. It is a pleasure to acknowl- 
edge the very able assistance of Bernice F. Van Cleave whose 
criticisms of the early drafts of the manuscript and aid in reading 
the proofs have been of very great value. 
Various publishers have granted the use of cuts or the per- 
mission to reproduce illustrations. Henry Holt and Co. and P. 
Blakiston's Son and Co. have supplied cuts for a considerable 
number of illustrations. The John Wiley and Sons, The Com- 
stock Publishing Co., Gustav Fischer, and the MacMillan Co. 
have granted permission to redraw illustrations and in some few 
instances have furnished cuts. Acknowledgement of these 
courtesies is further made in the legends accompanying the 
text-figures. 
H. J. Van Cleave. 
Urbana, Illinois, 
November, 1923. CONTENTS 
Page 
Preface ix 
CHAPTER I. Introduction 1 
The System of Classification 2 
Primary and subdivisions 2 
Basis for classification 2 
Primitive vs. degenerate 3 
Mutability of group concepts 3 
The law of priority 3 
Phylogenetic relationships 4 
Geological chronology for North America 6 
Reproduction in Invertebrates 7 
Asexual reproduction 7 
Sexual reproduction. . . . , 8 
Specialization of gametes 9 
Early Isolation of germ cells 10 
Gametogenesis and fertilization 11 
Composition and cleavage of zygote 14 
Cleavage patterns 14 
Blastula and gastrula 16 
Mesoderm and later development 16 
Parthenogenesis 17 
Paedogenesis and polyembryony 17 
Breeding habits *. 18 
Birth habits 18 
General References 19 
CHAPTER II. Phylum Protozoa 20 
The cell as an individual 20 
Metazoan tendencies 20 
Organization 21 
Habitats 22 
Food habits 22 
Cultures 23 
Reproduction 23 
Endomixis 25 
Colony formation 25 
Subphylum Plasmodroma 26 
Class 1. Mastigophora 26 
Subclass Phytomastigina 27 
I. Order Chrysomonadina 28 
II. Order Cryptomonadina 28 
III. Order Dinoflagellata 29 
xi xii 
CONTENTS 
Page 
IV. Order Euglenoidina 29 
V. Order Phytomonadina 30 
Subclass Zoomastigina 30 
I. Order Protomonadina 30 
II. Order Polymastigina 32 
III. Order Hypermastigina 33 
IV. Order Distomatina 33 
V. Order Cystoflagellata 34 
Class 2. Rhizopoda 34 
I. Order Amoebina 35 
II. Order Rhizomastigina 36 
III. Order Heliozoa 36 
IV. Order Foraminifera 37 
V. Order Radiolaria 38 
VI. Order Mycetozoa 39 
Class 3. Sporozoa 40 
Subclass Telosporidia 41 
I. Order Coccidiomorpha 41 
Suborder Coccidia 41 
Suborder Haemosporidia 41 
II. Order Gregarinae 45 
Subclass Neosporidia 45 
I. Order Cnidosporidia 46 
Suborder Myxosporidia 46 
Suborder Microsporidia 47 
Suborder Actinomyxidia 47 
II. Order Sarcosporidia 47 
III. Order Haplosporidia 48 
Subphylum Ciliophora (Infusoria) 48 
Class 1. Ciliata 48 
I. Order Holotricha 50 
II. Order Heterotricha 51 
III. Order Oligotricha 51 
IV. Order Hypotricha 51 
V. Order Peritricha 52 
Class 2. Suctoria 53 
Interrelationships of the classes of Protozoa 53 
References 54 
CHAPTER III. Introduction to the Metazoa 56 
A. The Mesozoa 57 
B. Histological differentiation 57 
Epithelial tissues. . 58 
Definition and classification 58 
Form vs. function 58 
Ciliated and flagellated epithelium 58 
Protective epithelium 59 
Sensory or communicative epithelium 60 CONTENTS 
xiii 
Page 
Germinal epithelium 60 
Connective or supporting tissue 61 
Definition 61 
Muscle tissue 62 
Nervous tissue 63 
Morphological changes in the nucleus 65 
C. The organ systems 65 
The digestive system 66 
The circulatory system 68 
The respiratory system 69 
The excretory system 71 
The reproductive system 71 
The locomotor system 73 
The nervous and sensory system 73 
References 76 
CHAPTER IV. Phylum Porifera 77 
Class 1. Calcarea 80 
Class 2. Hexactinellida 80 
Class 3. Demospongia 80 
References 81 
CHAPTER V. The Coelenterates and Ctenophores 82 
A. Phylum Coelenterate 82 
Class 1. Hydrozoa 84 
I. Order Leptolinae 86 
II. Order Trachylinae 87 
III. Order Hydrocorallina 87 
IV. Order Siphonophora 88 
Class 2. Scyphozoa 88 
I. Order Stauromedusae 90 
II. Order Peromedusae 90 
III. Order Cubomedusae 90 
IV. Order Discomedusae 90 
Class 3. Anthozoa • 91 
Subclass Zoantharia 93 
I. Order Actinaria 94 
II. Order Madreporaria 94 
III. Order Antipatharia 94 
Subclass Alcyonaria 94 
I. Order Alcyonacea 94 
II. Order Gorgonacea 94 
III. Order Pennatulacea 95 
B. Phylum Ctenophora 95 
References 95 
CHAPTER VI. Phylum Plathelminthes 98 
Class 1. Turbellaria 99 xiv 
CONTENTS 
Page 
Class 2. Trematoda102 
Subclass Monogenea102 
Subclass Digenea104 
Class 3. Cestoda106 
Subclass Cestodaria107 
Subclass Cestoda (s. str.)108 
Class 4. Nemertinea Ill 
References114 
CHAPTER VII. Phylum Nemathelminthes116 
Class 1. Nematoda116 
Class 2. Gordiacea120 
Class 3. Acanthocephala121 
References123 
CHAPTER VIII. Phylum Trochelminthes125 
Class 1. Rotifera125 
Class 2. Gastrotricha129 
References130 
CHAPTER IX. Phylum Coelhelminthes (Annelida)131 
Class 1. Chaetognathi131 
Class 2. Chaetopoda133 
Subclass Polychaeta135 
I. Order Errantia137 
II. Order Sedentaria 137 
Subclass Oligochaeta138 
Class 3. Hirudinea139 
Class 4. Archiannelida141 
Class 5. Gephyrea142 
References142 
CHAPTER X. Phylum Molluscoidea144 
Class 1. Polyzoa144 
I. Order Ectoprocta145 
II. Order Endoprocta146 
Class 2. Brachiopoda146 
Class 3. Phoronida148 
References148 
CHAPTER XI. Phylum Echinoderma150 
Subphylum Eleutherozoa152 
Class 1. Asteroidea152 
Class 2. Ophiuroidea157 
Class 3. Echinoidea158 
Class 4. Holothuroidea161 CONTENTS 
XV 
Subphylum Pelmatozoa 162 
Class Crinoidea 163 
References 165 
CHAPTER XII. Phylum Mollusca 166 
Class 1. Amphineura 167 
I. Order Aplacophora 167 
II. Order Placophora 167 
Class 2. Acephala' 168 
I. Order Protobranchia 178 
II. Order Filibranchia . . . 178 
III. Order Pseudolamellibranchia 178 
IV. Order Eulamellibranchia 178 
V. Order Septibranchia 174 
Class 3. Scaphopoda 174 
Class 4. Gasteropoda 174 
Subclass Prosobranchia 177 
I. Order Aspidobranchia 177 
II. Order Ctenobranchiata 177 
Subclass Euthyneura 177 
Class 5. Cephalopoda 178 
Subclass Tetrabranchia 180 
Subclass Dibranchia 180 
I. Order Decapoda 180 
II. Order Octopoda 181 
References 181 
CHAPTER XIII. Phylum Arthropoda-Introduction and Class 
Crustacea 183 
Class 1. Crustacea 185 
Subclass Trilobita 188 
Subclass Phyllopoda 189 
I. Order Branchiopoda 190 
II. Order Cladocera 191 
Subclass Copepoda 192 
Subclass Ostracoda 193 
Subclass Cirripedia 194 
Subclass Malacostraca 196 
I. Order Phyllocardia , ... . 197 
II. Order Schizopoda 197 
III. Order Stomatopoda 197 
IV. Order Decapoda 198 
V. Order Amphipoda 201 
VI. Order Isopoda 202 
References 202 
CHAPTER XIV. Phylum Arthropoda Exclusive of Crustacea 
and Insecta 204 
Class 2. Acerata 204 
Subclass Gigantostraca 204 xvi 
CONTENTS 
Page 
Subclass Arachnida 205 
I. Order Scorpionida 206 
II. Order Araneina 207 
III. Order Acarina 207 
IV. Other Orders of Arachnida 208 
Class 3. Pycnogonida 209 
Class 4. Tardigrada 210 
Class 5. Onycopho ra 210 
Class 6. Myrientomata 211 
(The Myriapods) 212 
Class 7. Diplopoda 212 
Class 8. Chilopoda 212 
Class 9. Symphyla 212 
Class 10. Pauropoda 212 
References 213 
CHAPTER XV. Phylum Arthropoda (Concluded) 214 
Class 12. Insecta 214 
References 230 
CHAPTER XVI. Phylogeny 231 
References 239 
Index 241 INVERTEBRATE ZOOLOGY 
CHAPTER I 
INTRODUCTION 
This book deals with that portion of the field of zoology which 
concerns all animals other than those included in the single 
phylum Chordata. Its scope is thus expressed in negative 
terms, for all animals lacking a backbone are invertebrates. 
Early in the development of modern classification the term 
Vertebrata was introduced as the name of a phylum or branch to 
include the four highest classes of the Linnean system. Popu- 
larly, then, all animals have become recognized as either verte- 
brates or invertebrates. More recently, three small groups of 
animals, including Amphioxus, the sea-squirts, and Balano- 
glossus, have been shown to possess characters which seem to 
point to definite relationships with the Vertebrata, even though 
they lack a vertebral column. One of these characters is the 
presence of a notochord, a structure which in the embryology 
of the vertebrates is the forerunner of the vertebral column. 
As a consequence, the Leptocardii, the Tunicata, and the Enter- 
opneusta as prochordates have been combined with the true 
Vertebrata to form a phylum which bears the name Chordata. 
Technically, then, the terms chordate and non-chordate furnish 
a more sound basis for distinguishing the highest phylum of 
animals from all the lower phyla combined but the widespread 
and popular acceptance of the terms vertebrate and invertebrate 
have operated against the general introduction of these terms. 
Strictly speaking, the prochordates are invertebrates but they 
are not discussed in this book. 
In the classification here adopted, twelve non-chordate phyla 
are recognized, as follows: Protozoa, Porifera, Coelenterata, 
Ctenophora, Plathelminthes, Nemathelminthes, Trochelminthes, 
Coelhelminthes, Molluscoidea, Echinoderma, Mollusca, and 
Arthropoda. 
1 2 
IN VER TEBRA TE ZOOLOG Y 
THE SYSTEM OF CLASSIFICATION 
Primary and Subdivisions.-It should be recalled that each 
phylum is made up of classes, orders, families, genera, and 
species in descending sequence, and that each primary division 
is capable of still further arrangement into sub- and supergroups. 
Thus, when a class comprises a large number of orders, it 
frequently becomes convenient to combine the orders within the 
single class into two or more groups by uniting those orders 
which seem to have essential features in common into a sub- 
class or a superorder. Each major or primary group may thus 
have within it secondary groups of greater scope than the next 
lower primary division. 
Basis for Classification.-The entire system of classification 
of animals is based upon interpretation and judgment. There 
are no absolute units of measure which determine how many 
species or how many classes any phylum is to include. Every 
attempt at classification in some measure aims to express varying 
degrees of relationship between different organisms, and every 
decision regarding relationships is based upon fixed premises. 
Proofs of phylogenetic relationships are not procurable through 
direct observation but evidences are offered in the structure and 
development of the individual and frequently through fossil 
remains of extinct forms which serve as bridges or connecting 
links between our modern groups. Man's knowledge of all 
organisms is, at best, but fragmentary. A comparison even of 
the known facts of structure, development, and habits of two 
organisms involves much interpretation in determining the 
relative importance which is to be ascribed to each set of facts. 
Some structures and organs are highly variable even among the 
offspring of the same parents, while other characters may be 
relatively fixed in members of one group and highly variable in 
those of another. Obviously, then, all facts are not of equal 
significance in determining relationship and there is no arbitrary 
means of predicting which are of value and which are worthless 
in making comparisons. 
In common parlance, one organism is said to be higher or 
lower than another without any conscious analysis of how the 
decision has been reached. Such an expression, in order to carry 
weight, is reached only after numerous comparisons have been 
made and a decision has been reached as to what differences are 
essential and what incidental. INTRODUCTION 
3 
Primitive vs. Degenerate.-Simplicity of organization is not 
always a safe guide in the interpretation of relationships for sim- 
plicity of form and structure may be either primitive or secon- 
darily derived as a consequence of degeneracy. A tapeworm is 
simpler in many points of organization than a planarian, because 
the tapeworm lacks a digestive system and special sense organs, 
both of which are present in the planarian. Primitively, every 
organism must be able to digest and assimilate its own food 
materials. Dependence upon some other individual for perform- 
ing a process essential to life cannot be a primitive condition but 
is secondarily acquired and has been accompanied by degeneration 
of the digestive organs in perfectly adjusted parasites. The 
tapeworm is consequently degenerately and not primitively more 
simple than the planarian. 
Mutability of Group Concepts.-Throughout the entire system 
of classification, different premises and definitions lead to wholly 
divergent conclusions. A group which has been considered as 
a class by one zoologist may be set apart as an independent 
phylum by another, just because the same observed facts receive 
different interpretation and the same terms are defined differently. 
In elementary zoology courses, a rigid system of classification is 
frequently taught but the advanced student must sooner or later 
appreciate the fact that group concepts are human made devices 
adopted for man's convenience in his discussions of organisms 
which seem to be related. Shift of emphasis or new facts and 
new interpretations play an important part in formulating any 
scheme of classification for, after all, group concepts are con- 
structed to include organisms and are not necessarily expressions 
of natural laws with which the organisms must of necessity agree. 
The Law of Priority.-Scientific names of the larger subdivisions 
of the animal kingdom are subject to considerable differences in 
usage, for there are no fixed rules governing the acceptance or 
rejection of names pertaining to phyla, classes, and orders. In 
contrast with this, the use of names for species, genera, and 
families is definitely controlled by rules or laws. . An Inter- 
national Commission on Zoological Nomenclature was established 
by the International Zoological Congress (1895) to formulate a 
code of rules or laws governing the problems of naming animals. 
One of the basic principles of this code has been the Law 
of Priority. According to this law the valid name of a genus or 
of a species can be only that scientific name under which it was 4 
INVERTEBRATE ZOOLOGY 
first described, providing that the name is binomial and has not 
been used previously for some other animal. The same specific 
name may be used for any number of different animals belonging 
to different genera. 
The tenth edition of Linnaeus' Systema Naturae was published 
in 1758. Since the appearance of this work marks the first 
general application of binary nomenclature in zoology, this 
date has been accepted by the International Commission as 
the starting point for application of the law of priority. 
It frequently happens that after certain generic and specific 
names come into general use some one discovers that an earlier 
writer had applied a different binomial name to the same animal. 
Then the generally accepted name has to be dropped in favor of 
the prior name. Confusion and inconvenience frequently result 
from the application of this law but it seems to be the only safe 
means whereby scientific names may be accepted. 
It also frequently happens that a generic name has been 
used previously for some other genus of animals and in this 
instance only the earliest use of the name stands as valid, for the 
second time the name is used it is considered as a homonym. 
The name Trichina was applied to a parasitic worm in 1835, 
but in 1830 the same name had been assigned to a genus of insects. 
It therefore became necessary to rename the worm and even 
after the name Trichina came into very general and popular 
use the worm was renamed Trichinella. 
Since the name of a family is formed by adding the suffix idae to 
the stem of the name of the type genus, family names are also gov- 
erned by the same laws which govern the use of generic names. 
Phylogenetic Relationships.-The arrangement of animal 
groups in a list or in a book of necessity follows a sequence which 
places the lowest at one end and the highest at the other. Fre- 
quently, this arrangement carries with it the idea that classifica- 
tion intends to express a linear relationship of all forms, that 
each group has arisen from or has evolved from the one preceding. 
Such a hypothesis was held by some of the early zoologists but 
the more commonly accepted idea of today postulates that our 
present day animals are not directly related but that two groups 
bear closer or more distant relationship to each other chiefly 
through an extinct form which is an ancestor of both. Thus, 
for example, among the flat worms the Turbellaria are regarded as 
the most primitive class in the phylum Plathelminthes while the INTRODUCTION 
5 
Trematoda are considered as a higher class. This does not mean 
that trematodes had their origin from the turbellarians but prob- 
ably signifies that both trematodes and turbellarians had a com- 
mon origin through some ancestral group of past time which was 
neither trematode nor turbellarian. 
Thus, instead of a linear arrangement illustrating the modern 
idea of phylogenetic relationship of the animal groups, this 
relationship is best ex- 
pressed as a branching 
tree-like structure (Fig. 
1) in which the various 
groups are represented 
by the branches. 
Closely related groups 
would be represented 
by a forking of a com- 
mon ancestral branch. 
One point wherein the 
tree comparison may 
not be carried too far 
is that many of the 
branches connecting 
modern day forms 
are dead or extinct. 
Through the field of 
paleontology we have 
an imperfect picture 
of these connecting 
branches in the records 
left as fossil animals-■ 
imperfect because 
there are so many 
groups like the naked Protozoa and the worms of which but few 
fossils are known. Because of the significance of these extinct 
animals in considering our modern fauna, attention is directed to 
some of the more important fossil forms throughout the body of 
this book. The necessity of considering our fauna as a product of 
the animal life of the past makes it seem advisable to include here 
a reference table showing the sequence of some of the more impor- 
tant geological periods with some of the dominant forms of life 
characteristic of each. 
THE PHYLUM PROTOZOA 
Fig. 1.-Phylogenetic arrangement of the classes 
in the phylum Protozoa. 6 
INVERTEBRATE ZOOLOGY 
Geologic Chronology for North America 
(Slightly modified from Lull) 
Eras 
Major divisions 
Periods 
Dominant life 
Psychozoic 
Age of Man 
Coenozoic 
Quaternary 
Glacial 
The Age of Mammals 
Tertiary 
Late Tertiary 
Early Tertiary 
Mesozoic 
Late Mesozoic 
Epi-Mesozoic 
Interval 
Rise of Archiac Mammals 
Cretaceous 
Extreme specialization of 
reptiles 
Comanchian 
Early Mesozoic 
Jurassic 
Rise of birds and flying 
reptiles 
Triassic 
Rise of Dinosaurs 
Paleozoic 
Late Paleozoic or 
Carboniferous 
Epi-Paleozoic 
Interval 
Extinction of ancient life 
Permian 
Rise of land vertebrates, 
modern insects and am- 
monites 
Pennsylvanian 
Rise of insects and prim- 
itive reptiles 
Mississippian 
Rise of echinoderms and 
sharks 
Middle Paleozoic 
Devonian 
Rise of Amphibia 
Silurian 
Rise of scorpions and lung- 
fishes 
Early Paleozoic 
Ordovician 
Rise of corals, nautilids, 
and armored fishes 
Cambrian 
Rise of shelled animals and 
dominance of trilobites 
Late Proterozoic 
Algonkian 
Age of primitive marine 
invertebrates but very 
few fossils known 
Early Proterozoic 
Neolaurentian 
Archeozoic 
Paleolaurentian 
No fossils. Probably only 
single-celled organisms INTRODUCTION 
7 
REPRODUCTION IN INVERTEBRATES 
Many of the early naturalists believed in spontaneous genera- 
tion of life, that is, that non-living matter gives rise to living 
organisms continuously. As early as the seventeenth century 
Redi and some other leaders in science advanced evidences that 
the more complex organisms cannot originate in this manner. 
It is only within the past century, however, that the spontaneous 
generation of the lower organisms was finally discredited and 
disproved. That all life comes only from living things was only 
then established as an axiom. Power of reproducing its like is 
an inherent property of protoplasm which sharply differentiates 
it from all lifeless matter. Though all forms of life possess this 
power of reproduction, the detailed steps in the genesis of new 
individuals are far from uniform throughout the animal kingdom. 
Of the varied methods of reproduction encountered among ani- 
mals, common features permit them all to be classified as either 
asexual or sexual. Any reproductive process which involves the 
genesis of new individuals through the functioning of specialized 
cells, termed the gametes or germ cells, is sexual. Conversely, 
any reproductive process which does not involve the functioning 
of germ cells is asexual. 
Asexual Reproduction.-In the simplest forms of animal life, 
the body of a single individual becomes divided into two or more 
parts each of which by the growth processes assumes approxi- 
mately the size of the original organism which produced it. This 
kind of a reproductive process wherein no germ cell functions is 
termed asexual, and depends upon the power of part of an organ- 
ism to reproduce the whole (Figs. 2-4). If the products resulting 
from such a division of the body are approximately equivalent, 
the term fission is applied to the process. Fission is further 
recognized as simple or binary (Fig. 2) and multiple (Fig. 3) 
depending upon whether two or numerous individuals result. 
In binary fission, the direction of the dividing plane is frequently 
indicated by specifying whether the fission is longitudinal or 
transverse. Fission is characteristic of many Protozoa and 
occurs in isolated instances through many metazoan groups, 
but in the latter asexual reproduction through the formation of 
buds is more frequent. In budding (Figs. 4 and 36), a relatively 
small part of the body of a parent individual becomes modified 
as a starting point of a new individual. Only after development 
has gone to certain stages is the bud recognizable as similar to 8 
INVERTEBRATE ZOOLOGY 
the parent. Ultimately, the bud may separate from the parent 
and become an independent organism, or, in some instances, 
it remains attached permanently and successive generations 
retain bodily connection thereby producing a colony. 
Buds usually occur on the external surface of the parent indi- 
vidual, but in some instances groups of cells within the body 
become surrounded by a membrane and form what are desig- 
nated as gemmules or internal buds. Typically, these are highly 
resistant bodies which are liberated by the disintegration of the 
body of the dead parent, and as resting gemmules tide the species 
over times and conditions which are unsuited for a vegetative 
period. The gemmules of sponges and statoblasts of the Polyzoa 
are among the best examples of internal buds or gemmules. In 
both origin and structure, these are multicellular. 
Sexual Reproduction.-There are relatively few Metazoa which 
rely upon asexual reproduction exclusively. More frequently a 
Fig. 2. 
Fig. 3. 
Fig. 4. 
Figs. 2-4.-Diagrams to illustrate 'the methods of asexual reproduction: 
2, binary fission in Paramecium; 3, multiple fission or spore formation in a proto- 
zoan; 4, budding in Acanthocystis. 
process involving the specialization of cells for reproduction is 
encountered. Any cell specialized for this purpose is designated 
as a gamete or germ cell, and reproduction through such an 
agency is termed sexual reproduction. Even among the Protozoa 
(Fig. 5) germ cells are encountered. Despite their relative sim- 
plicity in organization, there are comparatively few protozoans 
for which a complete developmental history is known. For 
some of the most commonly known forms, ignorance of anything 
beyond an asexual multiplication has led to a widespread belief 
that simple division is the only means of reproduction. How- 
ever, in the Protozoa which have been thoroughly investigated, INTRODUCTION 
9 
it has been found that an asexual cycle (schizogony) is frequently 
followed by a sexual cycle (sporogony). Presence of a compli- 
cated life cycle, involving an alternation of generations, has 
thus been established for some single-celled creatures. (See 
Fig. 29.) 
Fig. 5.-Eudorina elegans Ehrenberg. A, adult colony, X475; B, daughter 
colony produced by division of one of the cells of A, X730; C-E, development of 
spermatozoa from a mother cell; F, spermatozoa. (From Shull, LaRue, and 
Ruthven after West and Goebel). 
Specialization of Gametes.-In its most characteristic form, 
sexual reproduction involves the complete fusion of two special- 
ized gametes to form a zygote (Fig. 6). In some instances, 
however, the fusing cells are termed isogametes because they 
seem to be alike in form and function. Though isogametes have 
considerable significance in hypothetical discussions of the origin 
and differentiation of sex cells, they are relatively infrequent in 
occurrence among animals. Even in some instances in which 
fusing gametes are indistinguishable in size the two react differ- 
ently to cytological stains and thus give evidence of a probable 
differentiation even though morphological differentiation is 
lacking. 
The gametes of most animals show two distinct lines of differ- 
entiation. The enlargement of the cell through accumulation of 
yolk or deutoplasm is characteristic of macrogametes or egg 
cells. In some instances, the ovum does not contain all of the 
stored food material but is accompanied by special storage cells 
as yolk cells or follicle cells. Special protective envelopes or 10 
INVERTEBRATE ZOOLOGY 
shells are frequently developed about the egg cell. The male 
germ cell or microgamete is usually very minute and represents 
a specialization for effective locomotion. 
In many kinds of animals, but a single type of germ cell is 
produced by any given individual. Those which produce macro- 
gametes or ova are designated as females while those which pro- 
duce microgametes or spermatozoa are called males. When 
both kinds of germ cells occur in the body of the same individual 
such a one is said to be hermaphroditic. Usually this condition 
involves the presence of two distinct gonads but in some molluscs 
there is a hermaphroditic gonad which produces both eggs 
and spermatozoa. 
Fig. 6.-Diagram of fertilization and cleavage. It is assumed that maturation 
of the egg has been completed before the entrance of the spermatozoon. {After 
Sharpe). 
Early Isolation of Germ Cells.-Long before germ cells become 
functional they are readily distinguishable from the other cells 
of the body. The exact point at which this differentiation of 
germ cells from somatic cells is accomplished has been determined 
for only a few animals but in these instances the cells destined to 
form the gametes are distinguishable at a remarkably early period 
in development of the young. In Ascaris megalocephala, a 
nematode of the horse, the first division of the fertilized egg (Fig. 
7) separates two cells only one of which (s) retains all of its 
chromatin intact. Portions of the chromosomes in the other 
cell are cast off into the cytoplasm. It is only from the cell with 
unaltered nuclear content that the germ cells have their origin. INTRODUCTION 
11 
But even in this instance the separation of germ cells from somatic 
cells is not so direct as this narration might imply. In each 
of about the first five divisions of the cell with its full complement 
of chromatin one of the two resulting cells undergoes a chromatin 
diminution but at the end of the fifth or sixth cleavage the germ 
plasm has been isolated and in all subsequent divisions every 
blastomere gives rise to either somatic or germ cells. 
In several other instances, it has been noted that the cells 
which later produce the gametes are distinguishable early in 
embryonic life. In insect embryos, the cells from which the 
Fig. 7.-Two cell stage in development of the egg of Ascaris megalocephala, 
showing chromatin diminution in somatic cells. Germ cells are derived from 
blastomere marked s which retains all of its chromatin. (After Boveri). 
gametes are formed are distinctly larger than the somatic cells. 
Thus in the fly Miastor, the primordial germ cells (Fig. 8, gc) 
are readily distinguishable from the somatic cells (cZ) early in the 
cleavage of the egg. 
Gametogenesis and Fertilization.-Even though the cells 
which later go to form the gametes are early distinguishable from 
the somatic cells, they must pass through a complicated series 
of changes before they are capable of union in fertilization. These 
changes are collectively termed gametogenesis, or in the male, 
spermatogenesis and in the female, oogenesis (Fig. 9, B-D). 
Three periods are recognizable in gametogenesis: a multiplica- 
tion period wherein the relatively small number of primordial 12 
INVERTEBRATE ZOOLOGY 
germ cells is greatly augmented; a growth period, which involves 
fundamental changes in the nuclear organization of the cell and 
in relative size; and finally a maturation period during which the 
chromosomes in the gametes are reduced to one-half the number 
characteristic of the somatic nuclei. The chromosome number in 
cells which have not undergone maturation is said to be diploid 
because each cell contains two sets of chromosomes, one of which 
is derived from its male parent and the other from the female 
parent. Following maturation, 
the gametes are said to have the 
haploid or reduced number of 
chromosomes. 
The reduction in chromosome 
numbers is in preparation for 
fertilization or the union of two 
gametes to form a fertilized egg 
or zygote (Fig. 9). By this 
reduction phenomenon the ferti- 
lized egg, and consequently all 
of the cells resulting from its 
mitotic division, retain a con- 
stant number of chromosomes 
from generation to generation. 
Through fertilization a sperm 
cell with the haploid number of 
chromosome unites with an egg 
cell with the haploid number to 
form a zygote whose nuclear 
composition is diploid. 
In the maturation process, two 
kinds of chromosomes are usu- 
ally distinguishable. These are 
the ordinary chromosomes which 
are designated as the autosomes 
and others which differ from them both in appearance and in 
behavior and are termed the sex-chromosomes. 
In some instances, the immature germ cells of each sex have 
a pair of sex-chromosomes. Typically, the sex-chromosomes of 
the female are both alike and are called the X-chromosomes. 
On the other hand, in the immature germ cells of the male one 
of the sex-chromosomes is frequently smaller than the other 
Fig. 8.-Development of a centro- 
lecithal egg of the fly Miastor. 
Blastula stage showing germ cells (grc) 
at posterior extremity readily distin- 
guishable from the other blastomeres 
(cl). (From Shull, LaRue, and Ruthven 
after Hegner). INTRODUCTION 
13 
and is designated as the Y-chromosome. Under these conditions 
the autosomes of the maturing male germ cells are distributed 
evenly and equally among the mature cells but the sex-chromo- 
somes become segregated so that any individual cell in addition 
to its autosomes contains either an X- or a Y-chromosome, but 
never both. In the male, then, half of the sperm cells contain 
the Y-chromosome but no X, while the other half contain an 
X-chromosome but no Y. Every mature egg cell contains the 
autosomes and a single X-chromosome. Fertilization of an egg 
Fig. 9.- Maturation of the egg and fertilization in Ascaris. A, spermatozoon 
about to enter egg; B, spermatozoon inside egg; egg nucleus in anaphase of first 
maturation division; C, completion of first polar body; D, late anaphase in forma- 
tion of second polar body; E, masturation of egg completed, male and female 
pronuclei, each with two chromosomes, meeting; F, formation of first cleavage 
spindle from centrosome of spermatozoon, with two paternal and two maternal 
chromosomes in late prophase. (From Sharpe after Hertwig). 
cell by a spermatozoon containing the Y-chromosome produces a 
zygote from which only a male could develop but fertilization 
by a spermatozoon with an X-chromosome produces a zygote 
from which only a female could develop. 
The Y-chromosome may be entirely lacking in some species. 
Under these conditions, there are two kinds of male* sex cells 
formed, one with the autosomes plus an X-chromosome; the 
other with autosomes alone. 14 
INVERTEBRATE ZOOLOGY 
While the dimorphism in the germ cells is usually characteristic 
of the male, there are some instances recorded in which all of 
the male germ cells are alike, but the female germ cells show 
chromosome differences. 
Composition and Cleavage of Zygote.-Except for the possi- 
bility that one of the fusing germ cells may have one more 
chromosome than the other, the two gametes contribute equally 
to the chromatin content of the zygote. The cytoplasm of the 
zygote is that contained in the mature egg except for a practically 
insignificant amount brought in by the entering sperm cell. The 
stored food material, which provides the energy requisite to the 
life processes of the embryo, is furnished by the deutoplasm of 
the egg cell, except in those forms which have accessory yolk 
cells accompanying the egg and those which receive nourishment 
from the parent individual. The so-called middle piece of the 
sperm cell contains a centrosome. Barring some very unusual 
circumstances, this centrosome brought in by the male pronucleus 
forms a mitotic spindle within which the chromatin of both the 
male and female pronuclei becomes co-mingled. The mitotic 
division of the fertilized egg which ensues is followed by cleavage 
of the surrounding cytoplasm. Through a continued sequence 
of mitosis and cleavage, large numbers of cleavage cells or 
blastomeres are formed. 
Cleavage Patterns.-The relative size and arrangement of 
these blastomeres is greatly influenced by the amount and distribu- 
tion of the stored material within the egg. If the yolk is evenly 
distributed, the egg is said to be homolecithal and it undergoes 
a complete cleavage resulting in the formation of numerous cells, 
all of which are practically uniform in size. However, deuto- 
plasm is heavier than the surrounding cytoplasm, and in many 
instances tends to accumulate at the vegetative pole of the egg. 
The term telolecithal is applied to such an egg. Yolk serves 
as a mechanical obstruction to the paths of the cleavage planes. 
Consequently, if the vegetative pole is heavily yolk laden, cleav- 
age is restricted to a disc of cytoplasm at the animal pole. How- 
ever, in some telolecithal eggs, the entire cell cleaves, but, since 
the mitotic spindle tends to take a position in the center of the 
cytoplasm of the cell, spindles will be formed nearer the animal 
than the vegetative pole and as a consequence the resulting cells 
are unequal in size. The cells at the animal pole are much 
smaller than those at the vegetative pole. Some arthropod eggs INTRODUCTION 
15 
have the yolk collected in the center and are therefore said to 
be centrolecithal. Cleavage in such an egg is restricted to the 
layer of cytoplasm surrounding the yolk (Fig. 8) and is referred 
to as superficial cleavage. 
The blastomeres resulting from cleavage of the fertilized 
egg assume various arrangement patterns. In some groups of 
animals, the blastomeres fail to follow any orderliness in their 
formation and arrangement. Instances of this sort are desig- 
nated as indeterminate cleavage. This condition stands in sharp 
contrast with that found in some other groups, the members of 
which have cleavage processes so orderly that it is possible to 
predict with exactitude the direction which successive cleavage 
planes will take. Determinate cleavage, as this is called, renders 
it possible to trace the history of each blastomere, to follow 
Fig. 10.-Diagrams showing cleavage pattern in Mollusca and Annelida. 
a, blastomeres of Nereis {after Wilson)-, b and c, blastomeres of Crepidula. 
The stippled cells form the Molluscan Cross. The four unshaded cells at the 
pole of the embryo are termed the apical cells, while the unshaded cells radiating 
from these comprise the Annelidan Cross. {After Conklin). 
through a "cell-lineage" the succession of blastomeres, and to 
tell ultimately what organ or structure of the larva or adult is to 
be formed from any given blastomere. This recognition of a 
fixed arrangement of the cells is especially characteristic of anne- 
lids and molluscs (Fig, 10). The annelidan cross and the mollus- 
can cross are terms which are applied to the fixed cleavage pat- 
terns characteristic of these respective groups. The orderly 
processes which give rise to fixed relations of the cells of the 
embryo are carried throughout the entire life of some organisms, 
for scattered through the various phyla there are examples of 
animals which in the adult stage possess an absolutely fixed 
number of cells or of nuclei in all or in part of the organs of 
the body. Such a condition is spoken of as cell-constancy or 
nuclear-constancy. 16 
INVERTEBRATE ZOOLOGY 
Blastula and Gastrula.-All eggs pass through a stage called 
the blastula wherein the blastomeres are usually arranged in a 
single layer. Typically, the cells surround a central fluid-filled 
cavity, but in centrolecithal eggs the fluid-filled cavity is replaced 
by a solid yolk mass. The cells in certain regions of the blastula 
multiply more rapidly than the surrounding cells. Typically, in 
the homolecithal egg this unequal growth causes part of the wall 
of the blastula to be forced within the blastula cavity as an 
inpocketing or invagination. This stage in development is termed 
a gastrula and is composed typically of an outer layer of cells, 
the ectoderm, and an inner layer, the entoderm, which surrounds 
a central cavity, the gastrula cavity or archenteron. Primitively, 
this archenteron is in communication with the exterior through an 
opening, the blastopore. A gastrula such as that just described 
occurs only in the homolecithal type of egg. In telolecithal eggs, 
with total cleavage, the small blastomeres at the animal pole 
multiply much more rapidly than do the heavily yolk laden vege - 
tative cells. As a consequence, the cells from the animal pole grow 
down and surround those of the vegetative pole which thereby 
become the entoderm while the small surface cells are recog- 
nizable as ectoderm. In such a gastrula, there is no archenteron 
within the entoderm. Gastrulation by this method is termed 
epibolic gastrulation. In telolecithal eggs with only partial 
cleavage, a modified invagination occurs through a shoving in of 
cells near the edge of the cleavage disc. As mentioned above, 
blastula formation in the centrolecithal eggs gives rise to a layer 
of cells arranged around the central yolk mass. In later develop- 
ment, each cell of this blastula undergoes a cleavage parallel 
to the surface of the egg. Thereby a two layered condition or a 
gastrula is attained and the process is spoken of as delamination. 
Mesoderm and Later Development.-The addition of the third 
body layer, the mesoderm, between the ectoderm and the ento- 
derm is one of the most conspicuous and most significant features 
in later development of all animals above the coelenterates. 
Development from this point onward, is subject to so many indi- 
vidual differences that few general statements may be made. 
In many of the more generalized groups, the attainment of the 
gastrula stage marks the beginning of an independent existence 
by the young for through cilia it has the power of movement and 
through the blastopore takes food into the archenteron where it is 
digested and assimilated. However, in many groups and espe- INTRODUCTION 
17 
cially in terrestrial and fresh-water forms, the individual is carried 
far past the gastrula stage while still confined within the egg 
membranes. In the extreme of such cases, the individual 
which emerges from the egg is essentially like the adult except in 
size and stage of development of the reproductive organs. 
Throughout its development, the young of such a form would be 
referred to as an embryo. On the other hand, if the individual 
produced from the egg lacks some structures characteristic of the 
adult and possesses others which are lost in later development, the 
young is ordinarily termed a larva. Many different larval forms 
are encountered in the various invertebrate groups but these are 
so numerous and have such diverse forms that descriptions of 
them will be given in the discussion of the groups in which each 
type belongs. It should be mentioned, however, that these 
larval stages are frequently considered as having great phylo- 
genetic significance. Groups having fundamentally similar 
larvae are usually considered as having developed from a common 
ancestral form, for there are many evidences supporting the law 
of biogenesis which states that ontogeny is a brief recapitulation 
of phylogeny. 
Parthenogenesis is sexual development without fertilization. 
The eggs of many kinds of animals undergo a modified type of 
maturation and are then capable of development without fertili- 
zation. It is not uncommon for parthenogenesis and true sexual 
reproduction to alternate in the life cycle of the same species. 
The parthenogenetic habit has become thoroughly established in 
some species of animals. Frequently, in such instances, males are 
extremely rare and there are some species which reproduce 
parthenogenetically in which males have never been observed. 
Paedogenesis and Polyembryony.-The gonads of some indi- 
viduals become functional before the body reaches adult form. 
In some of the dipterous insects (flies of the genus Miastor, for 
example) the larvae become precociously mature and as maggots 
produce mature eggs. These eggs undergo parthenogenetic 
development within the body of the larva and a new generation of 
larvae is produced within the body of each. This type of pre- 
cocious parthenogenetic development is termed paedogenesis. 
It has been demonstrated that in some instances a single 
fertilized egg may give rise to more than one individual. This 
condition, which has been termed polyembryony, occurs in both 
invertebrates and vertebrates and is especially characteristic of 18 
INVERTEBRATE ZOOLOGY 
some insects. This power of development of an entire individual 
from a portion of an embryo calls to mind the fact that experi- 
mentally the blastomeres of many of the marine invertebrates 
may be isolated and each blastomere thus separated forms a 
complete individual. 
Breeding Habits.-Regarding breeding habits, numerous 
different conditions exist. In the Protozoa and among many 
lower Metazoa, isolated gametes are set free into the surrounding 
medium and fertilization occurs entirely apart from the bodies 
of the parent individuals. Motility, at least of the male gametes, 
and chemical emanations from the female gamete bring the two 
germ cells together and thus insure fertilization. Under such 
conditions, the larva or young of Metazoa passes through its 
embryological stages until it is capable of independent mainte- 
nance. Among many Metazoa, this stage is reached with the 
gastrula for at this time the larva contains organs differentiated 
sufficiently to enable it to ingest and utilize food from the outside 
world. For its metabolism prior to this time, the embryo has 
been dependent upon the food substances stored within the egg. 
Correlated with increase in quantity of stored food material 
within the egg, the young of many species undergo complete 
development upon the materials thus furnished, as mentioned 
above, and never lead an independent larval existence. 
Frequently, the eggs are fertilized while still within the body of 
the female. This may involve a copulation whereby sperm cells 
arc deposited within the body of the female by the male through 
some sort of an intromittent organ, or, in some instances, as in 
the rotifers and leeches, sperm cells are united in groups called 
spermatophores which penetrate the body wall of the female. 
Parts of the body of the female may be modified as a seminal 
receptacle for receiving the sperm cells from the male, though 
fertilization of the eggs may be deferred for some time even 
several years after copulation has taken place. In hermaphrodi- 
tic forms, there are several distinct methods of fertilization. 
Reciprocal copulation whereby sperm cells from one hermaphro- 
ditic individual fertilize the eggs of the other, occurs in many 
instances. Self-fertilization is also encountered in some herma- 
phroditic organisms. 
Birth Habits.-When eggs are discharged before cleavage has 
begun, the species is said to be oviparous. Frequently, the egg 
is retained within the body of the female until the young is fully INTRODUCTION 
19 
formed and the larva emerges from the egg membranes before it 
leaves the body of the parent. This condition is designated as 
viviparous. Any condition intermediate between these two 
extremes is termed ovoviviparous. In this condition, the egg has 
at least started to divide before it leaves the body of the parent. 
In popular, and sometimes in scientific, usage the term oviparous 
is applied to any condition in which the female brings forth eggs, 
regardless of the stage of development of the contained ovum or 
embryo. The term ovoviviparous is in this instance restricted 
to that condition in which the embryo is liberated from the egg 
membranes just before it leaves the maternal body. 
General References 
The references here cited contain information bearing upon most of the 
chapters throughout this book but are not repeated at the end of individual 
chapters. 
Bronn, H. G. (various authors). 1880- . Klassen und Ordnungen des 
Tier-reichs. Leipzig. 
Delage, Y. et Herouard, E. 1896-1903. Traite de zoologie concrete. 
Paris, Schleicher Freres. 
Harmer, S. F. and Shipley, A. E. (various authors). 1891-1909. The 
Cambridge Natural History. London, Macmillan. 
Hertwig, R. (translated and edited by Kingsley, J. S.). 1912. A Manual 
of Zoology. N. Y., Holt. 
Lang, A. (translated by Bernard, H. M. and M.). 1891. Text-book of 
Comparative Anatomy. London, Macmillan. 
Lankester, Sir R. (various authors). 1909. A Treatise on Zoology. 
London, Black. 
Parker, T. J. and Haswell, W. A. 1921. A Text-book of Zoology, Vol. 1. 
London, Macmillan. 
Sedgwick A. 1898. A Student's Text-book of Zoology. London, 
Sonnenschein. 
Verrill, A. E. and Smith, S. I. 1874. Report on the Invertebrate 
Animals of Vineyard Sound and Adjacent Waters. Rep. Commr. Fish 
and Fisheries, Vol. 1. 
Vogt, C. und Yung, E. 1885-1888. Lehrbuch der praktischen vergleich- 
enden anatomie. Braunschweig. 
Ward, H. B. and Whipple, G. C. (various authors). 1918. Fresh-water 
Biology. N. Y., Wiley. 
von Zittel, K. A. 1900. Text-book of Palaeontology, English translation 
bv Eastman, C. B. London, Macmillan. CHAPTER II 
PHYLUM PROTOZOA 
The Cell as an Individual.-The Protozoa represent the 
simplest organization of animal protoplasm to form independent 
units or individuals. Though in the typical examples the 
individuals comprise single cells, the cytoplasm of these cells 
has undergone various lines of differentiation and specialization. 
Thus these cells are far from simple or "undifferentiated" when 
considered from the point of view of adaptation for numerous 
different functions. The chief point of distinction between the 
cell of a protozoan and one of a many-celled animal (metazoan) 
lies in the fact that in the former all of the functions of a living 
animal are executed by a single mass of protoplasm. Any 
specialization for different functions in the single cell must be 
restricted to specialization of a part within that cell. On the 
other hand, the Metazoa are composed of numerous cells, groups 
of which have become specialized for limited functions. Conse- 
quently, for the rest of the functions characteristic of living matter 
these cells of limited function in the Metazoa become more or 
less dependent upon the other cells united with them to form an 
aggregate termed an individual. It thus becomes obvious that 
differentiation in Protozoa usually involves specialization of the 
parts within a single cell, while in the Metazoa entire cells 
become specialized for limited functions. In this light it is readily 
understood that "undifferentiated" as applied to a protozoan 
cell does not imply "unorganized" for the organization found 
there is frequently much higher than in many kinds of metazoan 
cells which have undergone histological differentiation. 
Metazoan Tendencies.-There is no broad gap separating the 
simple, single-celled Protozoa on the one side from the Metazoa 
on the other. Many Protozoa, especially among the Mastigo- 
phora and Ciliata, show distinct tendencies toward specialization 
of cells. Frequently, the cells resulting from the division of a 
single one remain attached, thus forming a group which functions 
as a unit and is termed a colony. Within such a group only part 
20 PHYLUM PROTOZOA 
21 
of the cells may retain the powers of reproduction while the 
others carry on all of the remaining functions for the colony. 
This marks an early separation of two kinds of cells, the germ cells 
as distinct from the body or somatic cells (Fig. 5). Separate germ 
cells or gametes occur in many different kinds of Protozoa, in fact it 
is not uncommon for two different kinds of gametes to make their 
appearance in this group as microgametes (male germ cells) and 
macrogametes (female germ cells), but in all these instances all 
of the somatic cells remain similar. Thus, there is a finely 
graded series of changes which connect the single-celled condi- 
tion and the colony bearing only one kind of somatic cells and 
one or more types of germ cells. Most Zoologists agree that this 
marks the limit of histological differentiation in the Protozoa. 
If histological differentiation of the somatic cells occurs, the 
organism is recognized as a metazoan. 
An inclusive definition of the Protozoa might then be given as 
follows: The Protozoa constitute that phylum of the Animal 
Kingdom which includes all single-celled animals and cell aggre- 
gates in which there is no histological differentiation of the 
somatic cells. 
Organization.-Because of their small size, Protozoa were 
entirely unknown to the early scientists. Their study dates 
from the introduction of the microscope. Most of the early 
observers maintained that Protozoa are made up of complete 
systems of organs such as are found in the higher animals. 
Dujardin denied the presence of organ systems. Definite com- 
parisons with single cells of the Metazoa were first made by 
de Bary as early as 1843 but it was left for von Siebold (1848) 
to describe them as unicellular. 
In lax usage, the term "organ" is still used in referring to those 
parts of the protozoan cell which have become adapted to special 
functions. More correct usage restricts this term to cell groups. 
Accepting this limitation, it becomes necessary to designate 
differentiated structures in Protozoa as ''cell organs" or "organ- 
ellae." This distinction has led to a rather common usage of 
terms such as cytostome (cell mouth), cytopyge (cell anus), 
cytopharynx (cell pharynx), in referring to the organellae of 
Protozoa. 
The classes of this phylum differ so widely in structure and 
degree of specialization of parts that little may be said that would 
apply equally to all animals included here. While most Protozoa 22 
INVERTEBRATE ZOOLOGY 
are small in size, the plasmodia of the Mycetozoa may cover a 
surface several inches in diameter. At the opposite extreme 
stand the Sporozoa, many of which pass through a spore stage 
in which the individual is less than 1 micron in diameter. These 
represent about the smallest cells known in the animal kingdom. 
The cytoplasm of a protozoan cell is usually divided into a 
covering ectoplasm and a more distinctly granular internal mass, 
the endoplasm. Within the ectoplasm, there are extreme differ- 
ences in organization for in some instances it is reduced to an 
extremely thin layer while in others it is stratified into several 
distinctly separable regions. In the Microsporidia, and some 
other forms, no differentiation of the cytoplasm into layers has 
been demonstrated. 
Habitats.-Protozoa are encountered in extremely diverse 
habitats. All classes, with the exception of the Sporozoa, have 
numerous species which occur as free-living organisms in both 
fresh and salt water. Soil inhabiting species are not uncommon. 
The parasitic habit has become the exclusive condition among the 
Sporozoa, but in each of the other classes the same habit is 
encountered to a greater or less degree. Some species have the 
faculty of leading either an independent or a parasitic existence, 
as opportunity is presented to the individual animal. 
Food Habits.-In food habits, the Protozoa display great 
diversity. Many of the chlorophyl-bearing Mastigophora are 
anabolic in their metabolism. Under the influence of sunlight 
their chlorophyl synthesizes food substances in a purely plant- 
like manner. These forms in which the food is built up from 
simple compounds by the processes of photosynthesis are 
designated as autophytic. 
Most of the Protozoa are katabolic in their metabolism and 
require complicated organic compounds as foods. These organic 
foods may be ingested as solid particles either through the action 
of pseudopodia as in Amoeba and other Rhizopoda or through a 
cytostome as in Paramecium and many other ciliates and many 
flagellates. In the endoplasm, these solid food particles un- 
dergo digestion in food vacuoles before they are assimilated. In 
the instances just cited the food material may be either living or 
dead plant or animal matter. However, in some species the 
predaceous habit is rather firmly fixed. Thus Didinium lives 
largely upon paramecia the bodies of which it ingests through a 
highly specialized cytostome located at the tip of a proboscis. PHYLUM PROTOZOA 
23 
The Suctoria through their hollow tentacles are enabled to suck 
out the protoplasm from other organisms and utilize it as food. 
Organic matter may be absorbed through the body surface in 
many Protozoa. This is especially true of forms which lack a 
cytostome. These organisms dependent upon the absorption 
of elaborated food stuffs may be either parasitic upon living 
organisms or may utilize decomposing organic matter. In 
the latter instance, they are said to be saprophytic. 
Cultures.-Under usual conditions, Protozoa are present in 
stream, pond, or lake water in relatively small numbers. 
Various factors in the environment cooperate in keeping the 
numbers of any given species from becoming excessive. If 
individuals of a given species multiply unusually, forms which feed 
upon this species, or depend upon it in other ways, will naturally 
increase, thereby tending to reduce the excessive numbers. Thus, 
the " balance of nature " works here even as among the higher forms 
of life. If food is present in excess and natural enemies are lack- 
ing, the balance is broken and immense numbers make their 
appearance. This is what happens when an infusion or culture 
medium, rich in food material, is allowed to stand in the labora- 
tory to produce a protozoan culture. 
Reproduction.-Various forms of reproduction are encountered 
in this group. The one most frequently found is that of binary 
Fig. 11. 
Fig. 12. 
Fig. 13. 
Figs. 11-13.-Diagrams to illustrate the methods of asexual reproduction. 
11, binary fission in Paramecium; 12, multiple fission or spore formation in 
a protozoan; 13, budding in Acanthocystis. 
fission. In this process, the nucleus usually undergoes an indirect 
division and this is followed by constriction and finally separation 
of the cytoplasm to form two new individuals (Fig. 11). Com- 
monly, structures become duplicated in the dividing individual 
before fission is completed. Fission not uncommonly occurs 
when individuals are in an inactive or encysted state. A type 24 
INVERTEBRATE ZOOLOGY 
of multiple fission (Fig. 12) is characteristic of some groups of 
Sporozoa and Mycetozoa. In this instance, the nucleus under- 
goes a series of divisions. Each of the nuclear masses thus 
formed becomes surrounded by a layer of cytoplasm and upon 
the rupture of the original cell wall numerous small cells called 
spores are liberated. 
In the indirect nuclear divisions mentioned above, there are 
frequently one or more points wherein the process differs from 
mitosis as it occurs in metazoan cells. When the centrosome 
occurs within the nuclear wall, the entire process of division may 
be accomplished without the disappearance of the nuclear 
membrane. Frequently the chromatin is present as one or more 
large bodies which are termed karyosomes. These constrict and 
divide without chromosome formation, in a manner somewhat 
resembling amitosis. The term promitosis has been applied to 
this primitive type of indirect nuclear division to distinguish it 
from the more elaborate mitosis. 
Many Protozoa have a definite life cycle more intricate than 
a simple succession of fission states. Frequently, this cycle 
involves two distinct types of adult individuals; sporonts, which 
give rise to gametes, and schizonts, which give rise to asexual 
individuals. There may thus be an alternation of generations 
involving two distinct types of developmental cycle; a cycle 
of sporogony, during which gamete formation and fertilization 
occurs, and one of schizogony, during which no sexual process 
is involved. These alternating cycles in the free-living forms are 
correlated with environmental conditions, especially seasonal 
changes, while in the parasitic species they are frequently 
correlated with change of host. In the malarial organisms (Fig. 
29), for example, schizogony continues during development of the 
organism in the vertebrate host and sporogony involving the 
fertilization of gametes is restricted to the sojourn of the parasite 
in the body of the mosquito. 
Budding (Fig. 13) differs from fission chiefly in the relative 
sizes of the resulting parts. In fission, two or more approxi- 
mately equivalent parts result from the partition of one individ- 
ual. Through budding, on the other hand, the identity of a 
parent organism is retained for the buds arise as smaller out- 
growths from the body of the producing individual. 
Conjugation in the Protozoa involves either the temporary or 
permanent fusion of two or more individuals of the same species. PHYLUM PROTOZOA 
25 
A simple type is found in the fusion of the cytoplasm of a number 
of individuals to form a plasmodium as in the Mycetozoa. More 
frequently the nuclei are involved in either a permanent fusion 
resulting in true fertilization or a temporary fusion involving an 
exchange of nuclear material. 
Endomixis. An intricate process of nuclear reconstruction 
without the fusion of individuals occurs in some Protozoa. 
Details of this phenomenon, which is termed endomixis, have 
been observed especially in Paramecium. The micronucleus 
undergoes a number of divisions and some of the resulting nuclei 
disintegrate and are resorbed by the cytoplasm. Authorities 
differ as to the exact meaning or 
significance of endomixis though some 
see in it a process in some degree com- 
parable with parthenogenesis of the 
Metazoa. 
Colony formation frequently results 
from incomplete separation of cells 
following division. The products of 
rapid division may remain in union 
for a short time to form a temporary 
aggregate which ultimately separates 
into its individual cells, or they may 
remain permanently associated to 
form a colony. Branching or arboid 
colonies (Fig. 14) frequently occur in 
the Peritricha and in some Mastigo- 
phora. Highly developed colonies 
(Fig. 5) are common in the Mastigo- 
phora, especially among the Euglenoidea. Because of the similar- 
ity between various mastigophoran colonies and the cleavage 
and blastula stages in the development of Metazoa, this 
group has frequently been cited as the one most directly 
in line with metazoan phylogeny. However, some workers 
are inclined to the view that while colony formation is 
less characteristic of the Ciliata, specializations found there 
demand consideration in any discussion of the phylogeny of the 
Metazoa. 
The classification of Doflein has been adopted in the present 
work. In this system, relationships are more clearly shown than 
in the older systems which involve recognition of four equivalent 
Fig. 14.-One of the colonial 
Chrysomonadina, Dino[bryon 
sertularia Ehrenberg. A, ar- 
rangement of cells in tree-like 
colony; B, individual in its cup- 
like sheath. (From Shull, La 
Rue, and Ruthven after Kent). 26 
INVERTEBRATE ZOOLOGY 
classes. In the remainder of this chapter, the following sub- 
phyla and their included classes are discussed: 
Subphylum Plasmodroma 
Class 1. Mastigophora 
Class 2. Rhizopoda 
Class 3. Sporozoa 
Subphylum Ciliophora 
Class 1. Ciliata 
Class 2. Suctoria 
SUBPHYLUM PLASMODROMA 
In the subphylum Plasmodroma, are assembled all Protozoa 
which never develop cilia while the subphylum Ciliophora 
includes those which have cilia at least during part of their 
existence. 
Class Mastigophora 
The presence of one or more flagella is practically the only 
character common to all Mastigophora. More than superficial 
examination is necessary to distinguish some Mastigophora 
bearing numerous flagella from ciliates and on the other hand 
the boundary between Mastigophora and Rhizopoda is obscured 
through the presence of temporary flagella on some of the 
Rhizopoda and of pseudopodia on some of the Mastigophora 
(Fig. 15). In addition to these confusing relationships with other 
classes of Protozoa, there are some forms which are so distinctly 
plant-like that they are claimed alike by zoologists and botanists. 
Body shape is far from constant in many forms, the degree of con- 
stancy depending upon the character of the body surface. In 
some, the pellicle is either wanting or so thin as to permit of 
free amoeboid movements. Limitation to change in shape is 
secured in some by the presence of supporting structures of organic 
matter such as the axostyle (Figs. 22 and 23). 
Each flagellum (Fig. 16) consists of a firm axial filament part 
of which is encased in a more fluid contractile sheath. Distally, 
this filament extends a short distance beyond the sheath and 
constitutes an "end piece." Proximally, the axial filament 
continues through the cytoplasm to the blepharoplast. This 
last named structure is similar to the basal granules of the ciliates. 
In fact, the entire flagellum very closely simulates the structure 
of an individual cilium. Among the more characteristic Masti- PHYLUM PROTOZOA 
27 
gophora, the flagella occur at the anterior extremity. If they 
are directed forward and by their movement pull the body 
along, they are called trachella; if they are directed backward and 
by their movement propel the body ahead, they are designated 
as pulsella. The trachella are the more common type. In many 
Mastigophora, flagella arising at the anterior extremity are 
directed backward along the side of the body as trailing flagella. 
These may be either free or fused with the side of the body as an 
undulating membrane as in the Trypanosomes (Fig. 20). 
Fig. 15.-Mastiyella vitrea. (After 
Goldschmidt). 
Fi g . 16 .-Isolated flagellum of 
Euglena showing axial filament within 
surrounding sheath. (After BiUschli). 
On the basis of relationships and general methods of metabolism, 
the Mastigophora may be divided into two subclasses. The 
distinctly plant-like forms are included within the Phytomastigina 
while the ones displaying more pronounced animal characters are 
grouped within the subclass Zoomastigina. 
Subclass Phytomastigina 
Members of this subclass have one to four flagella located at 
the anterior extremity. The cytoplasm is usually very finely 
granular, not heavily vacuolated, and without distinct boundary 
between ectoplasm and endoplasm. Typical members of this 
group possess chromatophores, frequently brown or yellow in 
the lower forms and green in the higher. Especially in the latter, 
chlorophyll is found. Contractile vacuoles are usually present 
in the fresh-water forms either as a simple pulsating vacuole or 28 
INVERTEBRATE ZOOLOGY 
as a more highly complicated system of vacuoles and reservoirs. 
Division is usually by longitudinal binary fission. 
I. Order Chrysomonadtna 
These are small forms of very simple structure, some with and 
others without a pellicle. One or two flagella are present and 
pseudopodia may also occur, especially as a means of securing 
food. Though most members of this order are autophytic, 
others feed upon bacteria and other unicellular plants and small 
Protozoa. A red eyespot is commonly located in the region of 
the root of the flagellum. Chromulina (Fig. 17), Chrysopyxis, 
Synura, and Dinobryon (Pig. 14) are characteristic genera. 
Fig. 17. - Chromulina Pascheri, 
one of the Chrysomonadina. (After 
Hofeneder). 
Fig. 18.-Chilomonas paramecium 
(After Dofleiri). 
II. Order Cryptomonadina 
Relatively constant body form with usually one or two chroma- 
tophores characterize these small flagellates. In habits, they 
are autophytic or saprophytic, occurring in both salt- and 
fresh-water and in some instances living symbiotically (genus 
Chrysidella) in Sarcodina. The body which bears one or two 
flagella usually has dorsal and ventral surfaces recognizable. 
At the anterior end, an oblique groove occurs on the ventral 
surface. In this groove or in the esophagus which communicates 
with it, arise the flagella. Of the several genera, Cryptomonas, 
Cyathomonas, and Chilomonas (Fig. 18) are characteristic. 29 
PHYLUM PROTOZOA 
III. Order Dinoflagellata 
Representatives of this order usually possess a shell-like lorica 
covering the body. Two flagella arise near the middle of the 
body. Of these, one extends posteriorly as a trailing flagellum 
and the other encircles the body. Each of these usually occupies 
a groove or sulcus in the lorica known respectively as the longitu- 
dinal and the circular groove or annulus. The circular flagellum 
because of its undulating movements was mistaken for a ring of 
cilia by the early workers who as a consequence called the group 
the Cilioflagellata. The plates of which the characteristic rigid 
armor is made are composed of a substance closely allied to 
cellulose. Spikes and prolongations of this armor frequently 
give to the body a helmet-shape or other bizarre form. The 
usual method of reproduction is by binary fission, each individual 
receiving half of the shell and regenerating the missing half. 
Rapid reproduction in some instances leads to 
chains of associated individuals. In some in- 
stances, multiple fission within the shell leads to 
spore formation. 
Members of the suborder Adinida lack the 
furrows characteristic of the typical Dinoflagellata. 
In these, the flagella emerge from an aperture 
between the two valves of the shell. This bivalve 
shell is perforated with numerous pores. Proro- 
centrum, Exuvaella, and Haplodinium (all marine) 
are typical genera. The typical Dinoflagellata, 
which include both marine and fresh-water forms, 
belong to the suborder Dinifera, of which Gymno- 
dinium, Peridinium, Ceratium, Amphisolenia, and 
Pyrocystes are characteristic genera. 
IV. Order Euglenoidina 
The Euglenoidina are relatively large and have 
practically constant body form except for squirm- 
ing or wriggling movements. At the anterior 
extremity of the body, occurs a deep esophagus from the 
wall of which the one or two flagella emerge. Into this 
same esophagus, empties the reservoir of the vacuole system. 
The reservoir does not pulsate but receives its contents from the 
smaller contractile vacuoles which communicate with it and 
r'ichPOph~. 
orum. {After 30 
INVERTEBRATE ZOOLOGY 
drains through the esophagus. A red stigma frequently appears 
in the anterior region. Of the numerous genera Euglena, Phacus, 
Trachelomonas, Colacium, and Peranema (Fig. 19) stand as 
examples. 
V. Order Phytomonadina 
This order of Mastigophora comprises the most distinctly plant- 
like forms. They are probably unicellular algae which have 
retained throughout their existence the flagellum characteristic 
of their early developmental stages. In nutrition, they are 
exclusively autophytic. Colony formation is very common, 
representing various degrees of complexity. Chlamydomonas 
and Haematococcus are solitary forms. Spondylomorum, 
Gonium, Pandorina, Eudorina (Fig. 5), Platydorina, Pleodorina, 
and Volvox are genera in which colony formation is found. 
Subclass Zoomastigina 
The Zoomastigina include the orders of flagellates the members 
of which are distinctly animal in their nutrition or receive their 
food by absorption. 
I. Order Protomonadina 
Very small Mastigophora with one or two flagella of equal or 
unequal length, one of which is frequently directed backward as 
a trailing flagellum, are included within the confines of this order. 
Members of the genus Monas represent the simplest organization. 
The Choanoflagellates, similar in appearance to the collared 
epithelial cells of Porifera and frequently cited in the discussions 
of the origin of Porifera, belong here. One of the most important 
groups is that of the Trypanosomidae (Fig. 20) which contains 
numbers of highly important animal parasites. Dendromonas 
forms branching colonies and Anthophysa occurs as masses of 
individuals attached to a stalk. 
Trypanosomes occur as blood parasites in all classes of verte- 
brates. In many instances, they are restricted to a single host 
species and insofar as has been observed these are harmless 
to the host. Such is the condition which exists between the 
rat host and its normal parasite Trypanosoma lewisi. In many PHYLUM PROTOZOA 
31 
instances, however, trypanosomes are not restricted to a specific 
host but are capable of developing in the blood of various species 
in addition to that of the normal host. Under such conditions, 
if a parasite innocuous to its normal host is introduced into some 
unusual host where it finds conditions favorable for development, 
it continues to multiply practically without limit until it produces 
serious or even fatal pathological conditions within its new host. 
The various species of non-pathogenic trypanosomes have certain 
features in common which unite them and distinguish them from 
the group of pathogenic species. Because the non-pathogenic 
species resemble the Trypanosoma lewisi, mentioned above, the 
entire group is spoken of as the lewisi-group. In similar manner, 
the pathogenic species in general resemble Trypanosoma brucei 
so the assemblage is termed the brucei-group. The last named 
species seems to be a normal parasite of some African game ani- 
mals but when it is introduced into 
the blood of domestic animals it 
produces fatal results. 
In their transmission from host 
to host, trypanosomes depend upon 
the agency of some blood sucking 
animal such as a fly or leech. Tse- 
tse flies of the genus Glossina are 
the inoculators of the T. brucei 
which has so seriously threatened 
extermination of all domestic 
animals in certain parts of Africa. 
Flies of the same genus also carry 
T. gambiense which is responsible 
for a fatal human disease in 
Africa known commonly as sleeping sickness. 
Fig. 20-Trypanosoma Theileri 
(Bruce), from blood of a cow. 
(After Lahe). 
The spirochaets represent a group of organisms of very great 
importance and yet of highly problematical relationships. Some 
investigators have thought that they are protozoans related to 
the trypanosomes but in structure, in habits, and in reactions 
they seem to be more closely related to the bacteria. Treponema 
pallidum, the spirochaet which causes syphilis, is one of the most 
important representatives of this group. Some of the species 
of the genus Spiroschaudinnia produce a disease in man known as 
relapsing fever. 32 
INVERTEBRATE ZOOLOGY 
II. Order Polymastigina 
These small, usually parasitic forms have two to six flagella 
at the anterior extremity, of which at least one is a trailing 
flagellum frequently united with the body to form an 
undulating membrane. Chromatic structures of highly vari- 
able form, termed parabasal bodies (Figs. 21 and 22), are 
characteristic. 
Costia is parasitic on the skin of fishes, Chilomastix (Fig. 21) 
occurs in man and Trichomonas (Fig. 23) is parasitic in man and 
Fig. 21.-Chilomastix mesnili (Wenyon). A, active stage; B, encysted. (After 
Kofoid and Swezy). 
in other vertebrates. The foregoing represent the simplest 
organization within the order but in the family Calonymphidae 
are united a number of genera which have numerous nuclei, 
axostyles, blepharoplasts, and parabasal bodies. Calonympha 
and Dienympha are examples of this peculiar family. Four 
flagella arise from each blepharoplast. The polynuclear and 
general multiple structure give to the members of these genera the 
appearance of colonial forms which lack cell boundaries. , PHYLUM PROTOZOA 
33 
III. Order Hypermastigina 
In this order, are united a number of genera of highly compli- 
cated and confusing structure, members of which live exclusively 
as parasites in the alimentary canal of insects. A bundle of 
long flagella usually arises from the anterior extremity and 
additional bundles may occur on other parts of the body. Super- 
ficially, these may look much like ciliates. Lophomonas of 
Fig. 22. 
Fig. 23. 
Fig. 22.-Giardia microti Kofoid 
and Christiansen. Parabasals are 
two curved, dark bodies behind 
the axostyle. {After Kofoid and 
Christiansen). 
Fig. 2 3.-Trichomonas augusta 
Alexieff. {After Kofoid and Swezy). 
cockroaches and Trichonympha of termites are characteristic 
genera. 
IV. Order Distomatina 
Four to eight or more flagella, usually arranged in two groups, 
are found in the Distomatina. Many of the species possess two 
nuclei. Both free-living forms and parasites in the alimentary 34 
INVERTEBRATE ZOOLOGY 
canal of vertebrates are found. Hexamitus and Giardia (Fig. 
22) are important genera. 
Giardia lives as a parasite in the intestine of man, rabbits, 
dogs, cats, and mice. Attachment to the epithelial cells is 
accomplished by means of the cup-shaped ventral surface of 
the parasite. It is uncertain whether a single or several species 
of this genus infest the various host species. Giardia is thought 
to be the causal agent of some dysenteric diseases though in 
some instances infestations by Giardia do not seem to be accom- 
panied by pathological conditions. 
V. Order Cystoflagellata 
Members of this order are exclusively marine. In the typical 
genus, Noctiluca, the body consists of a gelatinous sac, frequently 
more than a millimeter in diameter, with a concentration of cyto- 
plasm at one pole containing the nucleus and giving off the loco- 
motor organs. These organisms are free-floating and are highly 
phosphorescent. One of the most conspicuous structures is 
the tentacle, a cross-striated thread of protoplasm which lashes 
back and forth. Near the base of this tentacle, is located a single 
minute flagellum. In another genus (Craspedotella), the body is 
flattened and closely resembles a minute jelly-fish. 
Class Rhizopoda 
All rhizopods lack a true cell wall though in many instances 
skeletons or other protective coverings have been developed. 
The pseudopodia are the most characteristic feature of the rhiz- 
opods in spite of the extremely different appearances which these 
structures may present in the various orders. In most instances, 
pseudopodia serve both for locomotion and for the ingestion of 
food. The nucleus is usually single, though binucleate or multi- 
nucleate individuals are found frequently. The nuclei in these 
latter are essentially similar rather than manifesting the func- 
tional differentiation which is characteristic of the macro- and 
micronuclei of the ciliates. 
Binary and multiple fission and budding are all represented in 
the types of reproduction among the Rhizopoda and some forms 
pass through a definite life cycle involving a conjugation 
(Arcella). The relatively simple organization of the Rhizopoda 
has led many students to assume that only a simple cleavage of 
the cell is involved in the reproductive process. It is probable PHYLUM PROTOZOA 
35 
that simple fission is not the sole phenomenon in the ontogeny of 
many forms. 
Many important parasites, especially among the Amoebina and 
Mycetozoa, occur in this group. Especially, the Radiolaria 
(Fig. 27) and Foraminifera (Fig. 26) have been of great importance 
because of the part played by their shells in the formation of 
sedimentary rocks. 
I. Order Amoebina 
The Amoebina are naked rhizopods, that is, they possess no 
skeletal structures. Locomotion is accomplished either by the 
forward streaming of the entire cytoplasmic mass, in which case 
the entire body may be considered as a single pseudopodium, or 
through the extension of pseudopodia of various kinds. There 
is usually a fairly sharp differentiation of ectoplasm and endo- 
plasm. In most species the formation of pseudopodia is initiated 
by the ectoplasm. 
A single contractile vacuole is usually present, though marine 
and parasitic species usually lack the vacuole and in some other 
representatives more than one occurs. Binary fission involving 
mitotic division of the nucleus is the ordinary method of repro- 
duction. Multiple fission occurs in many species, usually during 
the encysted period. Various descriptions of conjugation in 
amoebae have been offered but the work has not been carried far 
enough to warrant generalization regarding the occurrence in the 
entire order. 
Species of this order live in greatly diversified habitats. A 
number inhabit fresh-water (Amoeba proteus, A. verrucosa, etc.), 
others live in the soil (A. terricola), and still others live as parasites 
in the bodies of man and of other animals (Entamoeba histolytica, 
E. coli, E. buccalis of man and Endamoeba blattae of cockroaches). 
Of the considerable number of amoeboid forms which have 
acquired the parasitic habit, the species infesting man are of 
greatest general interest. Some species, Entamoeba coli of the 
human intestine, for instance, are apparently harmless to the host 
but infection by other species of the same genus is accompanied by 
serious pathological conditions. Entamoeba histolytica (Fig. 24) 
is the causal agent of amoebic dysentery which is so prevalent in 
tropical and sub-tropical countries. In instances of infection 
by E. histolytica, lesions are formed in the rectum of the host, 
at first as small fluid-filled nodules containing degenerate tissue 36 
INVERTEBRATE ZOOLOGY 
and motile amoebae. These nodules later form ulcers which cover 
large areas of the intestine and even invade the liver, producing 
abscesses. The presence of non-pathogenic amoebae in the feces 
of both healthy and diseased individuals renders the specific 
determination of the cysts of various species in human feces of 
especial importance. 
There have been several different names applied to the 
pathogenic Entamoebae from the human intestine, and some 
workers maintain that there is more than one species. 
Fig. 24 .-Entamoeba histolytica 
Schaudinn. A, vegetative form; B, 
encysted. {Redrawn from Hartmann). 
Fig. 25.-Mastigella vitrea. (After 
Goldschmidt). 
II. Order Rhizomastigina 
Characteristics of both the Rhizopoda and Mastigophora are 
combined in the animals which belong to this order. In addition 
to the characteristic pseudopodia, each individual is supplied 
with one or two persistent flagella, thus differing from the tem- 
porary flagella with which the spores of some Rhizopoda are 
supplied. Mastigamoeba and Mastigella (Fig. 25) are typical 
genera. 
III. Order Heliozoa 
Arrangement of the numerous ray-like pseudopodia is respon- 
sible for the name sun-animalcules which is applied to these 
organisms. The pseudopodia are relatively fixed in position for a 
firm axial thread runs through the center of each. This filament PHYLUM PROTOZOA 
37 
is covered with a layer of cytoplasm, within which the movement 
of the pseudopodium is confined. Each axial filament either ends 
abruptly in the cytoplasm or continues through the cytoplasm 
until it reaches the wall of the nucleus. There is a sharp differ- 
entiation of ectoplasm and endoplasm. Contractile vacuoles 
occur in the heavily vacuolated ectoplasm. Some forms are 
polynucleate while others possess but a single nucleus. In many 
instances, the silicious skeleton is in the form of a lattice work 
sphere beyond which the pseudopodia extend radially. 
Both budding and fission occur. The bud, when liberated 
from the parent, may display a pair of flagella which function in 
the locomotion of the spore. Each spore may ultimately develop 
into a new individual or, as is the case in Actinosphaerium, 
there may be an actual fertilization preceded by a maturation 
process of the gametes. Actinosphaerium and Actinophrys are 
frequently found in mixed protozoan cultures. Acanthocystis 
(Fig. 13), Nuclearia, Vampyrella, Wagnerella, Raphidiophrys, 
and Clathrulina are also representatives of the numerous genera. 
IV. Order Foraminifera 
The Foraminifera or Thalamophora are rhizopods bearing 
shells of extremely diverse natures but usually closed at one pole 
and open at the other. In form, the shell ranges from a simple, 
single chamber (Monothalmia) to complex series (Fig. 26B) of 
spirally arranged chambers (Polythalmia). Growth of the latter 
is through the periodic formation of new chambers of increasing 
dimensions. Foramina serve to communicate between the 
various chambers of the shell, all of which are filled with proto- 
plasm. As the protoplasm increases in bulk, it protrudes beyond 
the limits of the old shell and a larger chamber is added to the 
chambers previously formed. Pseudopodia of different types 
protrude through either a single opening or through numerous 
small apertures. In some of the smaller forms, the shell is 
almost wholly of organic material, formed by the protoplasm. 
More commonly, silicious or calcareous matter is also present and 
frequently foreign bodies are incorporated directly into the sub- 
stance of the shell. Chalk deposits, the nummilitic limestones, 
and various other sedimentary rocks have been formed largely 
of the fossil shells of Foraminifera. 
In the suborder Monothalmia, the shells are usually relatively 
simple, of rounded, hemispherical, or ovoid form and composed of 38 
INVERTEBRATE ZOOLOGY 
chitinous or silicious matter. In the organization of the body, 
many of the Monothalmia are distinctly like Amoebae which have 
simply acquired a house. Reproduction in some of the thin- 
shelled forms involves a simple binary fission which also includes 
a division of the shell, but more frequently a portion of the proto- 
plasm is extruded and forms about itself a new shell. Ultimately, 
the cell body divides apportioning the protoplasm between the 
old and the new shell. A complicated life cycle has been demon- 
strated for Arcella. Other characteristic genera are Difflugia, 
Gromia, Chlamydophrys, Euglypha, and Lieberkiihnia. 
Members of the suborder Polythalmia (Fig. 26) are exclusively 
marine. The shells which are of extremely diverse types, may 
be either pure lime or partly 
of organic matter. Those of 
some fossiljtforms are rela- 
tively immense in size (Psam- 
monyx vulcanis, 5-6 cm.). 
Some idea of the rate of 
reproduction of these forms 
may be gained from the fact 
that chalk deposits many feet 
in thickness are composed 
almost exclusively of their 
shells (Biloculina). Rotalia, 
Polystomella, Siphogenerina, Cyclammina and Globigerina rep- 
resent modern genera. 
Fig. 26.-Shells of Foraminifera. A, 
Siphogenerina striatula; B, S. columnellaris, 
seen in longitudinal section; C, Cyclamina 
orbicularis, side view. (After Cushman). 
V. Order Radiolaria 
Radiolaria are frequently spherical, crown-, or disc-shaped, 
with numerous pseudopodia provided with axial filaments. 
The cytoplasm is divided into two regions; an external layer, the 
extracapsulum which is separated from the inner mass or central 
capsule by a skeletal formation called the capsular membrane. 
Perforations of this membrane allow the cytoplasm of the two 
regions to be in direct continuity. The capsular membrane is 
of organic matter similar to chitin but the accessory skeletal 
structures such as the rods and spines are usually of inorganic 
materials. One or many nuclei occupy the cytoplasm of the 
central capsule. In addition to food vacuoles, some forms con- 
tain unicellular algae as symbionts within the cytoplasm. Nutri- 
tion is distinctly holozoic. PHYLUM PROTOZOA 
39 
Reproduction is usually by fission which begins with the divi- 
sion of the nucleus. This is followed by a cleavage of the 
central capsule and of the capsular membrane and extracapsulum. 
Multiple fission in some instances gives rise to either isogametes 
or to micro- and macrogametes. Details of development in 
these instances are lacking. In some forms, it has been noted 
that fission proceeds so rapidly that several central capsules are 
contained within a single extracapsulum giving evidence of a 
tendency toward colony formation. 
Fig. 27.-Two characteristic forms of Radiolarian shells. (After Haeckel) 
Radiolaria, though exclusively pelagic, occur from the surface 
to the greatest depths, many forms being found regularly only 
at certain depths. 
Chiefly on the basis of the shape of the central capsule, and the 
number of perforations in the capsular membrane, several sub- 
orders are recognized. Acanthosphaera and Thalassicolla typify 
the suborder Spumellaria; Acanthometra and Sphaerocapsa are 
typical of the Acantharia. In Fig. 27, are shown drawings of 
two shells from Haeckel's classical monograph. 
VI. Order Mycetozoa 
The Mycetozoa or Myxomycetes are commonly known as the 
slime moulds or slime animals. The young stages are small 
amoeboid forms which may, by direct transformation, develop a 
flagellum. Transition between these two stages is accomplished 
very readily. Both reproduce by ordinary fission and each has 
the power of ingestion of solid food material and absorption of 40 
INVERTEBRATE ZOOLOGY 
organic fluids. The flagellate form may become a microcyst 
from which either the flagellate (myxoflagellate) or the amoeboid 
(myxamoeba) form may emerge. 
Recent investigations have brought forth evidence that 
gametes are formed with a haploid number of chromosomes. 
Following the fusion of two of these gametes to form a zygote, 
the fusion nucleus with its now diploid number of chromosomes 
undergoes a series of mitotic divisions which result in the forma- 
tion of a multinucleate body, the plasmodium. Apparently, 
through chemotactic response, plasmodia may be brought 
together and in such instances they fuse to form extremely large 
plasmodia which, as branching networks, cover surfaces several 
centimeters in extent. Color of the plasmodium varies in 
different species, often being characteristic for the species. 
Various adverse circumstances may cause the entire plas- 
modium to encyst. Even after several months or years the 
plasmodium may emerge from the cyst. Spore formation is 
also accompanied by encystment of either the entire plasmodium 
or of portions called sporangia or sporophores. Each sporan- 
gium is elevated from the substratum and is surrounded by a 
wall containing calcium carbonate and cellulose. Inside this 
wall (or peridium), are contained numerous uninuclear spores 
and a network of spirally twisted fibres called the capillitium. 
The encysted spores remain within the sporangium for an indefi- 
nite time. Rupture of the peridium releases the spores which 
are scattered to some distance and are frequently carried by the 
wind. The myxoflagellates, described earlier, emerge from these 
spores. After a period of reproduction, involving both the 
amoeboid and the flagellate forms, zygotes are formed which give 
rise to the plasmodia. 
Plasmodiophora brassicae is parasitic upon cabbage and related 
plants. 
Class Sporozoa 
The Sporozoa are Protozoa without locomotor structures in the 
adult stage. While representing great diversity of structure, 
habitats, and life cycle, they all agree in having the parasitic 
habit firmly established and in reproducing by spores which are 
usually enclosed in a firm shell. In some instances, especially 
where alternation of hosts is introduced, the spore shell may be 
wanting and in still other instances more than one spore may be PHYLUM PROTOZOA 
41 
included within a single shell. Alternation of generations is 
widely distributed in this group. Sporozoa are usually intra- 
cellular parasites, at least during the early stages in develop- 
ment. In most instances the spores give characters more readily 
available for identification than do the vegetative stages. Nutri- 
tion is exclusively by absorption through the body surface. 
Representatives of many groups ranging from Protozoa to 
mammals serve as hosts to sporozoan parasites. Pathological 
conditions in the host frequently result from infection by these 
parasites. 
Subclass Telosporidia 
In members of this subclass spore formation occurs only at the 
end of the vegetative period. Typically, the adult stage has a 
single nucleus but in some a multinuclear stage is found. Infec- 
tion of a new host is through a stage called a sporozoite which 
is usually an intracellular parasite. The sporozoite either 
develops directly into a sexual individual or first passes through 
an asexual multiplicative cycle. Following fertilization spores 
are formed which give rise to the sporozoites. 
I. Order Coccidiomorpha 
The vegetative stage of the Coccidia and the Haemosporidia 
is persistently intracellular and the sexual generation is also at 
least partially intracellular. In these points these two suborders 
differ from the remaining Telosporidia (the Gregarina). 
Suborder Coccidia 
The Coccidia are minute monocysted-gregarine-like Sporozoa 
which are permanent intracellular parasites of vertebrates, 
arthropods, and molluscs. Reproduction involves both schizog- 
ony and sporogony. Members of the genus Cyclospora are found 
in myriapods, Isospora in various vertebrates, Eimeria schubergii 
(Fig. 28) in myriapods and E. stiedae in rabbits and man. 
Suborder Haemosporidia 
The Haemosporidia live chiefly in the red blood corpuscles of 
vertebrates. The life cycle involves both schizogony and spor- 
ogony. As a typical example of the group stand the organisms 
(Plasmodium vivax, P. malariae, Laverania falciparum) which are 42 
INVERTEBRATE ZOOLOGY 
the causal agents of malaria. The amoeboid organisms of malaria, 
called schizonts (Fig. 29; 1-7), occur in the erythrocytes of 
persons afflicted with the disease. Each schizont grows until it 
almost fills the corpuscle (6). Then spore formation takes place 
Fig. 28.-Life cycle of Eimeria schubergi. Stages V to VIII, a period of 
schizogony; Xia and Xlla, beginning of development of male and female gametes 
in sporogony period of the cycle. (From Shull, La Rue, and Ruthven). 
and upon rupture of the corpuscle numerous spores or merozoites 
are liberated into the blood stream, ready, if they escape the 
attack of the leucocytes, to enter new erythrocytes and thus 
Fig. 29.--Life cycle of Plasmodium vivax, the tertian malarial parasite. Stages 
1-12 and 13c-17c in human blood stream; stages 13a-14a, 136-156, and 16-27, 
in body of mosquito. 1, the infecting stage for man, the sporozoite; 3-7, stages 
in schizogony resulting in the formation of merozoites (7). 9a-12a, formation of 
macrogametocyte; 13a-14a, maturation of macrogamete. 96-126, formation 
of macrogametocyte; 136-156, maturation of the microgametes. 16, fertiliza- 
tion; 17, zygote or ookinete. 18-25, stages in development within cysts in 
stomach wall of mosquito; 26, sporozoites liberated from cysts making their way 
through the body fluids to the salivary glands of the mosquito (27), where they 
are ready to infect a man bitten by the mosquito. 13c-17c, stages in formation 
of merozoites from macrogametocyte which fails to leave the human body. 9a 
and 96-(25) are stages in sporogony. (From Liihe after combinations of drawings 
by Grassi and by Schaudinri). PHYLUM PROTOZOA 
43 
Fig 29. 44 
INVERTEBRATE ZOOLOGY 
continue an autoinfection through an indefinite period. The 
chills characteristic of the disease are coincident with the periods 
of sporulation. 
Following a series of these asexual generations, the spore upon 
entering a corpuscle produces a gametoblast (9a, 96). In later 
development, each gametoblast develops into either a single 
female gamete or a group of male gametes. Further develop- 
ment of these gametoblasts depends upon their being taken into 
the body of a mosquito of the genus Anopheles which secures 
them with the blood of a malarial patient. The mosquito is not 
able to transmit malaria to a new human host immediately follow- 
ing the introduction of the organisms into the body of the 
mosquito. It is only upon completion of the sexual cycle (13 
to 27) of the parasite within the tissues of the insect that the bite 
of the mosquito may transmit the disease. In the stomach of 
the mosquito, each microgametoblast gives rise to a number of 
motile microgametes (156). Each of these upon fusion with a 
macrogamete (16) forms an ookinete or zygote (17). The zygote 
penetrates the wall of the stomach and becomes encysted (18). 
While in this condition sporoblasts are formed and each of these 
gives rise to numerous spindle-shaped sporozoites (25), These 
are liberated into the body cavity of the mosquito (26) and are 
carried by the circulatory system to various parts of the body 
including the salivary glands (27). From this location they are 
able to pass along the hypopharynx and thus are introduced into 
the blood stream of any human bitten by the mosquito. 
Several different kinds of malaria are distinguishable both from 
clinical evidence and from morphological and developmental 
differences of the causal organisms. Spring tertian or benign 
malaria, of which Plasmodium vivax is the cause, is characterized 
by the presence of a fever recurring on alternate days. The 
quartan type of malaria, which is caused by P. malariae, is dis- 
tinguished by fevers with two-day intervals between attacks. 
Infections of Laverania falciparum produce malignant or perni- 
cious malaria with daily recurrence of the fever. 
The haemogregarines, which occur as blood parasites especially 
in cold-blooded vertebrates, also belong to the Haemosporidia. 
Other representatives of this same suborder are: Piroplasma 
hominis, the causal agent of Rocky Mountain fever of man, and 
Babesia bovis, which produces Texas fever in cattle. Both of 
these organisms depend upon ticks for transmission from one 45 
PHYLVM PROTOZOA 
host to another. In this transmission by ticks there is a peculiar 
circumstance in that the tick which draws the infected blood from 
a host of the parasite does not reinfect new vertebrate hosts 
directly for it feeds upon but the one individual. The haemo- 
gregarines infect the eggs of the tick, then when young ticks hatch 
from these eggs and become attached to a mammal, the bite of 
the tick inoculates the new host. 
II. Order Gregarinae 
The asexual stage of the Gregarinae, which is the most conspicu- 
ous, is never intracellular throughout its entire development as 
in the Coccidiomorpha. A type of linear association called a 
syzygy may be followed by an encysted condition as a result 
of which numerous gametes are formed. The gametes pair to 
form zygotes, each of which becomes a single spore and later 
gives rise to several sporozoites. Typically the gregarine body is 
divided into an anterior protomerite and a posterior deutomerite 
separated by an ectoplasmic septum. Of these only the latter 
bears a nucleus. Frequently, especially in the young stages, 
there is an outgrowth of the protomerite carrying various 
structures for attachment to the host. This is termed the epi- 
merite. Since no mouth opening is present, all food is absorbed 
directly through the body wall. A myoneme layer within the 
deeper region of the ectoplasm renders body movements possible. 
The Eugregarinaria are the typical gregarines which in their 
life cycle involve only a propagative phase. This suborder is 
usually subdivided into two groups, the cephaline gregarines 
which bear an epimerite at least in the early developmental 
stages, and the acephaline gregarines which never possess an 
epimerite. The cephaline gregarines, of which there are 
numerous genera and species, occur briefly in the alimentary 
canal of arthropods. Gregarina, Stenophora, and Leidyana are 
a few of the numerous genera. The acephaline gregarines are 
chiefly coelom parasites, either lying free in the coelom or 
sometimes within organs located in the coelom. Monocystis 
from the sperm sacs of oligochaet worms is the typical genus. 
Subclass Neosporidia 
Members of this subclass are multinuclear in their adult stage. 
Spore production is continued over a considerable period of time 46 
INVERTEBRATE ZOOLOGY 
so that in any one individual, spores in different stages of develop- 
ment are to be found. Spore formation is indirect for the body 
forms a number of sporoblast mother cells. These in turn give 
rise to the sporoblasts which become transformed into the spores. 
I. Order Cnidosporidia 
Neosporidia with spores possessing one or more polar capsules 
are designated as Cnidosporidia. The amoeboid vegetative 
stages are parasitic in the cavities or tissues of the host. The 
spores of all cnidosporidians are composed of more than one cell. 
In the Myxosporidia and Microsporidia each spore contains a 
single amoeboid body within its bivalve shell. A three-valve 
shell covers the Actinomyxidian spore. Polar capsules are con- 
tained within the spore in addition to the sporoplasm. Each 
capsule contains a coiled polar filament which gives it an appear- 
ance strikingly like the nettling cells of the coelenterates. 
Suborder Myxosporidia 
The Myxosporidia have bivalve spores which usually contain 
one, two, or four polar capsules in addition to the amoeboid body 
called the sporoplasm (Fig. 30). The polar capsules are observa- 
ble in fresh material without previous treatment. Two nuclei 
Fig. 30.-Diagrammatical front and side views of a Myxobolus spore. (After 
Kudo). 
are found in the sporoplasm. The vegetative stage (Fig. 31) occurs 
either as a tissue parasite or in cavities. In the cavities the tropho- 
zoite forms plasmodia of various shapes. The tissue-inhabiting 
forms are usually oval, rounded or elongate, and may be either 
free or encysted. Spores are formed in the endoplasm of the 
plasmodial trophozoite. The Myxosporidia occur chiefly in PHYLUM PROTOZOA 
47 
fishes, though they occasionally parasitize reptiles and amphi- 
bians and there have been some very questionable records of 
their occurrence in invertebrates. 
Gall-bladder, uriniferous tubules, and urinary bladder are 
common seats of infection by the free organ-inhabiting forms, 
while gills and muscles of 
fishes are especially favorable 
tissues for the encysted forms. 
Cysts, which may attain a 
diameter of several millime- 
ters, result from the hyper- 
trophy of host tissues sur- 
rounding what was originally 
a single parasite. By endoge- 
nous budding of this parasite, 
spores in all stages of their 
development are being produced continuously. Leptotheca, 
Myxidium, Myxobolus, Henneguya, and Chloromyxum are 
characteristic genera. 
Fig. 31.-Vegetative stage of a myxo- 
sporidian, Leptotheca olmacheri. (After 
Kudo). 
Suborder Microsporidia 
In the Microsporidia, each spore contains a single polar capsule 
which is observable only after treatment with proper reagents. 
The Microsporidia are parasitic chiefly in arthropods, though 
they occur also in amphibians, fishes, some worms, and even in 
gregarines and in Myxosporidia. Nearly all are cell parasites. 
In the amoeboid vegetative stage they undergo asexual repro- 
duction. Nosema (N. bombycis of the silk worm, and N. apis of 
the honey bee), Glugea, and Thelohania are important genera. 
Suborder Actinomyxidia 
These forms reported only as parasites in oligochaets have a 
plasmodial stage represented by a binucleated amoeboid form 
which gives rise to eight sporoblasts, each of which forms a spore 
having three valves and three polar capsules. 
II. Order Sarcosporidia 
Sarcosporidia occur in the muscles of various mammals. The 
encysted form reaches a length of several millimeters and finally 
becomes a mass of sickle-shaped spores in clumps separated from 
each other by partitions. Detailed structure of the spore is 
not known. The encysted forms of the genus Sarcocystis within 48 
INVERTEBRATE ZOOLOGY 
the muscles of mammals have long been known under the name 
of Miescher's corpuscles but the complete life cycle is not 
understood. 
III. Order Haplosporidia 
The Haplosporidia are much simpler in structure than other 
Neosporidia. The spores, which are without polar capsules, are 
single cells, each with a single nucleus. Ichthyosporidium of 
fishes and Coelosporidium of the Malpighian tubules of cock- 
roaches are characteristic genera of this order. 
SUBPHYLUM CILIOPHORA (INFUSORIA) 
The subphylum Ciliophora includes all Protozoa which possess 
cilia at least through part of their active life, and in typical 
instances the nuclear material has become separated into two 
specialized bodies, one controlling the generative processes (the 
micronucleus) and the other directing the general activity of the 
cell (the macronucleus). In some degenerate forms (Opalina 
and Ichthyophthirius) there is no distinction of micro- and 
macronuclei for two or more similar nuclei occur. There are 
two classes within this subphylum; the Ciliata, in which cilia are 
retained throughout life and the Suctoria, in which cilia are lost 
after the young individuals become attached to some object. 
Class Ciliata 
As mentioned earlier in this chapter the ciliates represent 
a degree of complication of parts not found elsewhere even in the 
differentiated metazoan cells. It has been maintained by some 
that the body of the ciliate with its specialization and differentia- 
tion of the nuclei represents a differentiation analogous to the 
separation of somatic and germ cells in the Metazoa. Though 
the Mastigophora are commonly cited as the protozoan group 
through which relationships with the Metazoa are traced, there 
is considerable evidence that this phylogenetic significance is at 
least shared by the Ciliata. 
Both free-living and parasitic forms are common. Though the 
cell is commonly covered by a pellicle or cell wall there are definite 
openings for the ingestion of food and discharge of waste. A 
cytostome leads through a cytopharynx down into the endoplasm 
and a cytopyge for the elimination of solid waste is present 
though usually not observable except at the time of elimination. PHYLUM PROTOZOA 
49 
The cytoplasm is divided into an ectoplasm and an endoplasm 
of which the former comprises a number of rather sharply differ- 
entiated regions not uniformly encountered in the various groups. 
The alveolar layer is the outermost and is marked by a striated 
appearance due to the arrangement of the alveoli of the cyto- 
plasm. Beneath this lies the trichocyst layer in which the spindle- 
shaped trichocysts are embedded with their tips at the surface 
ready for discharge as long stiff organs of defense. A contractile 
layer underlies this one. It consists of myonemes which ordi- 
narily run parallel to the rows of cilia. Between or external to 
the myonemes are found the basal granules from which the cilia 
take their origin as they pass between the alveoli of the alveolar 
layer to the outer surface. A spongy zone of ectoplasm traversed 
by fluid-filled spaces and channels overlies the endoplasm and 
with the contractile vacuoles and their associated radial canals 
represents an excretory layer. 
The endoplasm is less highly organized. It comprises a fluid 
cytoplasm within which are contained the nuclei and various 
inclusions such as food and water vacuoles, excretory granules, 
and sometimes symbiotic algae. 
The macronucleus is extremely variable in shape, while the 
micronucleus is usually a single rounded mass. In the Peritricha, 
the macronucleus is frequently a long ribbon-shaped structure 
and in the Heterotricha it usually assumes a distinctly beaded or 
moniliform condition. An exceptional distribution is found in 
the genus Trachelius the members of which have a single large 
macronucleus and thirteen micronuclei. Reproduction is by 
binary fission, usually in the free state but in some forms accom- 
panying encystment. Preparatory to fission the micronucleus 
divides by mitosis and the macronucleus by amitosis. Mouth 
and other structures are frequently duplicated in the dividing 
individual before fission is completed. Conjugation of varied 
forms occurs in this group. The old idea of conjugation as 
essential to prevent senescence has been shown to be unfounded. 
Greater details of the process of conjugation will be discussed in 
connection with the treatment of the various orders. . 
On the basis of the arrangement of the cilia, the various 
orders of the Ciliata are recognizable. The cilia on the body sur- 
face are usually in meridional or spiral arrangement and occur 
either in furrow-like depressions or each cilium within the center 
of a small depression. In Paramecium, these depressions are 50 
INVERTEBRATE ZOOLOGY 
hexagonal or rhombic. At the angles of these polygons and in 
the middle of some of the sides are located the points of the 
trichocysts. Each cilium is composed of a firm axial filament 
covered by a sheath of more fluid contractile substance. The 
filament takes its origin from a basal granule and in many 
instances minute fibrils extend inward from the basal granules 
to the wall of the macronucleus. Various modifications of cilia 
occur. Non-motile tactile bristles have the same general struc- 
Fig. 32.--Diagram showing wave motion of cilia. (After Verworri). 
ture as the typical motile cilium. In the pharynx of many ciliates, 
all the cilia of a row become fused to form an undulating mem- 
brane or several adjoining rows may fuse to form a membranelle. 
In the Hypotricha, tufts of cilia are fused to form brush-like 
structures called cirri. Unmodified cilia are arranged in rows 
and move in rhythmical wave-like manner (Fig. 32). 
I. Order Holotricha 
Ciliates of this order possess cilia of approximately equal 
length evenly distributed over the entire body surface (Para- 
mecium, Opalina). In some highly specialized forms the arrange- 
ment may be modified by the development of more conspicuous 
bands or groups of cilia in certain regions (Didinium). The 
mouth is either a terminal pore or an opening on one side of the 
body which is then designated as the oral surface. Among 
some of the parasitic representatives (Opalina) the mouth is 
lacking. Trichocysts, characteristic of this order, are usually 
lacking in other ciliates. Typically the macronucleus is spherical 
or ovoid with the single micronucleus occupying a recess in its 
margin. In the two genera Opalina and Ichthyophthirius there 
is no differentiation of nuclei in the resting cell but in the 
latter genus a micronucleus makes its appearance at the time of 
reproduction. In Coleps the body is covered with a series of plates. 
In addition to the forms mentioned above Enchelys, Lacrymaria, 
Trachelius, Dileptus, Chilodon, Colpoda, and Urocentrum are 
characteristic genera. PHYLUM PROTOZOA 
51 
II. Order Heterotricha 
Representatives of this order have conspicuously large cilia 
or cirri arranged in a left-handed spiral leading to the mouth. 
The entire body is ciliated rather uniformly in addition to the 
oral spiral. Spirostomum, Bursaria, Nyctotherus, Balantidium 
(Fig. 33), and Stentor are representative genera. 
III. Order Oligotricha 
Members of this order were originally included with the 
Heterotricha with which they agree in the presence of the left- 
handed oral spiral of cilia. However, they lack general body 
ciliation. Halteria, Entodinium, and Cycloposthium are genera 
included in this order. 
Fig. 33.-Balantidium minutum 
Schaudinn. (After Schaudinri). 
Fig. 34.-Neuromotor system- in 
Euplotes harpa, showing system of 
fibrils connecting the cirri. (After 
Prowazek). 
IV. Order Hypotricha 
Hypotricha have a flattened ventral surface upon which they 
usually creep. Ciliation is chiefly on the ventral surface in the 
form of rows and of cirri. A powerful adoral membranelle is 
usually present. Long, non-motile, usually sessory cilia may 
occur on the dorsal surface. Urostyla, Stylonichia, Kerona 52 
INVERTEBRATE ZOOLOGY 
(commensal on Hydra), and Euplotes are the more important 
genera. A system of fibrils (Fig. 34) connects the cirri and 
seems to act as a neuromotor system in coordinating movements. 
V. Order Peritricha 
These typically sessile forms have a clockwise adoral spiral 
composed of two rows of cilia. This spiral leads into a vestibule 
at the base of which is located the mouth and into which the 
contractile vacuole and the anus empty. Locomotor cilia are 
frequently lacking or may be represented by a single permanent 
or temporary ring usually at the aboral pole. Trichodina (on 
Hydra and planarians) and Cyclochaeta (on fishes) are examples 
of the non-stalked forms. Vorticella, Carchesimn, and Zootham- 
nium are examples of contractile stalked forms and Epistylis 
and Opercularia of non-contractile stalked forms. 
In the Peritricha, a modified form of conjugation between a 
microconjugant and a macrocon jugant occurs. By two or three 
successive divisions, some individuals produce four or eight 
microconjugants which after acquiring an aboral ring of cilia 
become free-swimming. One of these microconjugants fuses 
with an ordinary individual, which is the macroconjugant. In 
brief, the micronucleus of the microconjugant undergoes division 
to form eight parts of which seven degenerate and only one per- 
sists. Later, this one divides into two. During this same time 
the micronucleus of the macroconjugant has divided into four 
parts of which three degenerate and the remaining one redivides 
to form two nuclei. In both conjugants, the macronucleus has 
undergone disintegration. In each conjugant, one of the two 
pronuclei which originated from the micronucleus disappears. 
The remaining nucleus of the microconjugant passes over into 
the cytoplasm of the macroconjugant and fuses with the single 
nuclear mass remaining there. As a consequence, the macro- 
conjugant now has a fusion nucleus and the microconjugant 
which now has no nucleus drops off and disintegrates. The 
nucleus of the remaining individual divides into eight parts of 
which one becomes a micronucleus and the other seven macro- 
nuclei. Through a series of divisions of the micronucleus and 
fission of the cytoplasm, seven individuals ultimately result 
from the conjugation. PHYLUM PROTOZOA 
53 
Class Suctoria 
The Suctoria are characterized by the complete lack of cilia 
in the adult stage and the presence of tentacles by means of which 
other organisms are captured and the protoplasm drawn into the 
body of the suctorian. Cilia occur on the body of the free-swim- 
Fig. 35.-Suctoria {after various writers). A, colony of Dendrosoma; B, 
Rhyncheta; C, Ophryodendron; D, Tokophrya; E, ciliated young of Sphaero- 
phrya; F, diagram of capitate and styliform tentacles arising from ectoplasm. 
{From Hertwig's Manual of Zoology by Kingsley, Courtesy of Henry Holt and Co.) 
ming larva (Fig. 35E) but are lost when the animal assumes the 
adult form after becoming sessile. The tentacles are hollow, 
usually terminating in a sucker-like knob. The macronucleus is 
extremely variable in form. In some 
colonial representatives its branches 
extend throughout the branches of the 
colony. A micronucleus has never 
been demonstrated for some suctorians. 
Binary fission, so common in the Ciliata, 
is rare in the suctorians. Both external 
and internal buds are of common occur- 
rence. The embryos when liberated 
are furnished with bands of cilia for 
locomotion. Conjugation, in nature 
similar to that found in Ciliata, may 
take place either between similar in- 
dividuals or between a fixed individual 
and a free-swimming bud suggesting 
the condition found in the Vorticella. 
Podophrya, Acineta, Ephelota (Fig. 
36), Tokophrya, and Dendrosoma are typical genera. 
Fig. 36 .-A suctorian 
{Ephelota biltschliana) with 
five daughter buds. {After 
Calkins). 
Interrelationships of the Classes of Protozoa 
The Protozoa represent the simplest present-day organization 
of animal life. It is probable, however, that there at one time 54 
INVERTEBRATE ZOOLOGY 
existed simpler living substance which would be considered 
animal in nature. This most primitive form of life was probably 
more homogeneous in its make up for morphological differentia- 
tion of the nucleus and cytoloplasm and regional differentiation 
of the cytoplasm such as we find in even the simplest Proto- 
zoa have probably resulted from long periods of progressive 
evolution. 
There are numerous evidences that plant life existed on the 
earth before animals came into being. The Mastigophora, 
through the Phytomastigina, display unquestioned relationships 
with the members of the Plant Kingdom. Because of this fact 
some biologists consider the Mastigophora as the most primitive 
class of the Protozoa. 
On the other hand, the rhizopod organization is, on the whole, 
less intricate than that of the Mastigophora or of any other proto- 
zoan. Consequently, on the criterion of simplicity of organiza- 
tion, some consider the Rhizopoda as the most primitive class of 
the phylum. It remains as a possible explanation that the 
Rhizopoda represent the simplifying effects of a regressive 
evolution. In their vegetative stages, the Sporozoa show con- 
vincing evidences of rhizopod affiliations through the presence 
of pseudopodia and in their general organization. Sporozoa 
probably had their rise through some ancestral form of our 
present day Rhizopoda. 
In the Ciliophora, is encountered the highest morphological 
and physiological differentiation found in Protozoa. Without 
doubt this subphylum stands at the apex of protozoan evolution. 
The Suctoria have been given off as a side branch from the main 
ciliate stem as evidenced by the fact that suctorians, in their 
development, pass through a ciliated larval stage. 
References 
Calkins, G. N. 1909. Protozoology. New York, Lea and Feabinger. 
Craig, C. F. 1911. The parasitic amoebae of man. Philadelphia, J. B. 
Lippincott. 
Dobell, C. G. 1921. The Intestinal Protozoa of Man. London, John 
Bale, Sons and Danielson. 
Doflein, F. 1916. Lehrbuch der Protozoenkunde, Ed. 4. Jena, G. 
Fischer. 
Haeckel, E. 1862-1888. Die Radiolarien (Rhizopoda radiosa) Eine 
Monographic. Berlin. PHYLUM PROTOZOA 
55 
Kent, W. S. 1880-1882. A Manual of the Infusoria (etc.). London, 
D. Bogue. 
Leidy, J. 1879. Fresh-water Rhizopods of North America. Rep. U. S. 
Geol. Survey, Vol. 12. 
Minchin, E. A. 1917. An introduction to the study of the Protozoa. 
London, Edw. Arnold. 
Prowazek, S. von (various authors). 1912. Handbuch der Pathogenen 
Protozoen. Leipzig, J. A. Barth. CHAPTER III 
INTRODUCTION TO THE METAZOA 
In the foregoing chapter, attention has been called to the ten- 
dencies which some Protozoa display toward the specialization 
of their cells. Most of this specialization is expressed in increased 
complexity of intracellular organization. In some instances, 
however, groups of cells, termed colonies, act as the unit or indi- 
vidual. Within colonies some of the cells frequently become 
specialized as gametes for reproductive purposes but all of the 
somatic cells remain alike. In the Protozoa, any specialization 
of somatic cells affects all equally so there is no differentiation. 
It seems probable that the Metazoa must, at some time, have 
originated from protozoan ancestors but all evidences of the 
means of this transition have been lost and both Protozoa and 
Metazoa have gone far along independent lines of evolution 
since the Metazoa came into existence. The lack of fossil forms 
bridging this gap is not surprising for both the Mastigophora 
and the Cihata, through the ancestors of which this genesis 
might have taken place, have no hard parts which could be 
preserved as fossils. 
There are a few present day animals which have been termed 
the Mesozoa because they have been considered as intermediate 
between Protozoa and Metazoa but their simplicity is probably 
due to degeneracy as is shown in the section of this chapter 
devoted to the Mesozoa. 
Embryology frequently offers a clue to and outlines the steps 
in phylogeny (Recapitulation Theory). All Metazoa, in their 
development, typically pass through a single-celled stage-the 
fertilized egg. This fact readily furnishes the basis for an analogy 
of metazoan origin from the single-celled organisms. In the 
development of the fertilized egg into the adult organism, there 
is a time when large numbers of fundamentally similar cells (the 
blastomeres) are produced through the partition of the single 
cell. It is only in later development that these cells through 
histological differentiation assume different forms and come to 
56 INTRODUCTION TO THE METAZOA 
57 
perform diverse limited functions. This subject of histological 
differentiation, which marks off the Metazoa from all Protozoa, 
is discussed in the second section of this chapter. A brief 
discussion of the organ systems of the invertebrates concludes 
the chapter. 
A. The Mesozoa 
A small group of relatively simple many-celled animals com- 
posed of a layer of ectoderm cells covering a single or several 
entoderm cells has frequently been considered as standing inter- 
mediate between the Protozoa and the Metazoa and has conse- 
quently been termed the Mesozoa. The phylogenetic significance 
which has thus been attached to this group is a subject regarding 
the tenability of which very grave doubts are justifiable, for all 
of the more important representatives of the group are internal 
parasites. It seems probable that their simplicity of organiza- 
tion is an accompaniment of degeneracy directly traceable to 
adaptation to the parasitic existence. Not even the known facts 
regarding the development of these forms aid in pointing out 
relationships with any other group of organisms. 
The Dicyemidae and the Heterocyemidae include species which 
are parasitic in cephalopods while Rhopalura which represents 
the Orthonectidae lives as a parasite in various invertebrate hosts. 
B. Histological Differentiation 
Differentiation of Tissues.-As emphasized in the foregoing 
chapter, all Metazoa differ from the Protozoa in that the cells 
of the former become specialized for restricted functions, that is, 
undergo histological differentiation. Certain groups of cells 
become similarly specialized for carrying out one specific function 
more effectively than that function may be executed by unmodi- 
fied protoplasm. Such a group of similarly specialized cells is 
termed a tissue. A tissue is made up not only of units of cyto- 
plasm and their contained nuclei, but frequently the cytoplasm 
forms substances which are essential in the effective functioning 
of the tissue. These substances, which are termed plasmic 
products, may be either inconspicuous or so prominent that the 
cells which produce them are obscured. 
Classification of Tissues.-Since the function of a tissue is one 
of its most important characteristics, the classification of tissues 
is usually built around this character. Even though differentia- 58 
INVERTEBRATE ZOOLOGY 
tion leads along numerous different lines in the metazoan body, 
the general results are so nearly uniform in different animal 
groups that but four classes of tissues are commonly recognized, 
namely; epithelium, connective tissue, muscle tissue, and 
nervous tissue. 
Both form and structure of an organism depend upon the 
grouping of the component tissues. Thus both morphology and 
physiology go back for their final basis to the study of the 
several tissues and frequently to the form and structure of the 
individual component cells. 
Epithelial Tissues 
Definition and Classification.-Any tissue which covers 
a surface is called an epithelium. Phylogenetically, tissues of 
this class are the most primitive for the Coelenterates are essen- 
tially nothing but epithelial in structure. In like manner, in 
individual development among all the Metazoa, the first division 
of labor for the cells of the embryo occurs with the formation of 
the gastrula stage when nothing but surface coverings is present. 
Form vs. Function.-Varied forms of cells and sundry func- 
tions occur in tissues in this group. If an epithelium comprises 
but a single layer of cells, it is termed simple while an epithelial 
layer two or more cells in thickness is designated as stratified 
epithelium. The form of the component cells leads to the use of 
such descriptive terms as cuboidal, columnar, and squamose 
but, since function is a much more important criterion for 
classification, epithelia are grouped as protective, formative or 
glandular, ciliated, communicative or sensory, and germinal. 
Ciliated and Flagellated Epithelium.-Even in the gastrula 
stage of ontogeny some or all of the cells are frequently provided 
with cytoplasmic threads, the movements of which are primarily 
for locomotion. Ciliated or flagellated cells as a means of loco- 
motion are only infrequently found in adult metazoans but do 
occur in the flatworms (Fig. 62), the rotifers (Fig. 72), and 
molluscs (Fig. 91). More frequently, however, cilia and flagella 
on epithelia of Metazoa produce movements of the medium (Fig. 
37) rather than affect locomotion. Thus in the sponges the 
flagellated cells of the entoderm produce the water currents which 
carry the food supply. The same is true of the ciliated mantle 
and gills of the mussels. INTRODUCTION TO THE METAZOA 
59 
Protective Epithelium.-Because of their location on external 
surfaces epithelia are frequently modified for a protective func- 
tion. Stiff fibrils in the cytoplasm (Fig. 38) and thickenings of 
the outer margin of the cell are two means whereby mechanical 
protection is secured. Frequently, an external epithelium may 
form extra-cellular materials which are deposited in a layer or in 
a succession of layers over the surface of the body. The cuticula 
characteristic of so many invertebrates is thus produced by an 
underlying epithelium which is frequently termed a hypodermis. 
Fig. 37.-Glandular and ciliated 
epithelium showing unicellular glands 
in entoderm of earthworm, Eisenia 
rosea. (After Schneider, courtesy of 
Gustav Fischer). 
Fig. 38.-Protective epithelium 
from a flatworm, Planocera folium. 
(After Schneider, courtesy of Gustav 
Fischer'). 
In many instances, inorganic salts are deposited in this cuticula 
until a shell-like armor of resistant plates is formed. « 
Glandular or formative epithelium represents one of the com- 
monest modifications of this class, of tissue. Even in the gastrula 
stage the entoderm of the embryo has begun to be specialized as 
a glandular tissue and the cytoplasm and its inclusions render the 
entoderm obviously different from the ectoderm. Cells scattered 
singly through an epithelium may become specialized as gland 
cells as in the case of the mucus cells (Fig. 37) of the earthworm. 
Frequently, these unicellular glands become enlarged and through 
elongation the single cells extend down into the underlying tissues 
much deeper than the ordinary epithelial cells which surround 
them. 
Flat surfaces or large areas of an epithelium may become 
differentiated for a glandular function, as in the foot of Hydra, 
but in most instances specialized glands require an increased sur- 
face for their functioning as secretory or excretory organs and 
consequently areas of glandular epithelium become invaginated 60 
INVERTEBRATE ZOOLOGY 
either as a simple tube or as a series of branched tubes and cham- 
bers the walls of which are composed of gland cells. A duct 
usually keeps such a compound gland in communication with the 
surface from which the gland cells were originally invaginated. 
Sensory or Communicative Epithelium.-Naturally, the only 
communication which an organism may have with the outside 
world is through its surfaces. Consequently, any sensory 
mechanism must be associated with the surface of the body and 
is therefore epithelial in origin. Sen- 
sory cells located in the epithelium 
have communication with nerve end- 
ings through which they are able to 
transmit stimuli to the central nervous 
system. Hairs or bristles on the sur- 
face of epithelial cells render them 
especially susceptible to touch stimuli 
so isolated cells or groups of cells thus 
modified form various tactile organs. 
An association of pigment with 
epithelial cells having rich nerve 
supply usually signifies an optic organ 
of some type and is designated as a 
retina. Balancing organs or 
statocysts (Fig. 39) are usually modi- 
fied epithelia specialized for receiving 
tactile stimuli through the movements 
of a. bodv ealled a, statolith or a.n otolith. 
Fig. 39.-Statocyst of Gomi 
onerous. (Orig.) 
Germinal Epithelium.-Germ cells of all Metazoa have their 
origin in an epithelium. In many of the groups, the presence of 
a germinal epithelium is readily observable. In some of the 
simpler Metazoa, as, for example, the coelenterates, the germ 
cells may arise as modifications of epithelial cells which are not 
even necessarily grouped to form a gonad. The gonads of many 
groups are bounded by an epithelium, the cells of which trans- 
form into the germ cells. Morphologically, the female germ cells 
become differentiated from ordinary epithelial cells chiefly 
through the addition of deutoplasm or yolk, and in many 
instances through acquiring additional membranes. On the 
other hand, the male germ cells become highly modified and 
undergo extensive rearrangements of their parts, chiefly as an 
adaptation for more effective movement. INTRODUCTION TO THE METAZOA 
61 
CONNECTIVE OR SUPPORTING TISSUE 
Definition.-Tissues which fill in between and connect other 
tissues, give support to them and to the body in general 
are grouped under the general category of connective or support- 
ing tissues. It has been shown that the individual cells in their 
form and function give the chief character to epithelial tissues, 
but in the connective tissues plasmic products are usually the most 
conspicuous elements. In some tissues these products are intra- 
cellular as in the spicules formed 
by mesoderm cells of the 
sponges. Fat or adipose tissue 
results from an accumulation of 
oil or fat droplets within the 
cytoplasm until they coalesce to 
Fig. 40.-Cartilage of an adult 
squid. (From Dahlgren and Reyner, 
courtesy of Macmillan Co.) 
Fig. 41.-Portion of a section of 
Cerebratulus marginatus, showing much 
branched connective tissue cells. 
(After Schneider, courtesy of Gustav 
Fischer). 
form a single large drop around which the cytoplasm of the cell 
is spread in a thin, attenuated layer. 
More frequently, the plasmic products are inter-cellular. 
In such instances, the cells of a connective tissue are embedded in 
a non-protoplasmic matrix, the specific nature of which may be 
highly variable. In its simplest condition, this matrix is com- 
posed of a gelatin-like mass within which the cells are embedded to 
form a homogeneous or gelatinous connective tissue (Fig. 40). 
Frequently fibrillae are formed within the gelatin (Fig. 41) 
giving greater firmness. Depending upon the arrangement of 62 
INVERTEBRATE ZOOLOGY 
these fibrillae, their composition, and composition of the matrix, 
several kinds of connective tissue are recognizable, as, for 
example; fibrous connective tissues, elastic tissue, cartilage, etc. 
Muscle Tissue 
Movement is an inherent property of all protoplasm. Even 
in the Protozoa, portions of the cytoplasm frequently become 
altered as fibers for movement more effective than that which 
Fig. 42.-Muscle fibers from sub-umbrella of jellyfish, Carmarina hastata. 
(After Schneider, courtesy of Gustav Fischer). 
characterizes undifferentiated, viscous protoplasm. These par- 
tial specializations of the protozoan cell for movement are termed 
myonemes. Partial differentiation of cells for contraction are 
not infrequent in the Metazoa. The inner margins of the ecto- 
derm cells of Hydra contain contractile threads within the cyto- 
plasm, and consequently the covering cells of the Hydra are 
frequently termed epithelio-muscular cells. In the 
nematodes, the mesoderm cells forming the inner- 
most layer of the body wall undergo a partial differ- 
entiation to form a series of longitudinal muscle 
fibers. The peripheral parts of these1 mesoderm 
cells become specialized for contraction while an 
undifferentiated mass of cytoplasm containing the 
nucleus protrudes into the body cavity. 
Frequently, mesenchyme cells become elongated 
or spindle-shaped and develop myofibrils within 
their cytoplasm. Such cells when grouped together 
form the commonest type of smooth muscle tissue. 
Striated muscles occur even in forms as low as 
the medusae (Fig. 42), where they remain on the 
surface of the body. They usually originate as 
epithelial cells. The nuclei of cells which are to form 
striated muscle undergo a series of divisions without sub- 
sequent division of the cytoplasm. In the cytoplasm of the poly- 
nuclear cells thus formed, numerous fibrils make their appearance. 
The fibers are surrounded by a sarcoplasm containing the nuclei 
Fig. 43.- 
Muscle fiber 
from adductor 
muscle of mus- 
sel, Anodonta 
muta biIis. 
(After Schnei- 
der). INTRODUCTION TO THE METAZOA 
63 
and the whole muscle element is covered with a sheath which is 
termed the sarcolemma. Each fibril is composed of two different 
kinds of substance, one called the isotropic substance which 
does not stain readily, and another, the anisotropic substance 
which is doubly refractive and stains deeply. The two substances 
alternate regularly along the fibril and in adjoining fibrils they are 
in alignment so that in stained preparations under the micro- 
scope a bundle of fibrils has a cross-striped appearance. Peculiar 
groupings of fibrils occur in some muscle cells (Fig. 43). 
Nervous Tissue 
Irritability, or the power of reacting to stimuli, is inherent in 
all protoplasm but even in the single-celled Protozoa it has been 
shown that parts of the cytoplasm become specialized as a neuro- 
motor apparatus. Such specialization has been demonstrated 
for some of the ciliates in which it seems 
probable that stimulation is propagated along 
certain tracts or fibrils more efficiently than 
through undifferentiated cytoplasm. In most 
Metazoa, a nervous system is developed for 
transmitting sensory and motor impulses 
through the linking together of highly special- 
ized nerve cells or neurons. Nervous impulses 
are propagated through a plexus of scattered 
cells in the hydroid stage of the coelenterates. 
Characteristically, a nerve cell consists of 
a nucleated cell body of cytoplasm from 
which one or more cytoplasmic processes 
are given off. There is a definite polarity in 
nerve cells. A given fibril propagates an 
impulse in only one direction. When there are several 
cytoplasmic processes from the cell body of a neuron those 
which transmit the impulse inward toward the cell body 
are termed dendrites. Normally, but a single process carries 
impulses outward from the cell body and this process is called a 
neurite or an axon. 
In most of the higher Metazoa, the nerve cells are grouped 
into masses which are called ganglia. The neurites and dendrites 
form part of the fibrous portion within the ganglia and continue 
in groups out from the ganglia as important constituents of 
the nerves of the peripheral nervous system. 
Fig. 44.-Neuro- 
fibrillae in ganglion 
cell of a leech. (.After 
Apathy). 64 
INVERTEBRATE ZOOLOGY 
By very special histological technic the cytoplasm of a neuron 
and its processes is shown to contain various fibrillar structures 
(Fig. 44). It is thought that nerve impulses are transmitted 
Fig. 45.-Nerve cell from ganglion of crawfish. (After Dolley) 
along these fibers rather than through the general cytoplasm. 
In addition, much of the cytoplasm of nerve cells contains deeply 
staining granules or masses of questionable 
significance, which are termed the tigroid bodies. 
Frequently, a region within the cytoplasm of the 
cell body directly continuous with the cytoplasm 
of the axon is devoid of tigroid bodies. This 
implantation cone, as it is sometimes called, is 
so marked in arthropods that sections of nerve 
cells (Fig. 45) have the appearance of being 
highly vacuolated. 
Interspersed between the neurons and fibers 
of a ganglion are varying amounts of mesodermal 
connective tissue and a highly modified type of 
ectoderm cells (Fig. 46) which are called neurog- 
lia. The neuroglia cells of invertebrates usually 
have some portion of the cell body on the 
surface of the ganglion while a fibre-bearing, 
supporting portion extends between the nervous 
elements of the ganglion. 
Fig. 46.-Neu- 
roglia cells from 
nerve cord of 
earthworm, Eis- 
enia rosea. (After 
Schneider). INTRODUCTION TO THE METAZOA 
65 
In function, structure, and origin, nervous tissue and the 
sensory epithelium discussed earlier are intimately associated. 
Morphological Changes in the Nucleus 
In the foregoing discussion, attention has been focused upon 
the changes in the form and structure of the cytoplasm and of the 
plasmic products which are involved in histological differentia- 
tion. Omission of the nucleus from this discussion does not 
imply that it is not concerned in differentiation. There are, 
however, few conspicuous changes in form or appearance of the 
nucleus in comparison with the radical changes in the extra- 
nuclear portions of the cells. As the controlling center of 
most of the activities of the cell, doubtless the nucleus must have 
varying functions depending upon the line along which the cell 
is specialized but there are few reflections of this in the morphol- 
ogy of the nucleus. The nuclei in some highly active tissues 
become irregular or even much branched but these morphological 
changes seem to be correlated with rate of activity rather than 
with actual differentiation of the nucleus in a particular kind of 
tissue. 
C. The Organ Systems 
Just as the individual cells in the many-celled animals become 
dependent one upon another through histological differentiation 
so also do the individual tissues become interdependent. Few 
tissues function as utterly isolated units for they become com- 
bined in groups which are known as organs. These organs are 
the readily recognizable morphological units of which the meta- 
zoan body is composed. All of the tissues which form an organ 
cooperate in the performance of some function. Obviously, not 
all tissues in an organ function to the same extent, but usually 
one kind of tissue becomes the dominant or essential tissue and is 
aided in carrying out its functions by the other tissues of the 
same organ. 
Thus, in a digestive organ the essential process of digestion is 
made possible through the presence of glandular epithelia which 
form substances necessary for the digestion of food. The 
epithelia are frequently aided in this function by cooperative 
muscles under whose action the content of the digestive tract is 
kept agitated so that all of the food material comes into contact 
with the digestive fluids produced by the epithelia. 66 
INVERTEBRATE ZOOLOGY 
When identical or similar organs are grouped for the perform- 
ance of the same or related functions, such a complex is termed 
an organ system. All of the activities which characterize the 
living animal are so fundamentally interrelated that the numer- 
ous organs and tissues of the metazoan body are capable of being 
grouped under a relatively small number of systems. Yet these 
major organ groups, the systems, have their interrelations and 
function not as independent units but as a correlated whole- 
the organism or the individual. 
Throughout the following sections of this book constant 
reference is made to the digestive, circulatory, respiratory, 
excretory, locomotor, reproductive, and nervous systems. The 
organs of which each system is composed in the various phyla and 
classes are not necessarily identical in structure or origin for 
frequently the same function is carried on by conspicuously 
different organs in different groups. A brief discussion of the 
modifications of the several organ systems is given here but for a 
full understanding and appreciation of this discussion, the 
student should again read this section after having completed the 
laboratory study of a number of metazoan forms. 
The Digestive System.-The power of transforming non-living 
food substances into living matter or protoplasm is a distinctive 
property of all living organisms. This is not accomplished 
by a direct transmutation or by a simple act of endowing the 
food with life. Since living protoplasm is in a liquid or semi- 
liquid state the process of digestion or liquefying of food is 
the essential first step in the transformation of food into living 
matter. In some degenerate forms of animal life which have 
become adapted to a purely parasitic existence as, for example, 
the tapeworms and the Acanthocephala, food taking is restricted 
to the direct absorption of liquids by the cells of the 
body. Organs of digestion are entirely wanting in such 
instances for the entire digestive process is performed by the 
organs of the host. 
In some of the protozoans and in a few of the Metazoa, solid 
food substances are taken directly into the protoplasm of the 
organism. These ingested particles, though within the proto- 
plasm, remain foreign to it until they have undergone physical 
and chemical changes which render them capable of assimilation 
by the protoplasm. This type of digestion is termed intra- 
cellular digestion. Among the sponges and some of the flat- INTRODUCTION TO THE METAZOA 
67 
worms, in addition to the Protozoa, the entire digestive process 
is intracellular. 
Ontogenetically, the archenteron of the gastrula stage represents 
the first organ specialized for digestion. In many animals, the 
larva upon reaching the gastrula stage is an independent organism 
capable of self maintenance. Food taken through the blastopore 
of such a gastrula is rendered liquid through the action of secre- 
tions formed by the entoderm cells. Thus the gastrula cavity 
becomes a space within which intercellular digestion takes place 
and is termed an archenteron or primitive digestive system. 
Among the Metazoa, there are some animals in which the 
digestive system of the adult never reaches a state of complexity 
appreciably higher than that of the archenteron. Among the 
Coelenterata, the polyp of the Hydrozoa is essentially similar to 
the archenteron for it is a simple entodermal sac with but a single 
opening, the mouth. In many other coelenterates, there is a 
beginning of a separation of the distributive function from the 
central digestive cavity. Thus in the jellyfishes, the gastro- 
vascular system consists of a central cavity from which a periph- 
eral system of pouches or canals passes to the outlying parts of 
the body and thereby aids in the distribution of the digested food. 
In most of the Metazoa above the coelenterates, the blastopore 
of the gastrula is not retained as the mouth of the adult for 
the blastopore closes during development of the embryo and for a 
time the entodermal digestive tube is but a sac without communi- 
cation with the outside. Communication is later established by 
an infolding of the body covering at the anterior extremity to 
form an ectodermal antechamber to the entodermal digestive sac. 
Such an entodermal digestive sac with a single opening lined with 
ectoderm (the stomodseum) is characteristic of the Turbellaria. 
In the nemertines and in all organisms higher than the 
flatworms, there is characteristically a second ectodermal 
invagination to meet the entoderm. This second ectodermal 
invagination is normally located at the posterior extremity as a 
proctodaeum terminating in an anal aperture. Forms in which 
both stomodaeum and proctodaeum occur are said to have a 
complete digestive system. In its simplest condition, this 
consists of a continuous tube the extremities of which are ecto- 
dermal and the true digestive middle portion of which is formed 
of, or at least is lined with, entoderm. Most of the highly 
developed digestive systems represent only modifications of this 68 
INVERTEBRATE ZOOLOGY 
simple tubular condition. Different regions become modified 
for limited functions. With these specializations changes both 
in form and in structure arise and regions such as esophagus, 
stomach, and intestine are marked off. Jaws or organs for the 
comminution of food are frequently developed in the stomodaeum. 
Glands grow out from the walls of the tube and remain attached 
to the digestive tract only through ducts. Muscles, connective 
tissue, and vessels of the circulatory system become associated 
with the entodermal tube to form a more highly complicated 
digestive system. In some instances, the proctodaeum receives 
the ducts of the reproductive system and is then termed a cloaca. 
Circulatory System.-Whether digestion is intercellular or 
intracellular, the products of digestion are directly available to 
only part of the cells of the body while the remainder of the cells 
must rely upon some agency external to themselves for furnishing 
them with the materials essential for anabolism. In some of the 
simpler Metazoa, this is accomplished by direct transfer of the 
materials from cell to cell but in the highly complicated organisms 
such transfer is not possible because some cells are so far removed 
from the organs where digestion takes place. In such instances, a 
special system for the distribution of digested food has made its 
appearance as a specialized circulatory system. 
Many steps in the development of complexity of the cir- 
culatory system are observable in the different animal groups. 
It has already been pointed out that the gastrovascular system of 
coelenterates serves both for digestion and distribution but even 
in this phylum many steps in the differentiation of the two sys- 
tems are found. Trematodes and Turbellaria among the flat- 
worms also represent the condition of a combined gastrovascular 
system. 
In its simplest form, as exemplified by the hydroid polyp, 
this system is a simple sac or pouch, the walls of which are formed 
of two layers of cells, the ectoderm and the entoderm. The most 
distant cells of such an organism are so slightly removed from the 
store of digested food material that distribution is by direct 
transfer from cell to cell. In the medusae of the same phylum, 
the digestive sac is relatively small and located near the center of 
a much enlarged body within which transfer from cell to cell is not 
so readily possible. Under these conditions a distributive system 
arises, intimately associated with but somewhat distinct from 
the digestive system. The tubes or pouches of such a gastro- INTRODUCTION TO THE METAZOA 
69 
vascular system mark the beginnings of a separate system for 
circulation. 
Among the coelomate animals, digested food materials fre- 
quently pass through the wall of the digestive tract into the 
coelomic cavities where it becomes recognizable as the body 
fluid. Only in the simple coelomate animals is this fluid entirely 
free within the body cavities for it becomes more or less confined 
within a system of definite channels comprising the vessels of the 
circulatory system. When the fluid is continuously within 
vessels, the system is termed a closed system but when in any 
part of its course the fluid is emptied into the body cavity or into 
sinuses the system is designated as an open circulatory system. 
Muscles become associated with at least part of the vessels which 
then function as a pumping organ or heart. In many inverte- 
brates, the most conspicuous pumping organ is the vessel, or 
part of the vessel, which lies dorsal to the digestive tract but 
hearts are frequently located in other regions or the body as, for 
example, the gill hearts of molluscs and the modifications of the 
circular vessels in the annelids. 
In some instances, notably in the molluscs, the heart becomes 
differentiated into various chambers. Those which receive the 
blood are termed auricles and those which force the blood out 
into the vessels are called ventricles. The heart presents an 
interesting instance of independent or parallel development in 
invertebrates and vertebrates. A three-chambered heart is 
fairly common in the molluscs while the fishes have but a two- 
chambered heart. 
Respiratory System.-The energy manifested by a living 
organism results largely from the union of oxygen with proto- 
plasm and its contained food substances. Highly complicated 
organic compounds are oxidized or broken down into simpler 
waste substances thereby transforming the potential energy of 
the complex chemical compounds into the kinetic energy of the 
living organism. The term respiration is applied to the process 
of admission of oxygen into living protoplasm and the subsequent 
giving off of carbon dioxide. More strictly this process should 
be termed aerobic respiration for there are some organisms which 
obtain their energy release in the total absence of oxygen and 
these are said to undergo anaerobic respiration. Since aerobic 
respiration is by far the more common, the term respiration when 
not modified is usually taken to mean this type. 70 
INVERTEBRATE ZOOLOGY 
Respiration is a process essential for the existence of every 
living cell, yet in the many-celled metazoans respiration as a 
cellular process becomes masked or lost sight of through the 
introduction of organs which facilitate the process for the entire 
organism. Since the respiratory process involves an exchange 
of oxygen and carbon dioxide, any surface which serves for this 
exchange must be moist and delicate in order to permit a diffusion 
of the gases. In animals which are diploblastic, as the coe- 
lenterates, conditions for respiration are not essentially different 
from those found in the single-celled Protozoa for practically 
every cell has a surface exposed to the water through which the 
gases may diffuse. Even in some of the coelomate animals, 
such as the earthworms, the body surface provides an area 
sufficient for the respiratory exchange but in these as well as in 
all the higher animals the body fluid plays an important part in 
that it absorbs or loosely combines the gases within the body. 
In many animals, the body surface is unable to supply all of 
the cells with oxygen because of insufficiency or because of dry- 
ness which inhibits diffusion. Special organs for respiration 
have been developed in all such forms. Roughly, in inverte- 
brates, these modifications may be classified as gills, tracheae, 
lung-books, and lung sacs. 
Of gills, there are two distinctly different kinds, blood gills 
and tracheal gills. The former usually consist of portions of the 
body wall drawn out into thin filaments or thin lamellae the 
cavities of which are continuous with the body cavity and 
are filled with body fluids or contain blood vessels through the 
walls of which the respiratory exchange takes place. Tracheal 
gills are evaginations or invaginations of the body wall within 
which or around which air tubes or tracheae are distributed. 
Gills occur on almost any part of the body, wherever their 
function may be carried out. 
Tracheae are invaginations of the body wall to form a system 
of air tubes which carry atmospheric oxygen directly to all parts 
of the body in insects. The intimate structure of the tracheal 
system is discussed in greater detail in Chapter XV. 
In the arachnids, an invagination of the body wall leads 
into an organ called a lung-book, the walls of which are 
composed of much-folded delicate membranes through which 
oxygen is taken from the air-filled cavity of the lung-book and 
carbon dioxide is given off. INTRODUCTION TO THE METAZOA 
71 
In some molluscs, the cavity which ordinarily bears gills 
lacks these structures and has become secondarily modified as a 
lung sac into which atmospheric air is periodically admitted and 
from which the air bearing carbon dioxide is ejected. 
Excretory System.-As a result of katabolic processes, wastes 
are formed within every living cell. The carbon dioxide men- 
tioned above is only one of the important waste substances. 
Water, ammonia, urea, and other nitrogenous compounds accumu- 
late within the living protoplasm. Many of these wastes 
are soluble in water and tend to diffuse through the walls 
of the cells. 
In many of the Protozoa, contractile vacuoles facilitate the 
collection and elimination of these wastes. Among the coe- 
lenterates and the Porifera, direct diffusion through the cell 
surfaces into the surrounding water suffices for the elimination 
of these katabolic products. In the Metazoa, the first specializa- 
tion for their elimination is the protonephridial system of flat- 
worms, rotifers, and of trochophore larvae. This system consists 
of a series of tubules associated with flame-cells. Such a system 
may be provided with an enlarged excretory vesicle within which 
the excretory matter accumulates before it is discharged through 
the excretory pore. 
A great many of the coelomate animals have a metanephridial 
system which picks up excretory wastes from the coelomic cavities 
and from the body fluid. The unit of structure in such a system 
is essentially a funnel-like nephrostome which communicates 
with the outside by means of a tubule. The nephridia are paired 
organs segmentally arranged in annelids, while in the Crustacea 
they are considerably modified in form and are limited to one or 
two pairs. 
A group of thin-walled Malpighian tubes communicates with 
the intestine of insects and discharges excretory wastes from the 
body fluid into the digestive canal. 
Reproductive System.-In some Metazoa, the gonads or 
essential organs of reproduction are the only organs involved in 
the reproductive process. Thus, in the coelenterates, the germ 
cells are dehisced directly from the body surface or into the 
gastrovascular cavity. Ducts which lead from the gonads to the 
exterior are present in most other Metazoa. These, and other 
accessory organs, comprise the reproductive system. The gonads 
may be single, paired, or multiple. Frequently, a gonad 72 
INVERTEBRATE ZOOLOGY 
becomes subdivided into follicles, each of which has more or 
less the appearance of an independent organ. 
In most instances, the ducts which carry the germ cells from 
the ovaries and spermaries are at least in part (Fig. 47) derived 
as modifications of excretory ducts. Vasa deferentia of the male 
are usually directly continuous with the gonads while the ovi- 
ducts of the female have communication with the ovaries only 
through the body cavity. A portion of the oviduct of the female 
is frequently enlarged as a uterus within which the eggs are held 
for either a brief time or through the full period of the develop- 
ment of the embryos. 
Fig. 47.-Different relations of nephridia and sexual ducts in chaetopods 
{after Goodrich'). I, hypothetical primitive condition, gonoducts {g) and proto- 
nephridium (p) independent; II, ciliated grooves discharge into duct of protone- 
phridium as in Phyllodoceids and Goniads; III, ciliated grooves and metanephri- 
dium (n) open independently as in Dasybranchus; IV, ciliated grooves open into 
canal of metanephridium as in Syllids, Spionids, etc. {From Hertwig's Manual 
of Zoology by Kingsley, courtesy of Henry Holt and Co.) 
Following copulation, sperm cells are frequently stored within 
the body of the female in a receptaculum seminis from which they 
escape to fertilize the eggs which are produced over a considerable 
period of time. The vitellaria are also accessory glands of the 
female. They furnish nutritive materials and in some instances 
supply the substances of which the egg shell is formed. In many 
animals, there are no separate vitellaria and frequently in these 
only a portion of the ovarian cells become functional ova while 
others act as nurse cells or follicle cells which supply the ova 
with additional reserve food material. INTRODUCTION TO THE METAZOA 
73 
Copulatory organs are of common occurrence. An intromit- 
tent organ known as the cirrus is frequently characteristic of the 
male. This may be a modification of the terminal portion of the 
vas deferens or, as is often the case, may be purely accessory 
structures such as spicules or appendages modified to aid in the 
transfer of sperm. Clasping organs are especially characteristic 
of the males of some arthropods and in the same group ovipositors 
are frequent accessories of the female system. 
Locomotor System.-Numerous specialized structures for 
locomotion are encountered in the single-celled organisms. 
Cilia, flagella, pseudopodia, and myonemes are characteristic 
partial differentiations of cells for movement. The first two of 
these especially are carried over into the Metazoa as modifications 
of epithelial cells for locomotion or the production of movement. 
Many larval Metazoa perform locomotion by means of cilia exclu- 
sively but the flatworms and the rotifers represent the highest 
adults in which locomotion results from ciliary action. In the 
Metazoa, movements and locomotion more frequently result 
from the activity of contractile fibers which represent different 
degrees of specialization or differentiation of muscle cells. In 
some instances, there are scattered contractile elements but more 
commonly the contractile elements are associated to form 
continuous sheets or bundles of muscles. 
The dermomuscular sac of flatworms represents a definite 
stage in the development of a locomotor system. Alternate 
contraction and relaxation of the longitudinally and circularly 
directed fibers in such a sac produce successive shortenings and 
elongations of the body. As a consequence locomotion results. 
When hard skeletal parts make their appearance, groups of mus- 
cle cells become attached to these hard parts and with fixed points 
of attachment perform more effective movements. Even the 
setae of an earthworm serve for muscle attachment and aid in 
locomotion, thereby suggesting an extremely early stage in the 
differentiation of a locomotor skeleton. The exoskeleton of the 
arthropods is the most highly organized locomotor skeleton 
found in the invertebrates. 
Nervous and Sensory System.-Histological elements differ- 
entiated for transmission of nervous impulses assume sundry 
forms of organization in the metazoan body. The hydroid or 
polyp stage of the coelenterates is usually provided with a 
scattered or diffuse arrangement of nerve cells but in most of the 74 
INVERTEBRATE ZOOLOGY 
Metazoa other than the sponges, definite nerve tracts are found. 
In these tracts, occur not only the cell bodies of the neurons and 
cytoplasmic fibers with their specializations as already described 
but also supporting elements of which the neuroglia cells are the 
most important. Usually, there is a tendency for the cell bodies 
of the neurons to become massed in certain parts of the nerve 
tracts. These regions are designated as ganglia. 
The nervous systems of medusae are interesting in that they 
exhibit a series of steps toward the centralization of the nervous 
system. In the hydromedusae, a nerve ring without any differ- 
entiation of ganglia parallels the margin of the bell. In the 
scyphomedusae, the nerve ring has become more specialized and 
shows localization of the cell bodies to form ganglia. 
Among metameric animals, there is a tendency for ganglion 
formation in each somite but this primitive scheme is in many 
instances altered by the fusion of two or more successive ganglia 
to form a single mass. This tendency is especially pronounced in 
the anterior region of the body where a single prominent ganglion, 
the brain, is located. 
In many of the worms, two or more lateral nerve cords pass 
through the body while in some of the annelids and in the arthro- 
pods these have been fused to form a single chain of ganglia 
ventral to the digestive tract. An examination of a cross-section 
of such a nerve cord, however, usually furnishes evidences of its 
double nature. 
That portion of the nervous system which contains the ganglia 
and consequently is the controlling center for directing the activ- 
ity of the organism, is designated as the central nervous system 
while from it fibers and bundles of fibers pass to the various parts 
of the body as a peripheral nervous system. Largely through the 
latter are the sense organs brought into relationship with the cen- 
tral nervous system. A sympathetic system in some of the higher 
arthropods controls the activity of some of the internal organs. 
As has been pointed out, there are numerous modifications of 
epithelial tissues for receiving stimuli and transmitting nervous 
impulses to the central nervous system of the invertebrates. 
Concerning many of these sensory organs our knowledge is but 
fragmentary for they are organs which have no counterpart in 
the human body and consequently direct knowledge of their 
functions is beyond the scope of human experience. In some 
instances, their functions may be inferred from the location and INTRODUCTION TO THE METAZOA 
75 
the mechanics of their structure and through the reactions of the 
organism when the organ in question is stimulated. 
Tactile organs are very commonly developed as hairs or bristles 
from the surfaces of epithelial cells or as in the case of bristles of 
arthropods they may be formed as outgrowths of specialized cells. 
Tactile organs may be scattered fairly uniformly over the 
surface of the animal but sometimes specific organs are especially 
adapted for receiving tactile stimuli, as, for example, the tentacles 
of coelenterates, worms and snails, and the antennae and palpi 
of insects and crustaceans. In many such instances, other sen- 
sory organs are associated with those of touch. Thus the 
olfactory organs of insects are commonly found on the antennae 
along with other sensory organs of uncertain function. In some 
instances, the antennae are provided with hairs which seem to be 
associated with an auditory function. 
Both taste and olfactory organs of the invertebrates are rather 
simple in their organization. They are especially developed in 
the insects where they frequently occur as a sac or depression 
within which there is a bristle or cone-like projection. 
Organs of sight, hearing, and balance are frequently much 
more highly organized than those organs previously mentioned. 
The association of pigment with sensory epithelial cells frequently 
denotes the presence of a retina or some sort of a structure 
for reception of light stimuli. Yet there are some organisms, 
such as the earthworms, which respond to light stimulation with- 
out having any specialized sense organs for the reception of light 
stimuli. Pigment spots near the bases of the tentacles in many 
jellyfishes are usually thought to have a light percipient function 
and in some instances even have lens-like bodies associated with 
them. 
In the turbellarians, there are eyes in which the sensory 
epithelium forming the retina has undergone considerable speciali- 
zation and in many of the marine annelids highly complicated 
optic organs are found. The highest development of an inverte- 
brate eye is found in cephalopods, the eyes of which very closely 
resemble those of vertebrates, though the two seem to have 
no phylogenetic relationship but apparently have arisen 
independently of each other. 
Compound eyes are highly characteristic of arthropods. A 
compound eye is composed of an aggregation of similar elements 
called ommatidia the number of which is indicated by the facets 76 
INVERTEBRATE ZOOLOGY 
or rounded or hexagonal markings on the surface of the eye. 
Each ommatidium is bounded externally by a biconvex cornea 
beneath which is located a fluid or a solid cone-like lens. 
Beneath this are one or more chitinous rods termed the rhabdomes. 
Pigment cells surround each ommatidium and nerve fibers pass 
off from the base of the rhabdomes. 
The term ocellus is frequently taken as synonymous 
with a simple eye but from 
point of view of structure there 
are two very distinctly different 
types of ocelli. Those of one 
type, exemplified by the median 
ocelli of insects (Fig. 48), are 
but little less complicated than 
the compound eyes while the 
lateral ocelli are much simpler. 
The simple, unpaired median 
eye of the lower crustaceans 
shows many features in com- 
mon with the eyes of the 
flat worms. 
Fig. 48.-Median ocellus of Acilius. 
(After Patten). 
There are numerous modifica- 
tions for auditory and balancing 
functions in invertebrates but most of these are recognizable as 
some sort of an otocyst or statocyst (Fig. 39), which depends upon 
the movement of a free or suspended body, the otolith or statolith, 
against the walls of the vesicle for stimulating the central nervous 
system and causing the organism to right itself. In some of the 
insects is found a highly specialized auditory organ in which a 
tympanum receives sound waves and transmits the stimulus 
through an underlying space to the nerve endings in the sensory 
epithelium. 
References 
Dahlgren, U. and Kepner, W. A. 1908. A text-book of the principles 
of animal histology. New York, Macmillan. 
Kellicott, W. E. 1913. A textbook of general embryology. New York, 
Holt. 
Korschelt, E. and Heider, K. 1895-1900. (English translation) Text- 
book of the embryology of invertebrates. London, Sonnenschein. 
McBride, E. W. 1914. Text-book of Embryology, Vol. 1. Invertebrata. 
London, Macmillan. 
Schneider, K. C. 1902. Lehrbuch der Vergleichenden Histologie der 
Tiere. Jena, Gustav Fischer. CHAPTER IV 
PHYLUM PORIFERA 
The word sponge usually calls to mind the sponges of commerce 
which are only one type of the skeletal remains from animals 
belonging to the phylum Porifera. A mass of parenchymatous 
tissue penetrated by numerous pores covers this framework in 
the living animal. Sponges vary so much in shape that little can 
be said regarding their form. Without exception, they are 
attached to some object and have lost powers of free locomotion. 
Cell Layers.-The bulk of the body is composed of mesoderm, 
the outermost layer of which is modified as a covering layer to 
take the place of the original ectoderm which is usually lost during 
development. The combined covering layer and mass of connec- 
tive tissue are frequently called the mesectoderm. Certain inter- 
nal cavities of the sponges are lined with entoderm, part of the 
cells of which are provided with flagella. Each flagellum is sur- 
rounded near its base by a prominent cup-like structure called 
the collar. These collared flagellate cells very closely resemble 
the cells of the Choanoflagellates. Upon the basis of the arrange- 
ment of the entoderm cells is based a system of classification of 
morphological types of sponge structure. 
Morphological Types.-Three types of sponge structure are 
usually recognized (Fig. 49). In the simplest sponges, of the 
Ascon type (A), the body is a thin-walled sac having a single 
large opening, the osculum, at one extremity and numerous 
minute incurrent pores through the body wall communicating 
with a central cavity. This central cavity, which is frequently 
called a stomach, is lined with entoderm cells, but since digestion 
is intracellular the cavity is not a true digestive cavity and there- 
fore not a stomach. Opening and closing of the pores leading 
into this flagellated chamber are under control of the animal. 
When the pores are open the action of the flagella causes water 
currents bearing food material to pass through the flagellated 
chamber and out through the osculum. During this passage, food 
is removed from the water and ingested by the entoderm cells. 
77 78 
INVERTEBRATE ZOOLOGY 
Leucosolenia and Olynthus are examples of this simplest type of 
sponge organization. 
The Sycon (Fig. 49, B) differs from the foregoing chiefly in the 
fact that the central cavity is lined with pavement epithelium 
and the entoderm cells have withdrawn to small radial chambers 
or ampullae embedded in the thickened wall. The central 
cavity is now a distinct cloaca with which each ampulla communi- 
cates by means of a small opening called the apopyle. As the 
name implies, the radial chambers are arranged radially about 
the cloaca. Alternating with the radial chambers are the incur- 
rent canals which pass from the exterior inward toward but not 
into the cloaca. Minute openings, the prosopyles, communi- 
cate between the incurrent canals and the adjacent ampullae 
thus allowing water currents bearing food material to pass 
Fig. 49.-Morphological types of sponges. A, Ascon type; B, Sycon type; C< 
Leucon type. (From Korschelt and Heider'). 
through the radial chambers, into the cloaca and out through the 
osculum. Grantia and Sycon are two typical genera whose 
representatives are built upon the Sycon plan. 
As they acquire greater bulk, some sponges reach a size 
wherein direct communication between the ampullae and the exte- 
rior and cloacal surfaces is no longer possible. Intricately 
branched incurrent and excurrent canals establish these surface 
relations for the deeply embedded ampullae (Fig. 49, C). Sponges 
of this character are of the Leucon type and are typified by such 
genera as Leucilia and Oscarella. 
Asexual reproduction through budding is common in sponges. 
By this means masses of individuals are produced thereby form- PHYLUM PORIFERA 
79 
ing colonies. These colonies may be either groups of distinctly 
separable individuals with only a common region near the base 
or they may be so intimately fused as to render the recognition 
of individuals impossible. Sponges 
possess great power of regeneration. 
In the commercial sponge fisheries, ad- 
vantage is taken of this fact and small 
fragments are "planted" and allowed 
to grow before harvesting. Gemmule 
formation is another type of repro- 
duction found in fresh-water sponges. 
Groups of cells called gemmules or 
internal buds become separated from 
the surrounding tissues by confining 
membranes. These gemmules are 
capable of withstanding desiccation 
and other adverse circumstances which 
are fatal to the living sponge and are 
therefore of great importance in the 
life cycle of sponges. 
Sexual Reproduction.-In regard to the sexual organs, sponges 
are usually hermaphroditic, but since in any individual the germ 
cells of the male usually mature before those of the female they 
Fig. 50.-Embryo of Gran- 
tia. Blastula stage within 
embryonic chamber of parent 
individual. (After Dendy). 
Fig. 51.-Skeletal structures of sponges {after Schulze and Maas). 1, spongin 
fiber surrounded by spongioblasts; 2-7, different types of spicules. (From 
Hertwig's Manual of Zoology by Kingsley, courtesy of Henry Holt and Co.) 
are said to be protandrous. Eggs are fertilized within the parent 
and there undergo segmentation (Fig. 50). Upon reaching the 
blastula stage, they are liberated as free-swimming larvae. 
Following gastrulation, the larva becomes attached to some 80 
INVERTEBRATE ZOOLOGY 
object, the blastopore closes and the osculum, as a new opening, 
breaks through the body wall at the opposite pole. 
Skeleton.-The skeletal structures (Fig. 51) which give the 
chief character to the Porifera are spicules formed by mesoderm 
cells called scleroblasts. In the classification of sponges, the 
nature and form of the skeletal structures are of great importance. 
Spicules may be formed of either calcareous or silicious material 
in a great variety of patterns. A network of a substance called 
spongin (Fig. 51, 1) is another type of supporting structure 
formed by cells called spongioblasts. This spongin, which is 
closely allied to silk in its chemical composition, may either 
appear alone or along with silicious spicules. 
Class Calcarea 
The calcareous sponges are so called because they have 
supporting skeletons of calcareous spicules usually tri-radiate in 
form. They are all shallow-water marine forms. All three types 
of sponge structure discussed earlier in this chapter occur in 
members of this class. The subclass Homocoela, of which the 
genus Leucosolenia is an example, includes forms of the Ascon 
type, while the subclass Heterocoela includes both Sycon type 
(genus Grantia) and Leucon type (genus Leucortis). 
Class Hexactinellida 
Sponges of this class are chiefly deep sea forms with silicious 
spicules, each with three rectangular axes. Euplectella, the 
Venus' flower basket and Hyalonema, the glass rope sponge are 
examples of this class. 
Class Demospongia 
In this class, are included three different conditions of skeletal 
structure. Some (subclass Myxospongia) lack skeletal structures 
entirely; others (subclass Ceratosa) have spongin fiber framework 
only; and still others (subclasses Monaxonida and Tetraxonida) 
have a framework made up of a combination of spongin fibers 
and silicious spicules. The Spongillidae, which include the 
only fresh-water sponges, belong to the Monaxonida. Spongilla, 
Ephydatia, and Carterius are common genera of wide distri- 
bution but frequently overlooked by collectors. PHYLUM PORI PERA 
81 
References 
(See references cited at close of Chapter I) 
Dendy, A. 1893. Observations on the Structure and Classification of the 
Calcarea heterocoela. Quart. Jour. Mier. Sci., 35: 159-257. 
Haeckel, E. 1872. Die Kalkschwamme, eine Monographie. Berlin. 
Moore, H. F. 1908. The Commercial Sponges and the Sponge Fisheries. 
Bull. U. S. Bur. Fish., 28: 399-511. 
Parker, G. H. 1910. The Reactions of Sponges, with a Consideration 
of the Origin of the Nervous System. Jour. Exp. Zool., 8: 1-41. 
Wilson, H. V. 1907. On Some Phenomena of Coalescence and Regener- 
ation in Sponges. Jour. Exp. Zool., 5: 245-258. 
. 1910. Development of Sponges from Dissociated Tissue Cells. 
Bull. U. S. Bur. Fish.,30: 1-30. CHAPTER V 
THE COELENTERATES AND CTENOPHORES 
A. PHYLUM COELENTERATA 
Coelenterates are radially symmetrical animals but slightly 
modified in fundamental structure from the typical gastrula stage 
which occurs in the development of all higher Metazoa. Because 
of their form, Cuvier included them along with the echinoderms 
in his type Radiata. Extreme differences in structure preclude 
the possibility of uniting these two groups on the basis of super- 
ficial agreement in disposition of their parts. The echinoderms 
have a true body cavity or coelom in addition to the digestive 
cavity. In the coelenterates, however, only one cavity is 
found. Phylogenetically, this single cavity represents the gas- 
trula cavity from which in higher forms of life both digestive 
cavity and body cavity arise during later development. Since, 
in the coelenterates, these two cavities are not differentiated, 
Leuckart suggested the name Coelenterata which they now bear, 
carrying with it the idea of lack of specialization of "coelom" 
from the "enteron." 
Morphological Types.-Body form among the coelenterates is 
usually referable to one of two types, the hydroid or polyp form and 
the mcdusoid or jellyfish form. While these two forms are clearly 
differentiated one from the other, in fundamental structure they 
are essentially alike. In each, tentacles are usually present and 
function in grasping food and bringing it into the mouth. These 
tentacles, and in some instances the body also, are supplied with 
nettling cells which are tubular threads coiled within a small 
bladder-like structure. As a means of defence or of offence, these 
threads are discharged from the nematocysts with a force suffi- 
cient to carry them even through the chitinous body covering of 
small crustaceans introducing into the wound an irritating fluid. 
By their use, some coelenterates, the Portuguese man-of-war for 
instance, are capable of inflicting painful injury even to man. 
Cell Layers.-The body wall is composed of an external layer 
of cells called the ectoderm and an internal layer lining the 
82 THE COELENTERATES AND CTENOPHORES 
83 
gastrovascular cavity called the entoderm. Between these lies 
a non-cellular, gelatinous substance termed the mesoglea. The 
extent and importance of this mesoglea in the various coelen- 
terates varies extremely. In some instances it is a very delicate, 
inconspicuous layer (hydro- and scyphopolyps), while in others 
(medusae) it has become the most conspicuous as well as the 
most bulky part of the whole organism. Structures having 
their origin in either ectoderm or entoderm tend to pass into 
the mesoglea, thus rendering it more highly complicated and 
causing it to partake of the nature of a definite mesoderm. The 
extreme of this tendency is found in the Ctenophora, a group 
which is frequently included as a class of the Coelenterata but 
here recognized as an independent phylum. 
Modifications of the Digestive System.-In its most primitive 
condition, the coelenteric or gastrovascular cavity of coelenterates 
is a simple bag with a single opening, the mouth. This is the 
condition in the polyp of the Hydrozoa. In the medusae, there 
are frequently diverticula from the central chamber of the 
digestive system which provide greater space for the digestion 
of food and serve for the delivery of digested food material to the 
more distant parts of the body. In many instances, these diver- 
ticula have the form of definite vessels, called the radial canals, 
which may be united at their distal ends by a common vessel 
termed the circumferential canal. Further complication of the 
digestive system is found in the Anthozoa wherein a definite 
infolding of ectoderm projects for some distance from the mouth 
opening into the digestive chamber as an esophagus. In this 
same group, the cavity becomes divided by a series of partitions 
called mesenteries or septa which provide additional surface for 
the processes of digestion and assimilation of the food material. 
Metagenesis.-Both types of individuals described above 
occur in the course of the life cycle of the Hydrozoa and the 
Scyphozoa. An asexual hydroid generation gives rise to a sexual 
medusoid generation. This condition of direct alternation 
between two generations of different type has been termed meta- 
genesis. The occurrence of hydroid and medusoid forms in 
the developmental cycle of a single species has not been under- 
stood long. It is, then, not surprising that all of the earlier 
treatises considered the hydroid and medusoid generations, 
even of the same species, as belonging to distinct and independent 
groups of the animal kingdom. With disclosure of the fact that 84 
INVERTEBRATE ZOOLOGY 
campanularian hydroids give rise to Leptomedusae in the course 
of their reproductive cycle, it became necessary to join the two 
groups, and in the face of the lack of a more appropriate name the 
combined groups have been given the compound name Campanu- 
lariae-Leptomedusae. Other groups have been united in this 
same manner following the discovery of their true relationships. 
In degree of differentiation, the medusoid represents much 
the higher type. Structures frequently either wanting or of 
low organization in the hydroid individuals are well represented 
in the medusae even of the same species. Muscle occurs as a 
partial differentiation of epithelial cells in hydroids of the Hydro- 
zoa and Scyphozoa but sheets and bundles of muscle tissue are 
recognizable in the medusae. Specialized sensory organs are 
almost exclusively associated with the more distinctly central- 
ized nervous system of the medusoid forms. In these, organs 
of equilibrium appear in extremely diverse stages of develop- 
ment, representing conditions varying from simple, exposed, 
modified tentacles or sensory clubs to the highly complicated 
statocysts with accessory protective vesicles (Fig. 39). 
Habitat.-In habits, the coelenterates are chiefly marine, 
though Hydra, Protohydra, Cordylophora, and a few rare 
medusae occur in fresh-water habitats. 
Class Hydrozoa 
The Hydrozoa are coelenterates usually having an alternation 
of generations of which the hydroid is frequently the more 
conspicuous. The hydropolyp lacks longitudinal folds of the 
entoderm and the mouth opens directly into the coelenteric 
cavity. The hydromedusa, when present, has a smooth bell 
margin; the concave surface is partially enclosed by a membrane, 
the velum, which extends inward from the edge of the bell. 
The gonads are ectodermal. 
The Gonophore and Its Reduction.-An individual among the 
coelenterates which bears the gonads is termed a gonophore. 
Ordinarily, this is a free-living medusoid individual (Fig. 52, A) 
with gonads either on the manubrium or on the radial canals of 
the gastrovascular system. In some representatives of the class 
Hydrozoa, the medusoid generation is lacking. This condition 
is usually the result of a process which is designated as gonophore 
reduction. Usually, the gonophores have their origin as products 
of asexual reproduction from the hydroid individuals. Under THE COELENTERATES AND CTENOPHORES 
85 
ordinary circumstances, they undergo development to a certain 
stage while still attached to the hydroid and are then liberated. 
In some instances, however, freedom is never gained, then the 
gonophores remain permanently attached to the parent hydroid 
(Fig. 52, B-D) and organs essential to the independent existence 
of the gonophore fail to develop to a functional stage or degen- 
erate entirely. By the selection of different examples, a finely 
graded series of steps in the degeneration of the medusoid may 
be secured. A greatly reduced gonophore comprises little more 
than an ectodermal bag within which the germ cells are arranged 
about a core of entoderm called the spadix. The spadix represents 
the remains of a degenerated manubrium. The term sporosac is 
Fig. 52.-Gonophore reduction. A, free gonophore, an independent medusa; 
B, a medusa which remains permanently attached to hydroid colony and fails 
to develop either tentacles or mouth; C, still further reduction of gonophore to a 
simple manubrium (the spadix) surrounded by gonads; D, gonads produced 
around a slight entodermal rudiment of the manubrium as in Hydra. (After 
Kingsley). 
applied to such a reduced gonophore (Fig. 52, C) attached to the 
body of a hydroid individual. The ultimate in gonophore 
suppression is found in members of the genus Hydra, wherein 
the gonads occur in the ectoderm with practically no remnant 
of spadix or other medusoid remains (Fig. 52, Z>). 
Suppression of the Hydroid.-In contrast with the gonophore 
reduction discussed above, stands the suppression of the hydroid 
generation characteristic of representatives of the order Trachy- 
linae. In these, development of the medusa proceeds directly 
from the larva derived from the fertilized egg without the inter- 
vention of the polyp generation. 
Histological Differentiation.-As was stated in the introduc- 
tory discussion of the characters of the phylum, the medusa 86 
INVERTEBRATE ZOOLOGY 
represents a higher type of differentiation than is found in the 
hydroid polyp. Scattered nervous elements and sensory cells 
characteristic of the polyp are replaced by a centralized nerve 
ring with which statocysts and light-percipient organs are asso- 
ciated in the hydromedusa. Hertwig has called attention to 
the fact that no less than eight distinct lines of differentiation 
occur in the ectoderm cells of the hydromedusa. There are 
myoblasts, nerve cells, indifferent cells, sensory cells, cnidoblasts, 
gland cells, pigment cells, and germ cells. Since a number of 
these are represented by several distinct types of histological 
differentiation, as for example the several distinct types of sensory 
and gland cells, these simple diploblastic animals become more 
highly organized than is ordinarily understood. 
An investing membrane surrounds some hydroid forms (Obelia) 
but is wanting in others (Hydra, Tubularia). This investing 
membrane may be either a delicate perisarc covering all or only 
part of the zooid or in some instances (Hydrocorallina) part of a 
massive calcareous skeleton. The expansion of perisarc around 
a vegetative zooid is termed a hydrotheca while that around a 
reproductive zooid is termed a gonotheca. 
I. Order Leptolinae 
The order Leptolinae includes those Hydrozoa which have a 
fixed hydroid stage. Within this group two suborders are recog- 
nized on the basis of the presence or lack of a hydrotheca surround- 
ing the vegetative zooids and upon the location of the gonads 
when a medusoid stage is retained. The Tubulariae-Anthome- 
dusae have zooids without thecae. When medusae occur they are 
without statocysts and bear the gonads on the manubrium. The 
hydroid stages have commonly gone under the name of the Tubu- 
larian hydroids and the medusae have been known as the Antho- 
medusae. Members of the genera Hydra, Clava, Cordylophora, 
Coryne, and Tubularia lack a free-swimming medusa, and repre- 
sent varying degrees of gonophore suppression. The suborder 
Campanulariae-Leptomedusae includes the Campanularian 
hydroids, forms which have a perisarc surrounding the zooids, and 
the Leptomedusae which bear the gonads on the radial canals. 
Typically, an exact alternation of generations, termed metagene- 
sis, occurs in members of this suborder. 
Life History.-The life cycle of Obelia serves well to illustrate 
metagenesis. The hydroid colony starts as a single polyp which, THE COELENTERATES AND CTENOPHORES 
87 
by continued budding, produces a series of individuals alternating 
along a common stalk. Such a colony is attached to some object 
by means of a root-like structure termed the hydrorhiza. In a 
young colony, all of the individuals are alike and all function as 
vegetative zooids. Starting near the base of the colony, reproduc- 
tive individuals begin to make their appearance. These are 
sac- or vase-shaped zooids, termed gonangia, budded off in the 
axils between the vegetative individuals and the common stalk. 
Each gonangium is composed of a central core upon which buds 
are formed. As development proceeds these buds display medu- 
san characters and upon reaching full development become 
detached and emerge from the open end of the gonotheca as free- 
swimming jellyfish. The medusae produce germ cells which, 
upon fertilization, undergo cleavage to form a ciliated larva, 
termed a planula, but little higher in organization than the 
gastrula. After a short period of free existence the planula 
becomes attached to some object and transforms into a hydroid 
through the development of a circle of tentacles and of a hydro- 
rhiza. From this single zooid, an entire colony is produced 
through repeated budding. 
II. Order Trachylinae 
The order Trachylinae includes two suborders of hydro- 
medusae which develop directly from the egg without passing 
through a polyp stage in the life cycle. In the suborder Trachy- 
medusae, tentacles arise from the margin of the bell and gonads 
are borne on the radial canals. The suborder Narcomedusae 
includes hydromedusae which bear the gonads on the manubrium 
and have tentacles arising from the exumbrella. Trachynema, 
Liriope, and Campanella are examples of the Trachymedusae; 
Cunina and Cunocantha of the Narcomedusae. 
III. Order Hydrocorallina 
Members of the order Hydrocorallina resemble the corals in 
the production of massive calcareous skeletons. Two kinds of 
zooids are lodged in calyces of the skeletal structure. Gastro- 
zooids bear a mouth and tentacles. Distally the cavity of the 
gastrozooids communicates with a series of branching coeno- 
sarcal canals which ramify through the colony. Either irregu- 
larly scattered or arranged about the gastrozooids occur the 88 
INVERTEBRATE ZOOLOGY 
dactylozooids which lack a mouth and usually have knobbed 
tentacles supplied with nettling cells. Gonophores develop on 
the coenosarcal canals as rudimentary medusae. Millepora, the 
staghorn coral, and Stylaster of the tropical seas, are examples 
of this order. 
IV. Order Siphonophora 
Extreme polymorphism is found in this pelagic order of Hydro- 
zoa. The individuals of a colony are united by a common tube 
of coenosarc at one end of which usually 
occurs an air-filled bladder-like float 
called the pneumatophore (Fig. 53, A). 
There is a division of opinion as to 
medusan or polyp relations of the 
individuals of the siphonophore colony. 
Most of the individuals are so highly 
specialized that they perform but 
limited functions and other rudimentary 
organs that would give a clue to the 
type of general structure are wanting. 
Nectocalyces or swimming bells (B) 
frequently occur just below the float. 
The rest of the colony is composed of 
assemblages of feeding zooids (H), 
dactylozooids (F and (7), sporosacs (E), 
and bracts or protective zooids (Z>). 
Polymorphism is so great and the 
specialization of individuals so complete 
that the entire colony gives the appear- 
ance of a single individual with its organ 
systems dissected apart one from 
another. In the Portuguese man-of- 
war (Physalia) the float is greatly enlarged and the swimming 
individuals are lacking. 
F i g . 53 .-Diagram of a 
siphonophore j colony (Phy- 
sophorida). A, pneumato- 
phore; B, C, swimming bells; 
D, protective zooid; E, sporo- 
sac; F, G, dactylozooids; H, 
feeding polyp; I, nettling 
cells. (After Claus'). 
Class Scyphozoa 
The Scyphozoa are coelenterates having an alternation of 
generations of which the medusoid is the more conspicuous. The 
scyphopolyp or scyphostoma when present is distinguishable 
from the hydropolyp through the presence of four longitudinal 
folds of the entoderm called the taeniolae. In the development THE COELENTERATES AND CTENOPHORES 
89 
of the medusae, these form the gastral tentacles or phacellae. 
The scyphomedusa lacks a velum, has a more or less notched 
margin, and bears entodermal gonads. Form and structure of 
the medusa are more readily understood when the development 
has been outlined. 
Life History.-The medusae of Aurelia produce germ cells, 
which, upon fertilization, undergo cleavage and develop into a 
ciliated larva called a planula. After a brief free existence, the 
planula becomes attached to some object and transforms into an 
individual superficially like a Hydra, to which the names scypho- 
stoma (Fig. 54, A) or scyphopolyp have been applied. By a series 
of transverse constrictions, the scyphostoma becomes separated 
into a pile of saucer-like structures (Fig. 54, B) and in this stage is 
Fig. 54.-Larval stages in the development of Aurelia. A, scyphostoma; B 
strobila; C, ephyra. (Orig.)i 
called a strobila. As the constrictions become deeper a series of 
ephyrae become detached from the strobila. At the time of their 
liberation, each ephyra (Fig. 54, C) has eight prominent arm-like 
projections arranged radially about the margin of the body. 
Growth of the medusa from this ephyra involves a rather con- 
spicuous metamorphosis. The eight arms mentioned above mark 
off the eight radii of the mature jellyfish. The four radii passing 
through the angles of the mouth are known as the perradii, the 
other four as the interradii. Axes falling between the bases of 
the eight arms of the ephyra are designated as the adradii. By 
more rapid growth of the adradial zones the notches of these 
regions become filled up, thus giving the young jellyfish a slightly 
crenated circular outline. The marginal tentacles make their 
appearance in the adradial regions. 90 
INVERTEBRATE ZOOLOGY 
I. Order Stauromedusae 
The Stauromedusae are conical or vase-shaped medusae 
which usually lack marginal sense bodies though in some 
(Halycystus) there are modified tentacles which seem to serve as 
special sense organs. In Halycystus and Lucernaria, the margin 
of the umbrella is drawn out into eight arms at the tip of each of 
which there is a bundle of small tentacles. In these same two 
genera, the aboral surface is drawn out into a peduncle by means 
of which the medusae become attached to objects. Representa- 
tives of this order belonging to the family Tesseridae are free- 
swimming. 
II. Order Peromedusae 
The Peromedusae are free-swimming, with a cup-shaped or 
conical body carrying four interradial sense bodies. Pericolpa 
and Periphylla occur in the open seas. 
III. Order Cubomedusae 
As the name implies, the Cubomedusae are more or less cubical 
in form. They bear four perradial sense bodies. Members of 
this order, of which the genus Charybdea is an example, are 
chiefly tropical. 
IV. Order Discomedusae 
The Discomedusae are by far the most abundant and most 
widely distributed of the Scyphozoa. The medusa is usually 
the shape of an inverted saucer. The margin of the bell is 
furnished with eight or more otocysts. Members of this order 
range from a few inches to seven feet or more (Cyanea arctica) 
across the bell, while the tentacles may be more than a hundred 
feet in length. The eight-rayed symmetry characteristic of so 
many forms is carried over directly from the eight-rayed ephyra. 
Two suborders are recognized, chiefly on the nature of the manu- 
brium. Members of the suborder Semostomae have four large 
perradial arms surrounding the centrally located mouth. In 
the suborder Rhizostomae there are eight large adradial oral arms. 
Grooves formed by folds of these arms open as numerous suctorial 
mouths instead of a single mouth opening into the gastrovascular 
cavity. Tentacles on the margin of the bell are lacking in this 
suborder. THE COELENTERATES AND CTENOPHORES 
91 
Class Anthozoa 
All organisms belonging to this class conform to a polyp form 
of organization. The anthozoan polyp differs fundamentally 
from the characteristic hydrozoan polyp in the presence of an 
ectodermal esophagus and of longitudinal partitions called 
septa or mesenteries (Fig. 55) partially dividing the gastrovascular 
cavity. Well developed bands of muscles are found in connec- 
tion with the septa. The mesoglea contains numerous cells, 
thus having the appearance of a simple connective tissue and 
supplying firmness and a fleshy consistency to the body in many 
forms. Both solitary and colonial forms occur in this exclusively 
marine group of almost universally sessile organisms. Examples 
of the solitary forms are the sea anemones and of the colonial 
forms are the corals, sea pens, and sea pansies. 
Fig. 55.-Section of a young sea anemone, ss, sagittal plane; tt, transverse 
axis; I, II, III, septa of first, second and third orders; ek, ectoderm; en, ento- 
derm; f, mesenterial filament; m, muscle; r, directive septa. (After Boveri, from 
Hertwig's Manual by Kingsley, courtesy of Henry Holt and Co.) 
The mouth is usually oval or slit-like and leads through an 
inturned tube of ectoderm, called the esophagus, into the gastro- 
vascular cavity. Like the mouth, this esophagus is usually 
compressed so that a sagittal axis is recognizable marking off a 
biradial symmetry. In some instances, one or two grooves run 
the length of the esophagus at the poles of the sagittal axis. 
These grooves (x, Figs. 56, 57) are termed the siphonoglyphes. 
The esophagus extends only part way from the oral toward 
the pedal surface. It is held in position by the primary septa 92 
INVERTEBRATE ZOOLOGY 
(Fig. 55, I), folds of ectoderm and mesoglea which extend from 
the body wall inward to meet the esophagus and thus at the 
oral extremity completely divide the gastrovascular cavity into 
a number of pockets or chambers. Since each chamber con- 
tinues beyond the body proper into a tentacle, there is usually a 
direct relation between the number of chambers and the number 
of tentacles. In addition to the primary septa, many others 
reach only part way toward the esophagus. These are of varying 
lengths and on the basis of their length and order of development 
are termed secondary, tertiary, and so forth (Fig. 55, II, III). 
The number of primary mesenteries is of considerable importance 
in distinguishing the various orders of the Anthozoa. In the 
Alcyonaria (Octocoralla), there are eight septa. In practically all 
representatives of this order, the muscle ridges of all septa are 
directed toward the same pole of the sagittal axis. Edwardsia is 
an exception to this rule and marks a step in transition from the 
Alcyonarian to the Zoantharian (Hexacorallan) type for in 
Edwardsia the position of the muscles on one pair of septa at one 
pole of the sagittal axis is reversed. The Zoantharia usually 
have one pair of primary septa at each pole of the sagittal axis 
and two lateral pairs on each side of the body. The two pairs 
lying in the main axis (Fig. 55, r) are called the directives. The 
muscle ridges on these face outward while in all the remaining 
pairs the ridges of one pair face each other. In development, the 
primary septa are the first to appear. Between these, come the 
secondary and later the tertiary septa. With the increase in 
number of septa, there is usually a corresponding increase in 
number of tentacles but this correlation is not absolute. 
In addition to the muscles, the gonads, mesenterial filaments, 
and acontia are important structures associated with the septa. 
The germ cells, which have their origin in the ectoderm, come to 
lie in the mesoglea near the free margin of the septa. The 
ruffled free margin of each septum is edged with a thickened mass 
of epithelial cells called the mesenterial filament, thought to be of 
use in holding and compressing food particles, thereby aiding in 
digestion. Near the pedal disc, these mesenterial filaments are 
frequently modified to form long thread-like organs, the acontia, 
which are provided with numerous nettling cells. Through the 
mouth or through minute pores in the body wall, the acontia 
are thrust to the outside of the body where they serve as efficient 
defensive organs. THE COELENTERATES AND CTENOPHORES 
93 
Sexual development involves the formation of a planula 
through cleavage of the fertilized egg. The planula, after a 
brief free-swimming life, settles down and undergoes a transforma- 
tion to the polyp form. Budding is of common occur- 
rence and gives rise either to separate individuals or to 
colony formation. 
Most Anthozoa, with exception of the Actinaria, possess some 
sort of skeletal structures, either as solid deposits formed by the 
zooids or as spicules in the colony wall. These structures are of 
such diverse natures in the different groups that they cannot be 
described here. 
Fig. 56.-Transverse section of Zoantharian. AB, plane of symmetry; x, 
siphonoglyphe. (From Hertwig's Manual by Kingsley, courtesy of Henry 
Holt and Co.) 
Subclass ZOANTHARIA 
Sea anemones and corals stand as examples of the Zoantharia 
or Hexacoralla of some authors. These forms have simple tubu- 
lar tentacles and numerous paired mesenteries (Fig. 56) typically 
occurring in multiples of six. Except for the directives (men- 
tioned above) the septa are arranged in pairs with the muscle 
bands of each pair facing one another. Edwardsia is an excep- 
tion with its eight mesenteries and sixteen or more tentacles. 
Various other modifications of the rule occur in other representa- 
tives of this subclass. 94 
INVERTEBRATE ZOOLOGY 
1. Order Actinaria 
The order Actinaria includes the sea anemones which usually 
occur as solitary polyps, though some are colonial. Numerous 
tentacles and mesenteries are present but skeletal structures 
are wanting. Metridium is a common example as is also Sagartia. 
II. Order Madreporaria 
In arrangement of the parts of the body, the Madreporaria 
rather closely resemble the Actinaria but in habits they are 
colonial and are surrounded by a calcareous skeleton. Most of 
the stony corals, Astrangia, Coeloria, Favia, Madrepora, and 
Porites are examples of genera included here. A common coeno- 
sarc over the surface of the skeleton communicates between the 
individuals of a colony. 
III. Order Antipatharia 
These branching colonies resemble the Gorgonacea. Members 
of the colony are united by a hollow, branched, horny axis 
surrounded by an epithelial sheath which 
lacks spicules. The black corals Antipathes 
and Cirripathes are examples. 
Subclass Alcyonaria 
Eight feathered tentacles and eight single 
mesenteries (Fig. 57) characterize members 
of this subclass which are also frequently 
referred to under the name Octocoralla. 
Colonial forms, frequently with polymorph- 
ism of the individuals are common. Budding 
to form a colony is usually stoloniferous. 
I. Order Alcyonacea 
Members of this order usually have scattered calcareous 
spicules and lack an axial rod. Alcyonium is the typical genus. 
II. Order Gorgonacea 
The Gorgonacea are sessile forms with a horny or calcareous 
axial rod. The sea fans (Gorgonia) and other tree-like forms as 
well as the precious coral (Cor allium rubrum} belong to this order. 
Fig. 57.-T r a n s- 
verse section of an 
Alcyonarian. (From 
Hertwig's Manual by 
Kingsley, courtesy of 
Henry Holt and Co.) THE COELENTERATES AND CTENOPHORES 
95 
III. Order Pennatulacea 
Colonies with one extremity modified for burying in the sand 
or mud of the sea floor are included in this order. The remainder 
of the colony bears polyps which may be arranged over an 
expanded disc (Renilla) or on lateral branches like a feather 
(Pennatula). Dimorphism of individuals within a colony is 
common. 
B. PHYLUM CTENOPHORA 
The Ctenophora have very commonly been considered as a 
class of the Coelenterata. The absence of marginal tentacles, 
lack of nettling cells, centralization of the nervous mechanism, 
and transformation of the mesoglea into a mesoderm mark the 
Fig. 58.-A ctenophore, Pleurobrachia rhododactyla Agassiz. A, lateral view 
B, aboral view. (After L. Agassiz). 
chief arguments in favor of considering the ctenophores as an 
independent phylum. The most striking external feature is the 
presence on the surface of the body of eight meridional bands of 
swimming combs or plates (Fig. 58) each of which is composed 
of a linear series of short rows of cilia. Two tentacles arising in 
lateral ectodermal depressions or pockets are characteristic of 
most representatives of this phylum. At the aboral pole occurs a 
slight depression within which is located a single otocyst. In the 
transverse plane this depression is continued as two narrow 
ciliated areas called the polar plates. 
Body form is not constant throughout the group. Many indi- 
viduals are ovoid, others have a somewhat shortened dorsoventral 
axis and still others are drawn out into a narrow band. 96 
INVERTEBRATE ZOOLOGY 
The nettling cells characteristic of coelenterates are wanting in 
the ctenophores. In their place structures called adhesive cells 
occur upon the tentacles. 
The mouth leads into a tube called the stomach, but since it is 
ectodermal in origin it is more properly a stomodaeum. This 
stomach opens into an entodermal sac, the funnel. The stomach 
and funnel are both flattened with the broad axis of one at 
right angles with that of the other. From the funnel are given 
off laterally two perradial vessels each of which divides dichoto- 
mously to form the interradial vessels, four in number. By 
another dichotomous division these form the eight adradial ves- 
sels each of which communicates with one of the meridional 
vessels directly underlying the rows of combs. Each perradial 
vessel gives off a branch called the paragastric canal which runs 
parallel to the stomach and ends blindly. At its aboral 
extremity the funnel gives off a funnel vessel which, after forming 
two or four branches, proceeds to the aboral pole and there 
empties through two or more openings called the excretory pores. 
On the presence of or lack of tentacles, two classes are recog- 
nizable; the Tentaculata and the Nuda. Of these, the former 
includes three orders and the latter a single order, the Beroida. 
Members of the order Cydippida (Pleurobrachia, Hormiphora, 
etc.) have ovoid or pear shaped bodies bearing two tentacles 
arising from tentacular sacs or sheaths and into which they may 
be retracted. The Lobata includes those which have two large 
oral lobes and numerous minute lateral tentacles. In the young, 
two large tentacles are present but in later stages only their bases 
are present and these are without a sheath. Venus' Girdle 
(Cestus) is an example of the order Cestida members of which 
have the body much compressed in the vertical plane. Repre- 
sentatives of the order Beroida lack tentacles and the wide 
mouth and gullet occupy most of the animal. 
References 
(See general -references at close of Chapter 1) 
Agassiz, A. 1865. North American Acalephae. lllustr. Cat. Mus. 
Comp. ZooL, Harvard, 2. 
Agassiz, L. 1850. Contributions to the Natural History of the Acalephae 
of North America. Mem. Amer. Acad. Arts and Sci., Boston, 4: 
221-374. 
-. 1860-1862. Contributions to the Natural History of the 
United States. Boston. THE COELENTERATES AND CTENOPHORES 
97 
Chun, K. 1880. Die Ctenophoren des Golfes von Neapel und der angren- 
zenden Meeres-Abschnitte. Fauna und Flora des Golfes von Neapel, 1: 
313 pp. 
Haeckel, E. 1879-1880. System der Medusen. I. Craspedoten, II. 
Acraspeda. Jena. 
Mayer, A. G. 1910. Medusae of the World. Carnegie Inst., Wash- 
ington, Publ. 109. 
. 1912. Ctenophores of the Atlantic Coast of North America. 
Carnegie Inst., Washington, Publ. 162, 
McMurrich, J. P. 1891. Contributions on the Morphology of the 
Actinozoa, III. The Phylogeny of the Actinozoa, Jour. Morph., 5: 
125-164. 
Nutting, C. C. 1901. The Hydroids of the Woods Hole Region. Bull. 
U. S. Fish Comm, for 1899: 325-386. CHAPTER VI 
PHYLUM PLATHELMINTHES 
The Plathelminthes comprise a phylum of worm-like animals 
with bodies usually flattened, devoid of body cavity, and lacking 
true segmentation. The bulk of the body is composed of a mass 
of connective tissue termed parenchyma within which the various 
organs are embedded. Externally, this is covered by either a 
ciliated epithelium or a non-cellular cuticula. A body muscula- 
ture is present, the fibers of which are usually circular, longitu- 
dinal, and diagonal in arrangement. Frequently, in addition, 
fibers are also found penetrating the body from dorsal to ventral 
surfaces. The nervous system usually consists of two or more 
longitudinal strands which, near one end of the body, bear a pair 
of ganglia called the brain. In many instances, the longitudinal 
nerve trunks are connected by cross-commissures which give 
the system a ladder-like appearance. 
Typically, the digestive system is composed of an ectodermal 
pharynx and a mesenteron, for the proctodaeum is lacking. 
Exceptions to this are found in the cestodes which lack all evi- 
dence of a digestive system and in the nemertines which have a 
complete system terminating posteriorly in an anus. Excretion 
is by means of a system of tubules, the protonephridia, which 
ramify throughout the body and end in minute structures called 
the flame-cells. Form, habits, and details of structure differ so 
profoundly in the members of the various classes that few features 
beyond those already mentioned are held in common by all 
members of this phylum. 
Interrelationships of the Classes.-The Turbellaria represent 
the most generalized group of the Plathelminthes. Modifica- 
tions in the classes Trematoda and Cestoda represent, chiefly, 
adaptation to the parasitic habit. Extreme development of 
organs of generation and fixation (Fig. 63) and reduction of 
locomotor organs, sensory apparatus, and other structures charac- 
teristic of free-existence characterize these two parasitic groups. 
Development.-Some of the flatworms, especially the fresh- 
water Turbellaria and the monogenetic trematodes, undergo a 
98 PHYLUM PLATHELMINTHES 
99 
direct development from the fertilized egg. More commonly a 
metamorphosis is involved. There are numerous different types 
of larvae characteristic of the various flatworm groups. Muller's 
larva with eight lobes, the margins of which are outlined by a 
continuous band of cilia, occurs in many polyclads. This seems 
to represent a primitive type of larva from which many others 
have been differentiated. 
The pilidium of nemertines characteristically undergoes a 
complicated metamorphosis during which but a portion of the 
larva is utilized in the transformation to the adult form while the 
remainder is cast off. 
Ciliated larvae are characteristic of many digenetic trematodes 
and of cestodes but in their organization these show many evi- 
dences of modification correlated with adaptation to the parasitic 
existence. The larvae of these groups will be considered in 
greater detail later. 
Relationships to Other Phyla.-Through the Ctenophora and 
the Turbellaria, the two phyla Coelenterata and Plathelminthes 
seem to have a fairly close phylogenetic relationship. Muscula- 
ture, nervous system, and reproductive organs which have their 
origin as purely epithelial structures in the lower coelenterates 
become associated with the mesoglea in the higher coelenterates 
and transform this layer into a true mesoderm in the Ctenophora. 
This trend of development and specialization of the mesoderm 
is carried still further in the highly organized parenchymatous 
body of the Plathelminthes. There are some organisms which 
show striking combinations of flatworm and ctenophore charac- 
teristics. Coeloplana and Ctenoplana are two such genera to 
which unparalleled phylogenetic significance has been attached. 
Each is a small creeping form with fundamental appearances and 
structure of a polyclad turbellarian. In addition, Coeloplana has 
a middorsal sensory apparatus and a pair of lateral tentacles 
similar to those of the ctenophores. Ctenoplana has much the 
same general body form but in addition to the general surface 
ciliation has eight rows of ciliated plates on the dorsal surface 
similar to the ctenophore combs. 
Class Turbellaria 
The Turbellaria range in size from minute, microscopic forms 
to some which attain several inches in length. The polyclads 
and triclads are usually distinctly flattened and in outline range 100 
INVERTEBRATE ZOOLOGY 
from disc-shape to lanceolate and long ribbon-like forms. Among 
the rhabdocoels is found even greater diversity of form. Some are 
spindle-shaped while others are distinctly flattened. The ecto- 
derm is covered with cilia (Fig. 62) which by their movement 
produce a smooth gliding locomotion. Currents produced by 
these cilia are responsible for the name of the class. Turbellaria 
are mostly free-living, aquatic organisms, though some have 
Fig. 59. 
Fig. 60. 
Fig. 61. 
Fig. 59-61.-General organization in the three classes of Turbellaria. 
59, a Rhabdocoel; 60, a Polyclad; 60, a Tri clad. The nervous system is shown in 
black; digestive system in cell outlines and nuclei; 59, reproductive system closely 
stippled, excretory system coarsely stippled; 60 and 61, reproductive system 
stippled on left side of body, excretory on right. (From von Graff"). 
acquired the parasitic habit and others have become adapted to 
living in or on moist soil. 
The Orders.-A pharynx and blind intestine comprise the 
digestive tract of the turbellarians. The muscular pharynx is 
frequently enclosed in a pocket within the body and in feeding is 
thrust out like a proboscis. 
Upon the nature of the intestine the three orders are distin- 
guishable. Among the Polycladidea (Fig. 60), the mesenteron PHYLUM PLATHELMINTHES 
101 
consists of a central space from which numerous branches pass 
into the parenchyma. These branches become greatly sub- 
divided and frequently anastomose. In the Tricladidea, three 
main branches (Fig. 61) lead off from the pharynx, one directed 
anteriorly and two posteriorly. Each of these has numerous 
lateral diverticula which nearly reach the margins 
of the body. In the Rhabdocoelida, the mesent- 
eron (Fig. 59) forms a simple sac-shaped or rod- 
like intestine. The so-called Acoela lack a cavity 
in the mesenteron which has been described as a 
digestive parenchyma. 
Nervous System.-A simple, linear type of 
central nervous system is characteristic. In the 
Polycladidea, there is a considerable plexus of 
nerve branches with a brain near the anterior 
extremity and a few main trunks. The triclad 
central nervous system is essentially a pair of 
lateral nerve trunks which pass posteriorly from 
the brain with irregular transverse commissures. 
In some forms (Gunda), these connecting branches 
are so regular that they constitute a distinctly 
ladder-type of system. The rhabdocoel nervous 
system is distinctly similar to that of the triclad 
type. Highly specialized sensory organs are not 
common in the Turbellaria. Eyes of a simple 
type are usually found above the brain. In the 
polyclads, these occur in large numbers and may 
also be developed along the margins of the body. 
In the triclads, a single pair of eyes is usually 
present as is also the case with the rhabdocoels, 
though these last may have two pairs or even a 
single eye. The sense of touch is very highly 
developed in the Turbellaria and tentacles are 
frequently found but the entire body is also 
highly sensitive, rendered so especially by the 
presence of sensorv hairs. 
Fig. 62.-A 
Rhabdocoel 
(Microstomum) 
in the process of 
fission into six- 
teen zooids. 
Roman numer- 
als indicate the 
order of the 
fi s si o n planes. 
(After von Graff). 
Reproduction.-Except for some rhabdocoels, 
the Turbellaria are hermaphroditic. The reproductive organs 
differ considerably in the different orders. In most of the fresh- 
water forms, the eggs undergo a simple, direct development but 
some of the polyclads have a more complicated developmental 102 
INVERTEBRA TE ZOOLOG Y 
cycle. The larval polyclad is called a Muller's larva. Asexual 
reproduction by transverse fission occurs in some rhabdocoels, 
frequently in the event of rapid fission, giving rise to a complex 
chain of individuals. Such a condition is shown in Fig. 62 in 
which the order of formation of the successive individuals 
is indicated. 
Class Trematoda 
The trematodes or flukes are exclusively parasitic, living either 
as ectoparasites upon or as endoparasites within the bodies of 
various animals. Since parasitism is an acquired and not a 
primitive mode of life, the bodies of all trematodes are more or less 
modified as a consequence of the development of the parasitic 
habit. Simplicity of structure in this class denotes degeneracy 
and not primitive simplicity. While there are no free-living 
forms, most trematodes in their fundamental structure present 
clear evidences of close relationship with the Turbellaria. They 
vary from less than one millimeter to several centimeters in 
length. 
A cuticula, frequently supplied with spines, covers the body 
surface of all adult trematodes while some larval forms (Fig. 65, A) 
possess a ciliated covering. The epithelium characteristic of 
the body covering of the Turbellaria has become profoundly 
modified in this group and the cells are scattered through the 
underlying parenchyma. 
Most flukes are hermaphroditic and the conspicuous repro- 
ductive organs are of prime importance in the classification of 
the group (Fig. 66, A). A few, the Schistosomes for example, are 
unisexual. Suckers, hooks, and spines are developed in varying 
combinations for securing attachment to the host. The number 
and arrangement of these structures are of particular significance 
in the grouping of the trematodes. Two subclasses, Monogenea 
and Digenea, are recognized. 
Subclass Monogenea 
The monogenetic trematodes are usually external body para- 
sites but in some instances they have migrated inward to loca- 
tions such as the mouth cavity, the respiratory organs, the cloaca, 
or the urinary bladder which are in direct communication with 
the body surface. The chief organ of attachment (Fig. 63) is 
usually located at the posterior extremity and consists of varied PHYLUM PLATHELMINTHES 
103 
forms of sucking discs and combinations of hooks and spines. 
These hermaphroditic individuals produce eggs which undergo 
direct development without complicated larval changes and 
without alternation of hosts. The young parasites may immedi- 
ately attach themselves to the body of the host which sheltered 
the parent fluke. 
Two orders are recognized on the basis of structure of the 
posterior organ of fixation. In members of the order Mono- 
pisthocotylea (Gyrodactylus (Fig. 63), Dactylogyrus, Ancyro- 
Fig. 63.-Gyrodactylus eleyans, a mono- 
genetic trematode. (After Lilhe). 
Fig. 64.-Opisthorchis felineus, a 
digenetic trematode. (After Stiles and 
Hassall). 
cephalus, Nitzschia), this posterior organ is a single structure 
provided with extremely varied combinations of hook and spines 
for attachment to the skin or gills of fishes. In the Polyopistho- 
cotylea (Microcotyle, Polystoma, Diplobothrium, etc.), each 
individual bears two or more posterior suckers supplemented by 
hooks or spines. Skin and gills of fishes and body surface, 
urinary bladder, and pharynx of amphibians and reptiles are the 
chief seats of infestation by representatives of this second 
order. 104 
INVERTEBRATE ZOOLOGY 
Subclass Digenea 
Digenetic trematodes are usually provided with one or two 
suckers the anterior of which is related to the mouth opening. 
Conspicuous chitinous hooks, so common in the Monogenea, are 
lacking in the Digenea, though relatively smaller spines are of 
common occurrence. The ventral sucker, frequently called the 
acetabulum, is usually near the middle of the body though it 
may be shifted to the posterior extremity (Diplodiscus and 
others) and in some instances it is entirely wanting (Mono- 
stomes). Development is through a complex series of stages 
involving an alternation of hosts, one of which is always a mollusc. 
In the adult worm (Fig. 66, A), the male reproductive organs 
consist normally of two testes which communicate with the 
genital pore through the vas deferens and a cirrus. The female 
organs comprise an ovary of variable form from which an oviduct 
leads through a modified region called the ootype into the uterus. 
Between the ovary and the ootype the oviduct may receive two 
canals, one from the vitelline receptacle and the other from the 
Laurer's canal and the receptaculum seminis. As ova pass 
from the ovary down the oviduct, spermatozoa from the recepta- 
culum seminis and yolk and shell material from the vitelline 
receptacle pass with them into the ootype. Here each fertilized 
ovum and a number of yolk cells become surrounded by a shell 
to form an egg. The ootype is a modified portion of the oviduct 
in the region of the so-called shell gland or Mehlis' gland. The 
vitelline material which supplies both the yolk cells and the 
substance of the shell is produced in glandular bodies, the vitel- 
laria, usually distributed along the margins of the body and 
communicating with a vitelline receptacle by means of smaller 
tubules which combine to form two transverse ducts. 
Life Cycle.-As an illustration of the developmental cycle of 
the Digenea, that of the sheep-liver fluke, Fasciola hepatica, has 
been chosen. The adult flukes live in the bile ducts of the sheep's 
liver where, by their presence and by their munching off portions 
of the lining of the ducts, they produce a diseased condition 
known as liver-rot. As eggs are discharged from the parent fluke, 
they pass down the bile duct into the intestine and out of the body 
of the host along with the feces. The embryo within the egg-shell 
develops into a small ciliated larva known as a miricidium (Fig. 
65, A) which is released from the shell only when surrounding con- PHYLUM PLAT HELM! NT IIES 
105 
ditions are proper. In swampy ground or following a heavy 
dew or rain, this miricidium is enabled to swim in search of a 
suitable snail before the trematode can go further in its develop- 
ment. Upon coming in contact with a snail of suitable species, 
the miricidium makes use of a small boring spine on its anterior 
extremity to penetrate the soft tissues of the snail and comes to 
lie in the liver of the snail. Here it undergoes a transformation 
becoming a bag-shaped structure called a sporocyst (B). Repro- 
duction occurs within the sporocyst 
to form numerous individuals called 
rediae. Each redia (C) is a simply 
organized individual possessing a 
pharynx and tubular intestine in 
addition to a single birth pore 
through which young are dis- 
charged. Within each redia, 
develop either a new generation 
of rediae or individuals of another 
type termed cercariae. A cercaria 
(Z>) is a minute fluke which in 
addition to the rudimentary organs 
of the adult contains also a strongly 
developed posterior tail for locomo- 
tion. The cercariae leave the body 
of the snail and for a time are free- 
swimming creatures. Finally the 
cercaria crawls onto vegetation 
where after losing its tail it 
becomes surrounded by a calcareous 
cyst wall (E). In this condi- 
tion, it remains inactive until the 
plant bearing the cyst is taken into the stomach of a sheep or 
other suitable animal. Under the digestive action, the young 
fluke is liberated from the cyst and occurs free in the digestive 
tract. As it passes into the intestine, its chance of ever reaching 
maturity rests upon the discovery of the opening of the bile duct 
and migration through it into the liver. The successful individ- 
ual becomes established in the bile passages where after a few 
weeks of growth it has reached adult size and begins to produce 
eggs, thus closing the cycle. 
Fig. 65.-Stages in the develop- 
ment of the sheep liver fluke 
(Fasciola hepatica). A, miricidium; 
B, sporocyst; with rediae develop- 
ing internally; C, redia, with second 
generation of rediae and cercariae; 
D, free cercaria; E, encysted 
cercaria. (Mostly after Thomas'). 106 
INVERTEBRATE ZOOLOGY 
In so far as is known, a mollusc is essential for the develop- 
ment of all digenetic trematodes. Frequently, other hosts arc 
added, either as essential links in the life cycle or as facultative 
adaptations, until the complete developmental cycle involves a 
number of different species which act as host to a single parasite. 
Orders and Families.-The majority of the Digenea have a 
mouth at or near the anterior extremity surrounded by an oral 
sucker. These comprise the order Prosostomata. A few (order 
Gasterostomata, including the genus Bucephalus) have an 
anterior sucker which is not perforate and not associated with the 
mouth. In these, the mouth occurs near the middle of the body 
on the ventral surface. Numerous families representing a 
number of suborders are included in the Prosostomata of which 
the following are examples: Fasciola hepatica of sheep, Para- 
gonimus with several species from the lungs of man and other 
mammals, Clonorchis sinensis from the liver of man and other 
animals, Schistoma japonicum and other species from the blood 
stream of man. 
Class Cestoda 
Like the trematodes, all cestodes are parasitic but through 
fundamental structures show marked relationships with the less 
highly specialized representatives of this phylum. The only 
sure criterion for the separation of Cestoda from other parasitic 
flat worms is their entire lack of a digestive system. While the 
typical cestode is made up of a chain of segments or proglottids 
and a scolex for attachment to the host, there are some, the 
monozoic cestodes or Cestodaria, in which the body is composed 
of but a single unit. Though these latter are usually not more 
than a few millimeters in length the segmented forms frequently 
attain a length of several meters and may be divided into several 
thousand proglottids. 
The question of the orientation of the cestode body has never 
been decided with certainty. Many zoologists maintain that the 
scolex is the anterior extremity of the chain because here the 
chief nervous centers are found. Other investigators contend 
just as stolidly that since in development from the larval stage the 
scolex is at the posterior extremity of the larva this scolex must 
represent the morphological posterior extremity of the adult chain. 
Another question upon which there is just as radical division of 
opinion is that upon the determination of what constitutes an PHYLUM PLAT HELM IN THES 
107 
individual. Is the entire chain an individual or a colony of indi- 
viduals? Continuity of nervous and vascular systems through- 
out the chain with some modifications at the extremities not 
found in the individual proglottids presents evidences of unity 
of the entire chain. On the other hand, since the reproductive 
organs are about the only structures remaining in the proglottids, 
complete duplication of these in each proglottid gives support to 
the argument that each proglottid is an individual, groups of 
which have remained united to form a colony as a result of incom- 
plete separation following asexual reproduction to form the chain. 
That part of the strobila which is located between the scolex 
and the proglottids is frequently not divided into segments but 
as a more or less sharply defined region is termed the neck. This 
is the budding zone where new proglottids are being formed. 
Thus in age the scolex is the oldest then come the proglottids in 
order from the free extremity toward the neck. Few structures 
are evident in the neck and in the small proglottids most recently 
formed for there is a gradual and progressive development of the 
organs (organogenesis) representing all stages between the fully 
formed organs of the terminal proglottids and the merest traces 
of fundaments in the proglottids just behind the neck. The 
sexual organs of the terminal proglottid are the oldest and con- 
sequently mature first. Upon reaching full maturity, the gravid 
proglottids are frequently severed from the remainder of the 
strobila. In some forms (Ligula), there are no partitions between 
the proglottids but the organs are duplicated successively in an 
undivided body. A secondary strobilization is known to take 
place in some cestodes wherein the original proglottids undergo 
a secondary subdivision, each giving rise to a number of proglot- 
tids which may eventually separate as independent strobila one 
of which becomes modified as a pseudoscolex. 
Subclass Cestodaria 
The Cestodaria or monozoic cestodes rather closely resemble 
trematodes but the lack of digestive organs necessitates their 
inclusion as a subclass along with the true cestodes. There is no 
sharp differentiation between scolex and proglottids in the Ces- 
todaria and but one set of reproductive organs usually occurs 
in each individual. They occur in both marine and fresh-water 
hosts. Members of the genus Archigetes become mature in 
oligochaetes, Gyrocotyle in the spiral valve of elasmobranchs, 108 
INVERTEBRATE ZOOLOGY 
and Caryophyllaeus, Glaridacris, and Amphilina in the body 
cavity or digestive tract of fishes. The organ of attachment is 
variously modified. 
Subclass Cestoda (s. str.)1 
The polyzoic cestodes include those tapeworms which have a 
scolex followed by a succession of organs usually divided in chain- 
fashion into a series of proglottids. Each proglottid is furnished 
with a complete set of reproductive organs (Fig. 66, B) and in 
some instances two sets occur in each. The male organs consist 
ofjrom one to more than a hundred testes embedded within the 
parenchyma and connected by vasa efferentia and vas deferens 
with the cirrus which communicates with the outside. The 
genital ducts of both sexes usually open in a common genital 
Fig. 66.-Diagrams showing the fundamental plan of organization of sexual 
organs in A, trematodes; B, cestodes. (Ong.) 
pore which is either on the lateral margin of the proglottid or on 
the ventral surface. When it is marginal there is no necessary- 
regularity in its disposition for in adjacent proglottids it may be 
either on the same or opposite margins. In cestodes which have 
two sets of genital organs, the pores occur on both lateral margins 
of each proglottid. 
The arrangement of the female organs differs greatly in the 
different genera. The vagina opens through the common genital 
1 Sensu strictu indicates that a word is used in a limited or restricted 
sense. In this instance the word Cestoda is used for the entire class but 
in a restricted sense the same word is used to designate the true cestodes 
from the Cestodaria. PHYLUM PLATHELMINTHES 
109 
pore and in addition in some instances the uterus has an inde- 
pendent orifice to the exterior. A typical arrangement is as 
follows: The vagina, as it passes through the proglottid, bears 
a dilated portion for the storage of sperm cells called the recepta- 
culum seminis or seminal receptacle. As the duct continues 
beyond this receptaculum it receives two side branches, the 
oviduct which connects it with the ovary and the vitelline duct 
which communicates with the vitellaria or vitelline glands. The 
oviduct frequently bears a modification called the oocapt for 
forcing the eggs along their course. When eggs and spermatozoa 
meet, fertilization occurs. The fertilized eggs and the substance 
from the vitelline glands pass into a structure similar to the 
ootype of the trematodes in the region of the shell gland. Here 
shells are formed around the eggs which are then passed on into 
the uterus. In older proglottids, the uterus frequently develops 
egg-filled pouches which occupy practically the entire volume of 
the proglottid. Eggs are rarely discharged from the uterus for 
more frequently the entire egg-filled proglottid is liberated and 
carried out of the body of the host. 
The nervous and excretory systems extend the length of the 
strobila and parts in individual proglottids do not represent 
complete units. Ganglia located in the scolex send nerve trunks 
backward through the proglottids, usually as two lateral branches 
fairly close to the lateral margins of the body. Parallel to the 
longitudinal nerve trunks run the main canals of the excretory 
system. These in turn receive smaller canals the branches of 
which ramify through the parenchyma and terminate in 
flame-cells. 
Reproduction.-Several different methods of development 
occur in the Cestoda. Typically, the egg undergoes cleavage 
within the shell to form an embryo, bearing six minute hooks, 
called an onchosphere. The onchosphere may in some instances 
be provided with cilia for locomotion (Dibothriocephalus) but 
more frequently it is borne within embryonic membranes as an 
immotile body. Upon introduction into a suitable host, the 
onchosphere develops into a larval form differing with the 
various groups of the cestodes. Among the lower cestodes, this 
larva is a small solid body called a plerocercoid (Fig. 67, A). A 
larval stage called the cysticercus or bladderworm stage (C) is 
found among the higher cestodes. The name cysticercoid is 110 
INVERTEBRATE ZOOLOGY 
applied to larval forms (B) which seem to be intermediate 
between the plerocercoid and the cysticercus. 
As an illustration of cestode development the life cycle of 
Taenia saginata, the beef tapeworm of man, will be outlined. 
The mature individual of Taenia saginata occurs in the intestine 
of man. As the older proglottids become gravid, they become 
detached from the end of the chain and pass out of the body 
along with the feces. These isolated proglottids have indepen- 
dent powers of movement and may crawl away from the feces 
onto grass or other vegetation where they might be taken into 
the stomach of a grazing cow, along with the grass. In the 
stomach of the cow, the walls of the proglottid are digested 
away and the shells of the embryos open, thus liberating the 
onchospheres in the digestive cavity of the cow. Each oncho- 
sphere by the action of its hooks bores into the wall of the diges- 
Fig. 67.-Schematic drawings of cestode larvae. A, plerocercoid; B, cysticer- 
coid; C, cysticercus. (Orifif.) 
tive tract and may enter the blood stream by which it is carried 
to remote parts of the body. Especially in the muscular and 
connective tissues, the onchospheres come to rest and undergo a 
transformation to large sac-like structures, the cysticerci, each 
bearing an inverted scolex. Each cysticercus is surrounded by a 
cyst wall formed by the action of the surrounding host tissues. 
Here it lies without going further in its development unless 
introduced into the body of another animal which is suitable as a 
host. Cysticerci in thoroughly cooked infected beef are harmless. 
If beef containing the living cysticerci is eaten by man, the 
digestive action liberates the cysticercus from its confining cyst, 
the inturned scolex becomes everted and secures attachment to 
the lining of the alimentary canal of the new host. The scolex 
is not affected by the digestive processes of the host but the cyst 
appended to its free extremity disintegrates, leaving only the PHYLUM PLATHELMINTHES 
111 
scolex and neck which, through growth, produce the entire 
strobila. 
In some instances (Echinococcus, e.g.), the cysticercus under- 
goes an asexual reproductive cycle resulting in the formation of 
numerous cysticerci from a single onchosphere, each of which 
bears numerous scolices. In the foregoing the budding is internal 
but in some instances there is an external proliferation of buds or 
branches in irregular masses (S'parganum). 
Orders.-The scolex furnishes valuable characters for the 
recognition of the orders of Cestoda. In members of the order 
Pseudophyllidea, the scolex is usually provided with two longi- 
tudinal sucking grooves (Marsipometra, Bothriocephalus, Aboth- 
rium, Ligula) though in some instances these are highly 
modified (Bothrimonus) and occasionally are united to form an 
unpaired terminal adhesive organ (Cyathocephalus). The scolex 
in members of this order is usually unadorned except in Triaeno- 
phorus which bears conspicuous chitinous hooks and in the 
primary scolex of Haplobothrium which bears four proboscides, 
therein resembling the Trypanorhyncha, Members of the order 
Trypanorhyncha, which occur chiefly in the digestive tract of 
marine fishes, have a proboscis of varied form provided with four 
terminal, long, cylindrical proboscides covered with minute spines 
(Tetrahynchus). In the order Tetraphyllidea, are included 
cestodes with a scolex bearing four cup-like bothria or suckers 
and with each proglottid bearing numerous vitelline follicles 
(Proteocephalus, Corallobothrium). The order Cyclophyllidea 
includes forms having four cup-like or saucer-shaped suckers and 
usually a terminal organ between the suckers termed a rostellum. 
In this order, the vitellaria are usually posterior to the ovary and 
occur in a single compact mass. This order of numerous families 
includes most of the cestode parasites of the higher vertebrates 
(Taenia, Anaplocephala, Hymenolepis, Dipylidium, Davainea, 
and many other genera). 
Class Nemertinea 
The nemertines are usually included as a class under the 
Plathelminthes because of their agreement with other flatworms 
in: (1) structure of the nervous system, (2) presence of a pro- 
tonephridial excretory system, and (3) the strong development of 
the mesoderm which renders the body highly parenchymatous. 
On the other hand, they differ from other flatworms in: (1) the 112 
INVERTEBRATE ZOOLOGY 
specialization of a vascular system so that distribution of nourish 
ment is not cared for by a combined gastro-vascular system, (2) 
the development of an anal opening at the posterior extremity of 
the digestive tract, and (3) the presence of a closed sac, the 
proboscis sheath, which surrounds the proboscis and by some is 
considered as representing the beginning of a coelom. In 
attempting to show relationships with the coelomate animals, 
this last point is of considerable importance. Other workers 
have thought the cavities of the gonads of nemertines are really 
coelomic sacs. 
Nemertines are chiefly marine, living usually in burrows in 
mud or sand, and in some species attaining a length of 90 feet. 
Fig. 68.-Diagram of a nemertine. b, brain; c, ciliated pit; d, dorsal nerve 
trunk; di, dorsal blood vessel; g, gastric ceca; i, intestine; I, lateral nerve trunk; 
Iv, lateral blood-vessel; p, proboscis, retracted; pm, proboscis muscles; pv, proto- 
nephridial tube; po, its opening; ps, cavity of proboscis sheath. (After Kingsley, 
courtesy of Henry Holt and Co.) 
Some of the smaller forms inhabit fresh-water or live in moist 
soil. The body surface is ciliated and frequently brilliantly 
colored. Numerous mucus glands produce secretions which 
may form a tube within which the animal dwells. The body 
wall contains an outer circular and an inner longitudinal layer 
of muscles which are so effective that a worm which is 15 feet or 
more when fully extended may shorten to less than two feet in 
length. 
The proboscis (Fig. 68) is one of the most characteristic struc- 
tures of the nemertine. This is a hollow muscular tube turned 
into the body at the anterior extremity and when thus inverted 
extends far back through the body within a sac-like cavity called 
the proboscis sheath. By contraction of the fluid-filled sheath, 
the proboscis is everted and thrust out from the anterior part 
of the body. At the tip of the extruded proboscis, there is fre- 
quently a sharp-pointed stylet which, of course, is at the extreme PHYLUM PLATHELMINTHES 
113 
posterior end of the proboscis when it is retracted and inverted 
within the sheath. Retraction of the everted proboscis is accom- 
plished by means of a retractor muscle which runs from the tip 
of the proboscis to the base of the sheath. Abundant nerve 
supply indicates that the proboscis is highly tactile. 
The mouth is located at the anterior extremity of the body or 
just ventral to the opening of the proboscis sheath. The tubular 
ectodermal esophagus opens into the tubular intestine (mesen- 
teron) which usually has paired lateral diverticula along its 
course to the anus. The anus may be either at the posterior 
extremity or in some instances the digestive tract does not enter 
the tail region. 
Reproduction.-The sexes are usually separate. The gonads, 
which are lateral in position occur between the intestinal diver- 
ticula. Each ovary or spermary is a sac-like organ which usually 
opens on the dorsal surface by a pore. Both eggs and sperma- 
tozoa are discharged from the body through the pores and 
fertilization takes place outside the body of the worm. Cleavage 
of the fertilized egg usually results in the formation of a helmet- 
shaped larva known as the pilidium. Cilia occur on the lapets at 
the lower margins of this larva and also in a patch at the opposite 
pole known as the apical plate. The apical plate is the chief 
nervous center of the larva. Development of the adult from 
this larva involves a complicated metamorphosis. By the 
growth of two infoldings of the ectoderm, a part of the body 
containing the digestive system of the larva is surrounded and 
cut off from the remainder of the body. In later development, 
these ectodermal infoldings form the body wall of the adult worm 
and only the parts of the larva enclosed by them are utilized 
in the production of the young worm for the remainder of the 
larva is cast off during the metamorphosis. In some instances, 
development is direct, without involving the pilidium while in 
still others a reduced creeping pilidium, frequently termed 
Desor's larva, takes its place. 
Vascular System.-Typically, the vascular system has three 
main longitudinal trunks, two lateral and a median dorsal vessel 
which lies between the intestine and the proboscis sheath. 
Transverse loops connect the two lateral vessels. The fluid 
contained in this system is usually colorless. This is the first 
instance in the animal kingdom where the function of distribution 
is taken over by an independent system for in lower forms the 114 
INVERTEBRATE ZOOLOGY 
functions of digestion and distribution are performed by a 
gastrovascular system. 
Excretory System.-The excretory system consists of two 
longitudinal tubules which run parallel to the lateral vessels of the 
circulatory system and through their course give off small 
branches which terminate in flame-cells. Either a single pore or 
several pores communicate with the exterior. 
Nervous System.-The central nervous system consists of a 
pair of ganglia from which two lateral and one median dorsal 
nerve pass backward through the body. Details of structure and 
arrangement of the nerve trunks differ considerably in the 
different orders. In some instances (Protonemertini), the 
nervous system remains in the superficial layers of the body 
external to the musculature. A pair of ciliated grooves on the 
sides of the head, frequently called cerebral organs, are closely 
connected with the brain and have a sensory function. Eyes 
and tactile organs are usually developed. 
Orders^-Four orders of nemertines are usually recognized. 
The nerve cords in members of the order Protonemertini 
are outside of the muscular layers. In members of the order 
Mesonemertini, they are among the muscle cells, while in the 
Heteronemertini and Metanemertini they are embedded in the 
parenchyma internal to the muscular layers. In the hetero- 
nemertines, the muscles are arranged in three or more layers 
while in the other three orders two muscle layers are character- 
istic. Stylets at the tip of the proboscis occur only in the 
Metanemertini. 
References 
(See general references cited at close of Chapter I) 
Braun, M. 1879-1893. Trematodes. Bronn's Klassen u. Ordnungen d. 
Tierreichs, Vol. 4. 
. 1894-1900. Cestodes. ibid, Vol. 4: 927-1732. 
Burger, O. 1890. "Untersuchungen uber die Anatomie und Histologie 
der Nemertinen. Zeitschr. f. Wissensch. Zool., 1: 1-277. 
■ . 1897-1907. Nemertini. Bronn's Klassen u. Ordnungen d. 
Tierreichs, Vol. 4, supplement. 
Curtis, W. C. 1902. The Life History, the Normal Fission, and the 
Reproductive Organs of Planaria maculata. Proc. Boston Soc. Nat. 
Hist., 30: 515-559. 
Cooper, A. R. 1919. North American Pseudophyllidean Cestodes from 
Fishes. Ill. Biol. Monogr., 4, No. 4. 
von Graff, L. 1882-1899. Monographic der Turbellarien. Leipzig. PH YL UM PL A THELMINTHES 
115 
. 1904-1908. Turbellaria. I. Abt. Acoela und Rhabdocoelida. 
Bronn's Klassen u. Ordnungen d. Tierreichs, Vol. 4. 
Hubrecht, A. A. W. 1883. On the ancestral forms of chordata. Quart. 
Jour. Mier. Sci., 23: 349-368. 
. 1887. Report on the Nemertea. Report Challenger, Zool., 
Vol. 19. 
Lang, A. 1884. Die Polycladen des Golfes von Neapel. Fauna u. Flora 
des Golfes von Neapel., 11 Monograph. 
La Rue, G. R. 1914. A Revision of the Cestode Family Proteocephalidae, 
Illinois Biol. Monographs, Vol. 1, Nos. 1 and 2. 
Leuckart, R. 1863. Menschlichen Parasiten und die von ihnen her- 
ruhrenden Krankheiten, Leipzig. 
. 1879-1889. Die Parasiten des Menschen. Leipzig u. Heidel- 
berg. 
Looss, A. 1894. Die Distomen unserer fische und frosche. Biblioth. 
Zool., 16: 1-296. 
Morgan, T. H. 1898. Studies on the regeneration of Planaria maculata. 
Arch. f. Entw.-mech. d. Org., 7: 364-397. 
Pratt, H. S. 1900 and 1902. Synopsis of North American Invertebrates, 
XII, The Trematodes. Amer. Nat., 34: 645-662 and 36: 887-910. 
Ransom, B. H. 1913. Cysticercus ovis, the cause of tapeworm cysts in 
mutton. Jour. Agr. Research, 1: 15-58. 
. 1920. Synopsis of the Trematode Family Heterophyidae. 
Proc. U. S. Natl. Museum, 57: 327-573. 
Silliman, W. A. 1885. Beobachtungen uber die Siisswasser-Turbelarien 
Nord-Amerikas. Zeitschr. f. Wissensch. Zool., 41: 48-78. 
Stiles, C. W. 1898. The Flukes and Tapeworms of Cattle, Sheep, and 
Swine with special reference to Inspection of Meats. U. S. Dept. 
Agr., Bur. An. Ind., Bull. 19. 
- . 1904. Illustrated key to the trematode parasites of man. 
U. S. Hygienic Lab., Bull., 17. 
Thomas, A. P. 1883. The Life History of the Liver-Fluke (Fasciola 
hepatica). Quart. Jour. Mier. Sci., 23: 99-133. 
Ward, H. B. 1895. The parasitic worms of man and the domestic 
animals. Ann. Rep. Nebraska Bd. Agr., 1894: 225-348. 
. 1903 and 1908. Data for the determination of human entozoa. 
Trans. Amer. Micros. Soc., 24: 103-138 and 28: 177-201. 
Wilhelmi, J. 1909. Die Tricladen des Golfes von Neapel. Fauna u. 
Flora des Golfes von Neapel, 32 Monograph. 
Wilson, C. B. 1900. Habits and Early Development of Cerebratuhis 
lacteus. Quart. Jour. Mier. Sci., 43: 97-198. CHAPTER VII 
PHYLUM NEMATHELMINTHES 
The Nemathelminthes or round worms are thread-like or cylin- 
drical worms which have a body cavity and lack segmentation. 
Since the splanchnic layer of the mesoderm is wanting, the body 
cavity is not a true coelom but is termed a pseudocoel. Three 
classes are usually recognized as belonging to this phylum, the 
Nematoda, the Gordiacea, and the Acanthocephala. There is 
considerable doubt as to the correct location of the Acantho- 
cephala in the system for evidences indicating flat worm relation- 
ships are not wanting. The three classes differ so fundamentally 
in structure that it seems best to offer individual treatment of 
the groups rather than discuss characteristics of the phylum 
further. 
Class Nematoda 
The nematodes or nemas, as they are frequently called, have 
characteristically elongated, cylindrical bodies covered by a 
resistant cuticula. They occupy almost every conceivable 
habitat capable of supporting life. Fresh-water, marine, and 
soil-inhabiting species are extremely numerous and as parasites 
both of plants and of animals they are of high importance. In 
length, they range from a fraction of a millimeter to more than a 
meter. There are no appendages and no segmentation though 
in some free-living forms striations, cuticular scales, or bristles 
may give a superficial appearance of segmentation. 
Histology.-In section, the body wall is seen to be composed of 
an external non-cellular layer, the cuticula, directly beneath 
which lies the subcuticula or hypoderm. A layer of partially 
differentiated muscular cells lines the outer wall of the body cav- 
ity and comprises the chief bulk of the body wall. Each of these 
epithelio-muscular cells has only a small portion of its bulk differ- 
entiated into contractile substance lying next to the subcuticula, 
while the remainder of the cytoplasm containing the nucleus, 
protrudes as a rounded mass into the body cavity. 
116 PHYLUM NEMATHELMINTHES 
117 
This muscular layer is not a continuous lining of the body 
cavity for it is interrupted by slight breaks in four regions which 
are designated as the dorsal, ventral, and lateral lines. The 
fairly conspicuous thickening of 
the subcuticula which stands in 
the middle of each lateral surface 
is known as a lateral line. Less 
pronounced intrusions of the sub- 
cuticula occur also in the mid- 
dorsal and mid-ventral lines. In 
the lateral lines, are borne the 
ducts of the excretory system. 
Near the anterior extremity of 
the body, these are united and 
communicate with the exterior 
through a single excretory pore 
on the ventral surface of the body. 
Fig. 69.-The nervous system of 
a nematode. {After Brandes). 
Fig. 70. - Semi-diagrammatic 
drawing of a female free-living 
nematode. {After Jdgerski&ld). 
In the dorsal and ventral lines, are found the two main longitu- 
dinal nerve trunks (Fig. 69) which connect with a nerve ring near 
the anterior extremity of the body. From this same ring, are 118 
INVERTEBRATE ZOOLOGY 
given off a number of smaller longitudinal branches some 
extending forward and others a short distance backward. 
Digestive System.-The digestive system is practically a 
straight tube (Fig. 70) with a mouth at the anterior extremity 
and the anus on the ventral surface slightly in front of the 
posterior tip. The mouth is usually surrounded by lip-like organs 
and, in the case of parasitic forms, frequently bears chitinized 
structures for grappling hold of the host tissues. Behind the 
mouth, there frequently occurs a muscular esophagus with an 
especially conspicuous pharyngeal bulb. The stomach-intestine 
continues posteriorly from the esophagus as a usually flattened 
tube. In the male, the genital ducts open into the posterior 
region of the digestive tube which is thus transformed into 
a cloaca. 
Reproductive Organs.-The male is usually recognizable by its 
smaller size and frequently by the more pointed and curved 
posterior extremity from the cloaca of which copulatory spicules 
are frequently protruded. Reproductive organs in the two sexes 
are very simple in structure for normally they consist of a gonoduct 
which is directly continuous with the gonad. This tube in the 
male is usually single, while in the female (Fig. 70) it is frequently 
bifurcated with the opening of the single vagina occurring on the 
ventral surface of the body. In some forms, the ducts and 
gonads are extremely long and coiled throughout the greater 
part of the length of the body while in others they are a single 
straight rod. 
Method of Reproduction.-Nematodes reproduce only sexu- 
ally. Eggs are fertilized within the oviduct of the female 
and are discharged either before development has proceeded far 
or are retained until the young are fully developed. In some 
instances, the eggs are not layed but are retained in the body 
of the female until the young have left the egg membranes and 
are thus brought forth viviparously. Parthenogenesis occurs in 
some instances, as mentioned below. 
Life Cycle.-A number of extremely interesting and important 
facts are connected with the developmental cycle of the parasitic 
nematodes. Some species are parasitic throughout their entire 
life (Trichinella, and blood Filariae) while many are free-living 
for at least part of their existence (hookworms as larvae, Mermis 
as adults). Still others are facultative parasites living either 
free or parasitic as opportunities are offered. Of this last condi- PHYLUM NEMATHELMINTHES 
119 
tion, Angiostoma nigrovenosum serves as an excellent example. 
The young of this species live in mud where they undergo sexual 
reproduction, both males and females being found. Larvae pro- 
duced by these sexual individuals may continue to produce in the 
same manner as the parents or in case they find their way into 
the lungs of a frog they become established as parasites and 
develop only parthenogenetically. The parthenogenetic eggs 
pass out with the feces and in the mud again pass through the dioe- 
cious phase of the life cycle as free individuals. 
The Ascaris which occurs in the intestine of hogs is not distin- 
guishable from the Ascaris lumbricoides of the human. Until 
very recently it has been assumed that infestation by this species 
is by direct introduction of the young into the digestive tract of 
a new host individual where it becomes established immediately. 
Recent investigations have demonstrated that the larvae of these 
ascarids, when introduced into the digestive tract of a new host 
individual, undergo extensive migration through the organs and 
tissues of the body, reaching the final position only after having 
passed by way of the circulatory system into the lungs. Heavy 
infestations by these larvae cause serious pulmonary disorders 
as the larvae penetrate the lung tissues and travel by way of the 
respiratory passages into the mouth and down the digestive 
tract to the intestine where they reach maturity. 
Extensive migrations are also carried on in the body of the host 
by the hookworms (Ancylostoma of the old world and Necator 
of the new world). The adult worms, which cause very serious 
loss of blood and affect the entire body of the host, occur in the 
intestine of man where the thin-shelled eggs are produced and 
eliminated along with fecal matter. In the soil, the young 
worms hatch and feed for a while on the fecal matter. A new 
skin forms beneath the old one, which is finally shed, and the 
larva enters upon the second period of its life. After another 
molt, the larva is ready to infest a new host individual. This it 
does by either entering the body along with contaminated water 
or food or by active penetration of the skin. Hands and feet 
are the chief inroad for the larvae which attack the skin exposed 
to the soil and follow along hair follicles or between epidermal 
scales to the lymph spaces. Once in the lymph stream they are 
carried passively to the subclavian, thence to the heart, and on 
with the blood stream to the lungs. Here the larvae leave the 
capillaries, enter the air sacs, and wander through the bronchi, up 120 
INVERTEBRATE ZOOLOGY 
the trachea into the esophagus, and down through the stomach 
to the intestine where they become attached to the wall by 
means of their specially adapted sucking mouth. About seven 
to ten weeks from the time the larva enters the skin, eggs begin 
to appear in the feces indicating that the worms have reached 
maturity and the life cycle is thus completed. 
Still a different condition is found in the life cycle of Trichinella 
spiralis. There are two distinct stages in the parasitism by this 
worm, the sexually mature worm which occurs in the digestive 
tract and the encysted larvae in the muscles. Both sexes occur 
in the intestine of man and of other mammals of which the pig 
and rats are the most important hosts. The gravid female 
pierces the wall of the intestine with her posterior extremity. 
With the genital pore inserted in a lacteal vessel, the young are 
liberated and carried by the lymph and blood streams into the 
tissues of the host's body. It is not known to what extent this 
migration is active or passive. Upon reaching muscular tissue, 
the larvae become encysted and all further development is 
contingent upon the muscles containing the encysted larvae 
being introduced into the digestive system of some other mammal, 
for the larvae have no independent means of ever leaving the 
body of the host individual which sheltered the mature female 
in its intestine. The chief source of the adult worms in the 
human intestine is from the larvae encysted in pork, while 
the hog in turn receives its intestinal form through eating the 
viscera of other hogs in slaughter yards or from eating rats 
which have become infested. 
Blood-inhabiting nematodes such as the Filariae are often 
carried from one host to another through the bite of blood-sucking 
insects. Some species which cause elephantiasis through the 
occlusion of blood and lymph vessels are carried by mosquitoes. 
There are numerous species of free-living nematodes represent- 
ing many genera. In the genus Iota the body is covered with 
scales which give it a superficial appearance of segmentation. 
Tylenchus, Dorylaimus, Mermis, and Rhabditis are names of 
characteristic genera of free-living nematodes though species in 
some of these genera may be parasitic. 
Class Gordiacea 
Superficially, the Gordiacea resemble the nematodes but in 
finer details of structure they have little in common with them. PHYLUM NEMATHELMINTHES 
121 
The adults live free in the water where the females lay strings of 
eggs which develop into small larvae. These larvae enter the 
body of some insect where they undergo development to the 
adult body form. 
The "hair-snakes," so commonly found in watering troughs 
and in ponds and streams, are the adults of Gordiacea which 
have escaped from the bodies of crickets or other insects. They 
bear no lateral lines and the nearly cylindrical body with blunt 
anterior extremity and irregularly roughened cuticula serve to 
differentiate the Gordiacea from the nematodes. 
Gordius, Paragordius, and Chordodes are genera in this class. 
Class Acanthocephala 
The Acanthocephala are absolutely parasitic in habits. As 
adults they occur normally in the digestive tract of vertebrates. 
The first larval host is practically always an arthropod, though 
young individuals have been found frequently in other hosts 
which probably act as intermediate hosts. In the digestive 
tract of the vertebrate, the acanthocephalan has an elongated 
flattened body form but upon removal to water or killing fluids 
the liquids distend the body to cylindrical form. One of the 
most characteristic structures is the proboscis (Fig. 71, A) at the 
anterior extremity of the body bearing hooks for grappling into 
the host tissues. This proboscis is frequently capable of inver- 
sion into the anterior extremity of the body inside an organ 
termed the proboscis receptacle or sheath. 
The body wall is composed of an external layer of cuticula 
which overlies a syncitial mass called the subcuticula. This 
subcuticula forms by far the greatest bulk of the animal and is 
provided with a few rounded giant nuclei, finely dendritic nuclei, 
or numerous small nuclei. Two muscle layers, one with the 
fibers directed longitudinally and the other circularly, mark the 
internal limit of the body wall and bound the body cavity. 
At the anterior extremity of the body proper, two organs of 
variable shape and of undetermined function, the lemnisci, 
extend into the body cavity alongside the proboscis receptacle, 
apparently as continuations of the subcuticula. 
The central nervous system consists of a single ganglionic 
mass within the proboscis receptacle. In the different genera, 
this occupies a position varying from the posterior extremity of 
the receptacle to a point near the anterior end of the receptacle. 122 
INVERTEBRATE ZOOLOGY 
Small branches are given off to the surrounding organs and 
usually a pair of structures called the retinacula pass through the 
wall of the receptacle out to the body wall. 
Internal Organization.-Inversion of the proboscis is accom- 
plished by a pair of invertor muscles which run from the tip of the 
Fig. 71.-Morphology of the Acanthocephala. A, general organization of 
male of Acanthocephalus ranae (Schr.); B, interior of caudal extremity of a young 
female of Neoechinorhynchus emydis, showing genital tract. (After Van Cleave'). 
proboscis to the base of the receptacle through which they con- 
tinue and pass on through the body cavity to an insertion on the 
body wall as the retractors of the receptacle. PHYLUM NEMATHELMINTHES 
123 
A sheath passes backward from the posterior extremity of the 
proboscis receptacle as the suspensory ligament which holds the 
reproductive organs in place. The male organs consist of a pair 
of testes and a group of cement glands which communicate with 
the cirrus. The cirrus is contained within an evertible structure 
at the posterior end of the body known as the copulatory bursa. 
When protruded, this is a bell-shaped structure in the center of 
which the cirrus is located. 
In the female, there is no persistent gonad. Egg masses are 
formed very early and after fertilization these are broken up into 
individual embryos each of which becomes surrounded by a series 
of three embryonic membranes. The hard-shelled embryos 
thus formed are usually ovoid or spindle-shaped in form. The 
embryos are held for some time within the body cavity of the 
female which becomes filled with them. Finally, they are dis- 
charged through an apparatus known as the selective apparatus 
(Fig. 71, B) which passes them down the uterus and out of the 
genital pore. 
All classes of vertebrates harbor these parasites. Macracan- 
thorhynchus hirudinaceus found in hogs is one the most commonly 
known species. The genera Echinorhynchus and Neoechinor- 
hynchus are represented by several species in American fishes. 
Several genera, including numerous species, infest the intestines 
of birds and mammals. The genus Moniliformis occurs normally 
in rodents but is also at least a facultative parasite of man. 
' References 
(See general references cited at close of Chapter I) 
Cobb, N. A. 1914. The North American free-living fresh-water Nema- 
todes. Trans. Amer. Micros. Soc., 33: 69-134. 
. 1915. Nematodes and their Relationships. U. S. Dept. Agr. 
Yearbook for 1914: 457-490. 
Hall, M. C. 1916. Nematode Parasites of Mammals. Proc. IL S. Natl. 
Museum, 50: 1-258. 
Hamann, O. 1891 and 1895. Die Nemathelminthen. Jena. 
Kaiser, J. E. 1893. Die Acanthocephalen und ihre Entwickelung. 
Biblioth. ZooL, 7: 1-136. 
Looss, A. 1905 and 1911. The anatomy and life history of Agchylostoma 
duodenale, Dub., a monograph. Cairo. 
Luhe, M. 1911. Acanthocephalen. Die Siisswasserfauna Deutschlands, 
Heft 16. Jena. 
Martini, E. 1916. Die Anatomie der Oxyuris curvula. Zeitschr. f. 
wissensch. ZooL, 114: 137-543. 124 
INVERTEBRATE ZOOLOGY 
May, H. G. 1920. Contributions to the life histories of Gordius robustus 
Leidy and Paragordius varius (Leidy). Illinois Biol. Monogr., Vol. 5, 
No. 2. 
Montgomery, T. H., Jr. 1903. The Adult Organization of Paragordius 
varius. Zool. Jahrb., 18: 387-474. 
Ransom, B. H. and Schwartz, B. 1919. Effects of heat on Trichinae. 
Jour. Agr. Research, 17: 201-221. 
Ransom, B. H. and Foster, W. D. 1920. Observations on the Life 
History of Ascaris lumbricoides. U. S. Dept. Agr., Bull. 817. 
Stiles, C. W. 1903. Report upon the Prevalence and Geographic 
Distribution of Hookworm Disease (Uncinariasis or Anchylostomiasis) 
in the United States. U. S. Hygienic Lab. Bull. 10. 
Van Cleave, H. J. 1919. Acanthocephala from the Illinois River, with 
descriptions of species and a synopsis of the family Neoechinorhynchidae. 
Ill. Nat. Hist. Survey, Bull. 13: 225-257. CHAPTER VIII 
PHYLUM TROCHELMINTHES 
Many of the higher invertebrates in their development pass 
through a larval stage known as the trochophore. In most 
instances where a trochophore is involved, it later by a meta- 
morphosis gives rise to an adult animal which in organization 
is fundamentally different from the simple larva. There are, 
however, a few organisms which in their adult state are not 
essentially different from the trochophore type of organization. 
The most characteristic of these is the group of the Rotifera. 
In addition to the rotifers, there is a small group of minute 
fresh-water organisms known as the Gastrotricha which are in 
some respects similar to the Rotifera. These two groups are 
united to form a phylum to which the name Trochelminthes is 
frequently applied. 
By some, representatives of this phylum are thought to repre- 
sent precociously mature larvae, or an instance of what might 
be termed phylopaedogenesis. 
Class Rotifera 
The Rotifera, or wheel animalcules, are microscopic animals 
which in fundamental structure closely resemble the trochophore. 
In size and superficial appearance, they might be mistaken for 
Protozoa and were so considered by many of the early workers. 
Close observation reveals in them miniature organ systems and 
demonstrates their true metazoan natures. Most rotifers live in 
fresh-water though a few dwell in seas. No body of water is too 
large or too small for them for tiny temporary pools frequently 
support a varied fauna of these minute organisms. Many are 
capable of withstanding desiccation and one type of eggs is highly 
resistant. These two facts go far toward explaining the prac- 
tically cosmopolitan distribution of many species for the dried 
individuals or eggs could be transported great distances or 
might even be carried by the winds. 
Gross Morphology.-The body, which is extremely variable in 
shape, usually consists of a trunk and a tail. The anterior 
125 126 
INVERTEBRATE ZOOLOGY 
extremity of the trunk is usually modified to form a structure 
known as the trochal disc (Fig. 72) which is one of the most 
characteristic structures of the rotifers. This disc bears cilia in 
highly variable arrangement and is capable of retraction within 
the anterior region of the trunk. The mobile posterior extremity 
is usually recognizable as a tail with some sort of adaptation for 
Fig. 72.-A rotifer, Hydatina senta, viewed from ventral surface. (Ori'g.) 
attachment, though in many forms (Asplanchna, Pedalion, etc.) 
this is lacking. Individuals of some species are permanently 
attached. In this case (Melicerta, Floscularia), they may secrete 
a tube or may form a case, partly of foreign matter, within which 
the rotifer can withdraw when disturbed. In free-swimming 
forms, the trochal disc is the chief organ of locomotion though at PHYLUM TROCHELMINTHES 
127 
times the rotifer may loop along like a leech and in some instances 
outgrowths from the body wall form hollow limb-like appendages 
(as in Pedalion) by means of which the animal skips through the 
water. 
Trochal Disc.-The plan of ciliation of the trochal disc is 
usually reducible to various modifications of one or two bands 
of cilia. In the simplest condition, a single circle of cilia edges 
the margin of the circular disc. Distortion of this circle at 
certain points results in the formation of either blunt ciliated 
lobes (Floscularia) or long ciliated arms (Stephanoceros) but in 
each of these instances the cilia are arranged in a single continu- 
ous row. In many instances, a second band of cilia is introduced 
parallel to the first and in some of these also the ciliated bands 
may become lobed. Almost always when two rows of cilia are 
present the mouth occurs between the two on the ventral surface 
of the disc. In Trochosphaera, which lacks a trochal disc, there 
is an equatorial preoral circle of cilia with a few cilia postoral 
in distribution. This condition, as well as the general internal 
organization of Trochosphaera, corresponds very closely to 
conditions found in the trochophore larvae of higher inverte- 
brates. It is because of this close agreement, that some contend 
that rotifers phylogenetically represent trochophore larvae 
which have attained full sexual development precociously. 
Digestive System.-The digestive system comprises a mouth 
located on the ventral surface of the trochal disc, an esophagus 
with its elaborate mastax for triturating food, a stomach and an 
intestine which opens near the posterior extremity through an 
anus. The mastax is highly characteristic of the rotifers, but is 
subject to considerable modifiability of form. In many instances, 
there are three heavily chitinized parts discernable; an incus and 
two mallei, but either of these elements may be wanting. By 
action of the muscular esophageal wall, the parts of the mastax 
are worked together as an effective crushing organ which reduces 
the food ready for digestion, when it passes on into the stomach. 
A pair of digestive glands is usually associated with the stomach. 
Both the stomach and intestine are lined with cilia. In the genus 
Asplanchna, the stomach ends blindly for there is no intestine. 
Excretory System.-Coiled nephridial tubes lying in the 
body cavity bear flame-cells at the ends of their lateral 
branches. These excretory tubules discharge their waste into 
the cloaca. 128 
INVERTEBRATE ZOOLOGY 
Nervous System.-A single ganglion, usually located in the 
anterior dorsal region of the body, gives off nerves to the sur- 
rounding organs and from it a pair of lateral nerve trunks pass 
posteriorly to the tail. One or more simple pigment spots, and 
sometimes more complicated eyes, are often associated with the 
brain and aside from tactile hairs represent about the only 
development of sensory apparatus. 
Reproduction.-Rotifers are bisexual, though usually the male 
is much reduced and in some instances has never been observed. 
At times, the male lives as a parasite on the female. Since they 
have no digestive organs, many males are very short lived. In 
describing rotifer structure, the body of the female is considered 
as typical. Most rotifers are oviparous or ovoviviparous, 
though some (Asplanchna, Philodina) are viviparous. Partheno- 
genetic development as commonly found here frequently .involves 
two different sizes of eggs, of which the larger produce only 
females and the smaller only males. Insemination of the female 
to produce fertilized eggs seems to be accomplished by perfora- 
tion of the body wall at any point to introduce spermatozoa into 
the body cavity. Fertilized eggs thus produced differ from the 
parthenogenetic eggs in the possession of heavy, resistant shells, 
and are designated as winter eggs. 
Sexual Organs.-The female organs usually consist of a single 
ovary (two in Philodina) and a vitellarium of highly variable 
form, though in a few cases there are two gonads with no dis- 
tinction between ovary and vitellarium. In parthenogenetic 
development, the young are produced within the body of the 
female and are usually liberated by rupture of the body wall of 
the parent. Winter eggs are usually carried inside the body some 
time before they are discharged through the oviduct and then lie 
dormant for a period before the young are hatched. Organiza- 
tion of the male is frequently simple, due to the degeneration of 
the digestive organs which occur as degenerate strings of tissue, 
near the posterior region of which the male gonad is attached. 
A special copulatory organ in the form of a protrusible cirrus 
is often present. 
Nuclear Constancy.-For a number of rotifers, it has been 
shown that each organ is built of a fixed number of cells or at least 
contains a constant number of nuclei. An especially thorough 
study along this line has been published by Eric Martini for 
Hydatina senta, each individual of which contains a total of 959 PHYLUM TROCHELMINTHES 
129 
nuclei distributed in fixed numbers through the various organs 
and tissues of the body. 
Class Gastrotricha 
Though the Gastrotricha are here included as a class along 
with the Rotifera under the phylum Trochelminthes, their 
relationships with the rotifers are far from firmly established. 
Some zoologists maintain that the Gastro- 
tricha are more directly related to the 
Nematoda. They are rarely more than 0.5 
mm. in size and though they occur relatively 
frequently in protozoan and rotifer cultures, 
their small size and rapid movements render 
close examination difficult. Though widely 
distributed, they are restricted to fresh-water. 
In most instances, there is a head set off 
from the body proper by a slight constric- 
tion. The body is flattened on the ventral 
surface and convex on the dorsal. The 
ventral surface is furnished with two longi- 
tudinal bands of cilia near the median line, 
by the action of which locomotion is accom- 
plished. The body proper may be either 
smooth or covered with plates, spines, 
or bristles. The mouth, which is borne at 
the anterior extremity of the head, is usually 
surrounded by a circle of delicate oral bristles. 
In addition, there are frequently lobes on 
the sides of the head from which groups of 
sensory hairs protrude. 
The internal organization (Fig. 73) is rela- 
tively simple. The digestive tract runs as 
a straight tube through the axis of the body. 
In the body musculature, only a few longi- 
tudinal strands of muscle but no circular 
muscles have been demonstrated. The brain 
occupies much of the head region. The 
excretory organs are protonephridia. 
Only females are known, yet it is uncertain whether these are 
truly parthenogenetic or are hermaphroditic and the male 
gonads have never been observed. The ovary occupies the 
Fig. 73.-Chaeton- 
otus maximus, one of 
the Gastrotricha, in 
ventral view. Ex, kid- 
ney; M, muscles; B, 
brain; E, egg; O, esoph- 
agus; I, intestine; Ov, 
ovary. X400. (After 
Zelinka, from Ward and 
Whipple's Fresh-water 
Biology, reprinted by 
permission of John 
Wiley and Sons, Inc.) 130 
INVERTEBRATE ZOOLOGY 
posterior region of the body cavity and as fully formed eggs 
push anteriorly in the body cavity they frequently distort the 
shape of the gravid female. 
Chaetonotus and Lepidoderma are the most representative 
genera in the North American fauna. 
References 
(See general references cited at close of Chapter I) 
Harring, H. K. 1913. Synopsis of the Rotatoria. U. S. Nat. Museum, 
Bull. 81. 
Hudson, C. F. and Gossfi, P. H. 1889. The Rotifera or Wheel Animal- 
cules. London. 
Jennings, H. S. 1896. The Early Development of Asplanchna Herrickii 
DeGuerne. Bull. Mus. Comp. Zool., Harvard, Vol. 30, No. 1. 
. 1900. Rotatoria of the United States with Especial Reference 
to those of the Great Lakes. Bull. U. S. Fish Comm., 19: 67-104. 
Martini, E. 1912. Studien uber die Konstanz histologischer Elemente, 
III. Hydatina senta. Zeitschr. f. Wissensch. Zool., 102: 425-645. 
Shull, A. F. 1918. Effect of environment upon inherited characters in 
Hydatina senta. Biol. Bull., 34: 335-350. 
Stokes, A. C. 1887. Observations on Chaetonotus. The Microscope, 7: 
1-9, 33-43. 
. 1896. Aquatic Microscopy for Beginners. 3d. Ed.: 178-193. CHAPTER IX 
PHYLUM COELHELMINTHES (ANNELIDA) 
The Coelhelminthes or Annelida comprise a number of worm- 
like forms which possess a coelom and are usually segmented. 
This group contains many of the forms included under the 
"Vermes" of Linnaeus and other early workers. It has been 
said frequently that the "Vermes" constituted a waste basket 
into which forms which could not be placed elsewhere were 
dumped. Much of the advance in modern classification of the 
animal kingdom has centered around the recognition of groups 
of worm-like forms bearing common characteristics and establish- 
ing for them rank as independent phyla and classes. Thus the 
Plathelminthes, Nemathelminthes, Molluscoidea, and Trochel- 
minthes have been removed from the old group "Vermes" and 
each has been elevated to the rank of an independent phylum. 
There yet remains an assemblage of segmented worms, arrow 
worms, and gephyrean worms which seem to have enough features 
in common to warrant their retention for the present in a single 
major group of the animal kingdom. These forms, which 
collectively are known as the Coelhelminthes, agree in possession 
of a coelom and in the general type of the nervous system. An 
excretory system is lacking in members of the class Chaetognathi 
and in some representatives of the other groups, but character- 
istically a metanephridial system is found. The Gephyrea, 
which lack segmentation, agree with the typical annelids in 
passing through a larval stage known as a trochophore. 
Class Chaetognathi 
The chaetognaths, or arrow-worms, are small forms with body 
shape admirably adapted for life at the surface of the ocean 
where they move about with great rapidity in search for food. 
The body is divided into three segments; head, trunk, and tail. 
Each of these divisions is separated from adjoining segments by 
a transverse septum and a longitudinal mesentery divides each 
coelomic cavity into a right and a left part. From the sides of 
131 132 
INVERTEBRATE ZOOLOGY 
the head (Fig. 74) there extend a series of bristles which act as 
jaws in seizing prey, hence the name Chaetognathi, or bristle 
jaws. In preserved specimens, these jaws are frequently folded 
close against the sides of the head. 
Locomotion is accomplished by horizontal extensions of the 
body called fins, surrounding the tail and occurring as one or two 
pairs of lateral fins on the trunk. The central nervous system 
consists of a cerebral ganglion on the dorsal surface of the esopha- 
gus which is connected with a ventral ganglion near the middle 
of the trunk by two long 
esophageal connectives. A 
pair of eyes lie close behind 
the brain with which they 
are connected by a pair of 
nerves. Papillae scattered 
over the surface of the body 
probably have a tactile 
function. A peculiarly 
modified region o n the 
dorsal surface just behind 
the head has been inter- 
preted as olfactory. 
The individuals are her- 
maphroditic. Ovaries 
occur in the posterior part 
of the trunk cavity and communicate with the exterior 
through an oviduct on each side which opens laterally 
near the posterior extremity of the trunk segment. The testes 
are contained in the coelomic cavities of the tail segment. Sper- 
matozoa, liberated directly into the cavity, are carried out 
through a delicate sperm duct which is frequently dilated near 
its extremity to form a seminal vesicle. 
In development, the fertilized egg undergoes cleavage to form 
a typical gastrula. Two lateral folds of the entoderm extend 
down into the archenteron dividing this cavity into three parts. 
Of these, the two lateral cavities, lying between each fold and 
the body wall, later become the coelomic cavities, while the 
central space between the two folds constitutes the mesenteron. 
During the gastrula stage, two entoderm cells opposite the blasto- 
pore become recognizable as the rudiments of the gonads. By 
a single division, these two cells form two pairs of which the 
Fig. 74.-Head of Sagitta showing bristle- 
like jaws extended. (Orig.) PHYLUM COELHELMINTHES (ANNELIDA) 
133 
anterior pair later forms the female gonads and the posterior 
pair the male gonads. 
Sagitta, the arrow-worm, is the most characteristic genus of 
the Chaetognathi of which Spadella and Krohnia are other 
recognized genera. 
Class Chaetopoda 
The Chaetopoda are among the most characteristic of Coel- 
helminthes. Metamerism is sharply marked. The coelom is 
divided into successive chambers by transverse septa which 
correspond with the external constrictions of the body wall and 
even internal organs such as nervous, excretory, and circulatory 
systems bear the marks of metamerism. Most of the segments 
bear bristles or setae which, by their number and arrangement, 
give a basis for classification into subclasses. In the Polychaeta, 
the setae occur in outgrowths of the body wall called parapodia 
which function in locomotion. 
The digestive system, though a straight tube, usually shows 
specialization into regions. The mouth typically lies on the 
ventral surface very near the anterior extremity beneath the 
terminal somite called the prostomium. In the two subclasses, 
the prostomium is highly variable in degree of specialization. 
Among the Oligochaeta it is frequently a small, inconspicuous 
lobe with generalized sense organs similar to those found on 
other segments of the body while among the Polychaeta it more 
frequently bears highly specialized tactile organs and eyes. 
The circulatory system consists of at least two main longi- 
tudinal trunks, one dorsal and the other ventral, connected by 
lateral vessels in each segment. Frequently, additional longi- 
tudinal vessels occur. Some of the lateral vessels in the anterior 
region of the body are specialized as pumping organs or hearts. 
These, with the dorsal vessel, propel the blood through the system 
by their pulsations. 
The central nervous system consists of a ventral chain with a 
ganglion in each segment. This chain, which is really composed 
of two fused cords, frequently shows its double nature in its 
ladder-like appearance and in cross-section the arrangement of 
cells and fibers give still further evidence. In the anterior region 
of the body, the two cords separate to pass around the pharynx 
on the dorsal surface of which occurs the largest ganglion, called 
the suprapharyngeal ganglion or brain. 134 
INVERTEBRATE ZOOLOGY 
Excretion is by means of metanephridia in the adults and 
protonephridia in larvae of forms which have the trochophore. 
Characteristically, each somite is provided with a pair of meta- 
nephridia the funnel or nephrostome of each of which is attached 
to the posterior septum while the tubule penetrates the septum 
and passes into the cavity of the adjacent somite before opening 
to the outside through the nephridiopore. Modified nephridial 
Fig. 75.-Different relations of nephridia and sexual ducts in chaetopods 
{after Goodrich'). I, hypothetical primitive condition, gonoducts and protone- 
phridium independent; II, ciliated grooves discharge into duct of protonephri- 
dium as in Phyllodoceids and Goniads; III, ciliated grooves and metanephridium 
open independently as in Dasybranchus; IV, ciliated grooves open into canal of 
metanephridium as in Syllids, Spionids, etc. {From Hertwig's Manual of 
Zoology by Kingsley, courtesy of Henry Holt and Co.) 
ducts (Fig. 75) are frequently utilized for the discharge of the 
germ cells. Primitively, separate ducts called ciliated grooves 
or coelomoducts serve for the discharge of the germ cells but 
these frequently become associated with the nephridia, the tubules 
of which then becomes the gonoducts. 
Embryology.-Annelid eggs undergo cleavage of determinate 
type which results in the formation of a definite pattern in the 
arrangement of the blastomeres. From polar view the cells are 
arranged in the form of a cross. So characteristic is this pattern 
(Fig. 10) that the term "annelidan cross" has been applied to PHYLUM COELHELMINTHES {ANNELIDA) 
135 
it. Later development results in the formation of a trochophore 
except in fresh-water and terrestrial forms. The structure and 
transformation of this larva will be described later. Asexual 
reproduction is not uncommon. In some forms, there is but 
slight specialization of the individual somites. This condition, 
known as homonomous metamerism, facilitates asexual repro- 
duction and frequently leads to the formation of a chain of 
individuals through the differentiation of the metameres of a 
single worm. 
Dimorphism.-Among the polychaetes, two different types of 
individuals are frequently encountered in the same species, 
the atoke or sexless and the epitoke or 
sexual individual. Before their relation- 
ships were understood, these phases have 
been frequently considered as distinct 
species and even as different genera. 
Some forms which reproduce only sexu- 
ally have immature young atokes which 
develop from the larva and undergo con- 
pi cuous changes to the epitoke form 
upon approach of sexual maturity. The 
"Heteronereis" and "Nereis" forms in 
species of the genus Nereis illustrate this 
condition well. In some instances, the 
atoke (Fig. 76) has the power of budding 
to produce sexual epitokes which become 
separated as free individuals. This is 
the condition found in the palola worm 
(Eunice viridis) the epitoke of which 
appears in extreme numbers in the 
tropical South Seas and is relished by the Samoans as a food. 
Fig. 76.-A Palola worm, 
Eunice viridis (Gray) show- 
ing differentiation of body 
into an enlarged atoke and 
a posterior epitoke. {After 
Woodworth). 
Subclass Polychaeta 
The sexes are separate in the polychaetes. Usually, the anterior 
somites are rather highly specialized to form a distinct head 
bearing eyes and tactile organs. The somites have outgrowths 
from their lateral margins which are known as parapodia and 
serve as rudimentary appendages. Each parapodium is sup- 
ported by numerous bristles or setae which are arranged in two 
bundles. The lobe of the parapodium surrounding the ventral 
bundle of setae is called the neuropodium while that surrounding 136 
INVERTEBRATE ZOOLOGY 
the dorsal bundle is the notopodium. In Nereis, the notopodia 
are supplied with numerous blood vessels and function as gills. 
Among other polychaetes, gills are frequently developed as long 
filamentous outgrowths, either along the sides of the body or 
restricted to certain areas. Each lobe of the parapodium 
frequently bears a fleshy sensory projection termed the cirrus. 
In some instances, dorsal scales covering the back of the worm 
represent modified dorsal cirri. 
Practically all polychaetes are marine and in development pass 
through the trochophore stage. In organization, the polychaetes 
represent a more primitive 
condition than the oligo- 
chaetes. Though the oligo- 
chaetes are frequently simpler 
in structure, their simplicity 
is the result of degeneracy for 
it seems probable that in 
becoming adapted to life in 
fresh-water or to the terres- 
trial existence they have de- 
parted from the generalized 
structure and mode of develop- 
ment which characterize the 
more primitive members of 
the class. 
The trochophore (Fig. 77, 
A) is typically pear shaped or 
in the form of two cones 
joined by their bases, one 
dorsal and the other ventral 
in position. Surface ciliation 
of this larva may follow any 
one of a number of patterns. A tuft of cilia at the apical 
plate and a preoral circle near the equator of the larva 
are the most constant, but other bands may also appear. The 
digestive system consists of a mesenteron which communicates 
with the mouth opening through an esophagus and with a 
terminal anus through an intestine. The region between the 
digestive tube and the outer body wall is filled with a gelatinous 
substance through which run strands of muscle and nerve and 
the tubules of the protonephridial system. Near the posterior 
Fig. 77.-The development of Poly- 
gordius. A, young trochophore; B, elonga- 
tion of posterior cone; C, D, stages in 
transformation of trochophore; E, anterior 
extremity of young Polygordius following 
metamorphosis. (After Fraipont). PHYLUM COELHELMINTHES {ANNELIDA) 
137 
extremity of the early trochophore, there usually occur a pair 
of cells called the teloblasts which are the forerunners of the 
mesoderm. As the larva elongates, these teloblasts continue 
to divide, forming bands of mesoderm cells on either side of the 
digestive tract. With the elongation of the posterior cone of the 
trochophore in the transformation of the larva into the adult 
worm these mesoderm bands become divided into primitive 
segments (Fig. 77, C) within which the coelomic cavities later 
make their appearance. The segments of the young worm thus 
formed are usually provided with provisional setae which are 
later thrown off and replaced by the permanent setae. 
Both free-living and sessile polychaetes are found. Modifica- 
tions of body form associated with these differences in habits have 
furnished a basis for the separation of two orders. 
I. Order Errantia 
The Errantia are free-swimming forms with a well-developed 
head but all the remaining segments practically homonomous and 
bearing parapodia of approxi- 
mately uniform character. The 
pharynx is evertible and frequently 
bears a pair of formidable jaws 
which are used in capturing prey. 
Nereis, the clam worm, is one of 
the commonest examples of this 
order. Aphrodita has the dorsal 
surface so covered with bristles 
as to warrant the common name 
sea mouse. Lepidonotus bears 
twelve pairs of broad overlapping 
scales on its back. Syllis repro- 
duces by means of lateral clusters 
of buds and Autolytus undergoes 
asexual reproduction through the 
formation of buds at the posterior 
extremity. 
Fig. 78.-Lateral view of a ser- 
pulid worm, Spirorbis, removed from 
its shell. The ovoid bodies within 
the club-shaped operculum are 
developing embryos. {After 
Claparkde). 
II. Order Sedentaria 
Members of the order Sedentaria are tube-dwellers. Some 
live in burrows in the sand (Arenicola), others form membranous 
tubes (Myxicola, Manyunckia), and still others live in calcified 138 
INVERTEBRATE ZOOLOGY 
tubes within which they withdraw (Hydroides, Spirorbis) and 
close the opening with a modified tentacle, the operculum. The 
Sedentaria lack the jaws characteristic of the free-moving 
Errantia and display much greater diversity in structure of the 
anterior and posterior regions ot the body. Numerous filamen- 
tous gills and tentacles frequently adorn the head and anterior 
body somites which usually protrude from the tube or burrow 
(Fig. 78) while the parts constantly encased have weakly devel- 
oped parapodia. 
Subclass Oligochaeta 
The oligochaetes are as characteristically terrestrial and fresh- 
water inhabitants as are the polychaetes marine. In many ways, 
they bear evidences of degeneracy as a consequence of adaptation 
to their environment. Pelagic larvae and parapodia are entirely 
lacking in all members of the subclass while gills occur in only a 
few forms and the sensory apparatus represents a very low stage 
of specialization. Setae occur in several distinct orders of 
arrangement. The sexes are never separate. The male and 
female gonads occur in different segments. In the Naididae 
and Aelosomatidae, asexual as well as sexual reproduction 
occurs. 
Near the anterior end of the worm, usually not far removed 
from the openings of the gonoducts, the body wall of a number of 
somites is supplied with numerous glands which in the height of 
sexual development forms a thickened collar-like band over the 
dorsal and lateral surfaces of the body known as the clitellum. 
This clitellum produces secretions which harden to form a capsule 
or cocoon for containing the eggs after they are laid. In the 
earthworms, fertilization is reciprocal. During copulation a 
spermiducal pore of each individual is opposite the opening of a 
receptaculum seminis of the other so that sperm cells of each 
individual pass into the receptaculum of the other, thereby 
accomplishing a cross fertilization. 
Various systems of subdividing the Oligochaeta have been 
proposed, most of which have as a basis either the habits or 
structural characters correlated with aquatic or terrestrial habits. 
Members of the order Microdrili are small oligochaetes of 
relatively few segments and usually aquatic in habits. Eyespots 
are frequently present. (Tubifex, Nais, Dero, Chaetogaster, 
Aelosoma, Pristina, Mesenchytraeus.) PHYLUM COELHELMINTHES {ANNELIDA) 
139 
In the order Megadrili are found the larger oligochaetes which 
are commonly known as earthworms. The bodies of these con- 
tain numerous segments. (Moniligaster, Perichaeta, Microsco- 
lex, Diplocardia, Helodrilus, Lumbricus.) 
Fig. 79.-Organization of a leech, Placobdella 'parasitica. I-XXVII, somites; 
phg, pharyngeal glands; oe, esophagus; s, atrium or spermatophore sac; v.s, 
seminal vesicle; t, testes; ov, ovary. Redrawn from Whitman in Ward and Whipple's 
Fresh-water Biology and reprinted by permission of John Wiley and Sons, Inc.) 
Class Hirudinea 
The Hirudinea or leeches are annelids with a fixed number of 
somites (generally 34) but superficially each somite is subdivided 
by constrictions (Fig. 79) into a number of annidations. The 
coelom is very greatly reduced but the small pouches furnish the 
basis for the determination of the number of somites. With 140 
IN VERTEBRA TE ZOOLOG Y 
the exception of one genus (Acanthobdella) leeches have no setae. 
The posterior extremity bears a sucker on its ventral surface and 
usually a second sucker is developed around the mouth. Both 
of these are organs of attachment and in addition the oral sucker 
aids in the ingestion of food. In addition to a graceful undulating 
swimming, free in the water, leeches may also progress by fixing 
the anterior sucker to some object then drawing the body into a 
loop and fixing the posterior sucker near the same spot. 
Slight development of the coelom and the dorso-ventral flat- 
tening of the body give leeches a distinct "flatworm" appearance. 
Segmentation, presence of a coelom, development of a clitellum, 
presence of a distinctly annelid type of nervous system and 
general organization, however, demonstrate close relationships 
with the Oligochaeta. Michaelson proposes a group Clitellata 
to include the Oligochaeta and Hirudinea as two coordinate 
groups. 
The digestive system does not lie loosely in a spacious coelom 
as does that of the Chaetopoda for the coelom is largely 
encroached upon by a parenchyma which leaves a system of 
blood-filled sinuses in addition to the definite circulatory system. 
The terminal buccal chamber opens into a muscular pharynx 
which, in turn, leads by way of an esophagus into a stomach or 
crop. This latter may be either a straight tube or may give rise 
to from one to twenty pairs of lateral diverticula before emptying 
into the intestine. A small rectum leads to the anus on the dorsal 
surface near the posterior extremity. 
A metanephridial system similar to that found in the Oligo- 
chaeta serves for excretion. Usually, not more than seventeen 
pairs of nephridia occur for they are lacking from the somites of 
both extremities and from some of the clitellar somites. The 
central nervous system consists of a brain and a ventral chain of 
ganglia of which there are frequently twenty-three. 
Leeches are hermaphroditic. Fertilization occurs either by 
reciprocal copulation or through the implantation of spermato- 
phores on the skin. Spermatozoa escape from these and pene- 
trate the tissues to the ovarian sacs where fertilization 
occurs. Development from the egg is direct. 
Hirudinea get their vernacular name of blood-suckers from 
the fact that many species are permanent or temporary ecto- 
parasites on the bodies of other animals. Many species, and 
especially the young, are predaceous in habit, feeding upon other PHYLUM COELHELMINTHES (ANNELIDA) 
141 
small organisms, and resort to the parasitic habit only when 
opportunity is offered. Most leeches are inhabitants of fresh- 
water though some are marine and a few are terrestrial. Those 
which feed upon blood have either three jaws supplied with sharp 
teeth (Gnathobdellida) which lance the skin of the host or a 
conical proboscis evertible from the pharynx (Rhynchobdellida) 
for piercing. Glandular secretions from the leech hinder coagu- 
lation of the blood and render its wounds difficult to staunch. 
Two orders are commonly recognized, the Rhynchobdellida 
and the Gnathobdellida. Of these, the former are jawless 
while the latter have jaws. Species of the genus Piscicola which 
live on fishes, Glossiphonia, and Placobdella are characteristic 
of the first order while the medicinal leech, Hirudo medicinalis, 
and Macrobdella which so commonly attacks bathers are 
examples of the Gnathobdellida. 
Class Archiannelida 
As the name signifies, the Archiannelida seem to represent 
a primitive type of annelid organization which may have con- 
siderable significance in solving the problem of the origin of the 
higher annelids. The body, which shows only slight indica- 
tion of segmentation externally, has a coelom completely divided 
into somites. Both parapodia and setae (Fig. 77, F) are lacking. 
The nervous system is distinctly more simple than that character- 
istic of other annelids for it remains in direct contact with the 
epidermis and shows no centralization to form ganglia. In the 
genus Protodrilus, there are two ventral nerve cords connected 
by transverse commissures but in Polygordius there is a single 
cord. Representatives of both of these genera are exclusively 
marine. The prostomium bears a pair of tentacles and in addi- 
tion to these a pair of ciliated grooves are the only structures 
which seem to have a sensory function. 
In the development of Polygordius, a typical trochophore 
(Fig. 77) occurs, the formation and metamorphosis of which have 
been worked out in great detail. 
Appendix to the Archiannelida 
Dinophilus and some other simple worm-like forms are of 
questionable systematic position. Some zoologists maintain 
that they show possible relationships with the Archiannelida while 
others consider them as more closely related to the Trochel- 142 
INVERTEBRATE ZOOLOGY 
minthes. Members of the genus Dinophilus are minute marine 
worms which live among seaweeds. The body consists of a head, 
five or six trunk segments, and a tail segment. The adult worm 
rather closely resembles the larva of marine polychaetes. 
Class Gephyrea 
The Gephyrea are marine worms which differ from the remain- 
ing annelids in the lack of segmentation, parapodia, and setae. 
Development involves a modified trochophore larva. The 
undivided coelom contains a complete digestive system the intes- 
tine of which extends posteriorly some distance then coils back on 
itself to a dorsal anal opening toward the anterior extremity of 
the body. The extended anterior extremity of the body is pro- 
vided with tentacles or a lobed tentacular fold within which the 
mouth is located. The entire anterior region is capable of inver- 
sion within the body. 
A single pair of nephridia comprise the excretory apparatus 
and serve as gonoducts. The nervous system originates in the 
anterior extremity as a dorsal cerebral ganglion which is joined 
with the ventral longitudinal nerve cord by a pair of lateral 
branches. The ventral nerve cord bears no ganglia but gives off 
lateral branches to the body wall and to the internal organs. 
Though the sexes are usually distinct, persistent gonads 
are not present. The germ cells have their origin in masses or 
ridges of cells in the lining of the body cavity and either undergo 
full development in this location or are discharged early into the 
coelom where development is completed. 
Sipunculus and Phascolosoma are characteristic genera of 
the Gephyrea. 
References 
(See general references cited at close of Chapter I) 
Claperede, E. 1868. Les Annelides Chetopods du Golfe de Naples. 
Ehlers, E. 1864-1868. Die Borstenwiirmer (Annelida, Chaetopoda) 
nach Systematischen und Anatomischen untersuchungen. Leipzig. 
Fraipont, J. 1887. Le genre Polygordius. Fauna u. Flora d. Golfes 
von Neapel, Monograph 14. 
Gamble, F. W. and Ashworth, J. H. 1900. The Anatomy and Classifi- 
cation of the Arenicolidae with some Observations on their Post-larval 
Stages. Quart. Jour. Mier. Sci., 43: 419-569. 
Gerould, J. H. 1906. The Development of Phascolosoma. Zool. 
Jahrb., 23: 79-162. PHYLUM COELHELMINTHES (ANNELIDA) 
143 
Hatschek, B. 1878. Studien uber Entwicklungsgeschichte der Anneliden. 
Arbeiten Zool. Inst. Wien, 1: 277-404. 
Hertwig, O. 1880. Die Chaetognathen, Ihre Anatomie, Systematik, und 
Entwicklungsgeschichte. Jena. 
Moore, J. P. 1901. The Hirudinea of Illinois. Bull. Ill. St. Lab. Nat. 
Hist., 5: 479-546. 
. 1905. Hirudinea and Oligochaeta collected in the Great Lakes 
Region. Bull. U. S. Bur. Fish., 25: 157-171. 
Quatrefages, A. de, 1865. Histoire naturelie des anneles marins et 
d'eau douce. Paris. 
Sedgwick, W. T. and Wilson, E. B. 1904. An Introduction to General 
Biology. New York, Holt. 
Vejdovsky, F. 1884. System und Morphologic der Oligochaeten. 
Prague. CHAPTER X 
PHYLUM MOLLUSCOIDEA 
The phylum Molluscoidea contains three classes which agree 
in the possession of a ridge called the lophophore at the anterior 
extremity of the body bearing a crown of ciliated tentacles. The 
three classes; Polyzoa, Brachiopoda, and Phoronida, present so 
many individual peculiarities that few statements may be made 
that would apply equally to the organization of the members of 
all three classes. Consequently, the structure and character- 
istics of the individual classes will be discussed separately. 
Fig. 80.-Diagrams to show contrast in general organization in; A, an endo- 
proct bryozoan; B, an ectoproct bryozoan. The digestive system is shown in 
solid black. Compare the position of the anal opening in the two figures. 
(Oriff.) 
Class Polyzoa 
Individuals of the class Polyzoa (or Bryozoa) bear very 
close superficial resemblance to hydroid polyps from which they 
are readily distinguishable because the polyzoan zooids possess a 
complete alimentary tract terminating in an anus near the 
144 PHYLUM MOLLUSCOIDEA 
145 
anterior extremity of the body. The position of the anal opening 
with reference to the circle of tentacles serves as a basis for the 
discrimination between the two orders (Fig. 80) which show 
extreme differences in internal structure. In the order Ecto- 
procta, the anus occurs outside the tentacular ring while in the 
Endoproctoa it is within the circle formed by the tentacles. The 
lack of a coelom in the Endoprocta renders their relationship to 
the other Polyzoa open to question. Some investigators have 
considered that the Endoprocta possibly bear relationship to the 
Rotifera. The majority of the Polyzoa are colonial. The 
individuals of a colony are bound together by an organic con- 
necting material. The individuals are in many instances cov- 
ered with a gelatinous, horny, or calcified layer forming an 
exoskeleton. Polyzoa are most abundant in salt-water though a 
number of genera appear in fresh-water. 
I. Order Ectoprocta 
The Ectoprocta are colonial forms which frequently attain 
considerable size. Individuals of a colony are variously arranged 
in branching pattern (Bugula, Plumatelia), in flat mats (Crista- 
tella), as encrusting layers (Microporella, Cribrillina), or as 
solid gelatinous masses (Pectinatella), which may attain a size 
of a foot or more in diameter. The individual zooids uniting to 
form these different types of colony assume a number of dis- 
tinctly different shapes. The lophophore is capable of retraction 
within the anterior part of the zooid through the agency of special 
retractor muscles. In addition to the action of this introvert, 
the entire zooid is frequently able to withdraw into the interior 
of the colony. In some instances, either an operculum or a series 
of lobes is drawn into the aperture to close it when the zooid 
is retracted. 
Modified individuals occur in many Ectoprocta. Avicularia 
are modified zooids shaped much like a bird's head, the beak of 
which is capable of grasping objects and holding them until 
they disintegrate. The food fragments are then secured by the 
tentacles. Vibricularia, another type of modified individual, are 
long whip-like structures which, in their development, seem to be 
modified avicularia. 
The funiculus is a double strand of tissues which passes from 
the bend in the alimentary canal through the coelom to the 
aboral extremity of the zooid. In addition to the budding which 146 
IN VERTEBRA TE ZOOLOG Y 
gives rise to the colony formation, sexual reproduction may also 
take place. Ovaries and spermaries make their appearance 
either in the lining of the coelom or in the tissues of the funiculus. 
The gonads dehisce into the coelom where fertilization takes 
place. In some species special chambers, called ovicells, are 
provided for containing the developing embryos. Following 
cleavage, a free-swimming larva of variable form in the different 
species makes its appearance. This larva undergoes a trans- 
formation to form a zooid from which a colony later develops by 
budding. Statoblasts, or internal buds, are characteristic of 
many fresh-water ectoprocts. These are surrounded by 
chambers which upon drying become filled with air and serve to 
float the statoblasts. In the fresh-water genera, Pectinatella 
and Cristatella, the free larval stage has been retained. 
II. Order Endoprocta 
In members of this order, the anal opening occurs within the 
circle of tentacles. With the exception of one genus, Urnatelia, 
the entire group is marine. The body is usually cup-shaped, 
enclosing at its open end a cavity called the vestibule (Fig. 80, A), 
which contains the mouth and the anus. The rim of this 
cavity bears the tentacles which are capable of being withdrawn 
into the vestibule. The space between the alimentary canal and 
the body wall is filled with a gelatinous matrix. This lack of an 
undisputed coelom, together with the fact that the excretory 
system is protonephridial, furnish ground for doubting any close 
relationship between the Endoprocta and the Ectoprocta. 
Pedicellina, Urnatelia, Loxosoma are characteristic genera. 
Class Brachiopoda 
Because of the presence of a bivalve shell, brachiopods are 
frequently confused with molluscs. The shell of the brachiopod 
is, however, composed of a dorsal and a ventral valve (Fig. 81) 
while the valves of the bivalve molluscs are lateral. In the 
brachiopods, the valves are articulated in their posterior regions 
and from the posterior end of the animal a stalk or peduncle for 
attachment is frequently developed. The valves are lined by a 
mantle by whose action the shell is secreted. The bristles borne 
in the edge of the mantle are of the same type as those found in 
annelids and seem to point to a relationship between brachiopods PHYLUM MOLLUSCOIDEA 
147 
and annelids. The ventral shell in many cases bears a short 
beak-like projection posterior to the hinge and it is through this 
that the peduncle passes. The shells so closely resemble the 
most primitive type of oil lamp that the common name "lamp- 
shell" is very generally applied to shells of this group. 
In members of the order Inarticulata (Lingula, Crania, and 
Discina), the foregoing description does not apply for the two 
valves are similar and the hinge is wanting. In the Articulata, 
where a- hinge is present, the valves are not held open by an 
elastic hinge ligament as in the Acephala of the molluscs but 
both opening and closing of the valves is accomplished by muscu- 
lar action. Closure of the shell is by means of a pair of adductor 
muscles which are attached to the dorsal shell but unite to form 
Fig. 81.-Semidiagrammatic sagittal section of a brachiopod (Magellania 
lenticularis). (Redrawn from Parker and Haswell, with the permission of Mac- 
millan Co.) 
a single muscle before reaching their insertion on the ventral 
valve. Two pairs of divaricator muscles pass between the ventral 
valve and that part of the dorsal valve posterior to the hinge. 
By their contraction the valves are opened. From each valve 
a pair of muscles known as the adjustors pass to an insertion on 
the peduncle. It is through contraction of these last muscles 
that the animal is able to shift the position of the entire body. 
Much of the space within the shell is occupied by a pair of 
conspicuous, spirally coiled arms or lophophores. In the order 
Articulata, the dorsal valve frequently bears a calcareous loop 
which supports the lophophore. Each arm on its outer margin 
bears a longitudinal groove bordered by a row of small tentacles. 148 
INVERTEBRATE ZOOLOGY 
Water currents produced by the cilia on the tentacles and in the 
groove carry food particles toward the mouth. 
The U-shaped digestive tract consists of a mouth opening in 
the middle of the lophophore, a dorsally directed gullet which 
empties into an expanded stomach, and from this a ventrally 
directed intestine which ends blindly except in members of the 
order Inarticulata. Cilia line the entire digestive tract. 
Two transverse septa divide the coelom into three somites but 
the shortening of the chief axis of the body has been accompanied 
by a coiling of the digestive tube and consequently the arrange- 
ment of the septa is somewhat confused and difficult to observe. 
The coelomic cavities extend into the arms and the mantle 
lobes. One or two pairs of nephridia communicate with the 
coelomic pouches and serve as both excretory and reproductive 
ducts. The gonads are borne chiefly in the coelomic cavities of 
the mantle. The sexes are usually separate. 
Brachiopods are exclusively marine. Though represented by 
relatively few living species, they reached an extreme state of 
species formation in the Silurian and Devonian periods. Tere- 
bratulina, Terebratula, and Waldeheimia are characteristic 
modern genera of the Articulata. 
Class Phoronida 
The relations of members of the single genus Phoronis have 
been much under discussion among zoologists. Worm-like in 
form, these marine organisms dwell in membranous or leathery 
tubes. The body is long, cylindrical, and unsegmented, at one 
extremity bearing numerous ciliated tentacles arranged in the 
form of a lophophore as characteristic of members of the phylum 
Molluscoidea. Both mouth and anus occur on the extremity 
bearing the lophophore. The larva, which is known as Actino- 
trocha, is a modified trochophore. 
References 
(See general references cited at the close of Chapter I) 
Allmann, G. J. 1856. Monograph of the Fresh-water Polyzoa. Ray 
Soc., London. 
Davenport, C. B. 1890. Cristatella: The Origin and Development of the 
Individual in the Colony. Bull. Mus. Comp. ZooL, Harvard, 20: 
101-152. PHYLUM MOLLUSCOIDEA 
149 
. 1893. On Urnatelia gracilis. Bull. Mus. Comp. Zool., Har- 
vard, 24: 1-44. 
Morse, E. S. 1902. Observations on Living Brachiopods. Mem. Boston 
Soc. Nat. Hist., 5. 
Osburn, R. C. 1910. The Bryozoa of the Woods Hole Region. Bull. 
U. S. Bureau Fish., 30: 203-266. CHAPTER XI 
PHYLUM ECHINODERMA 
The Echinoderma are radially symmetrical, coelomate animals 
with usually a skeleton of calcareous plates and a vascular system 
known as the ambulacral system. AU representatives of this 
phylum are marine in habits. The radial symmetry is not per- 
fect for some structures are distinctly bilateral in their arrange- 
ment. In the classification of Cuvier, the echinoderms were 
included along with the coelenterates within the group Radiata. 
The superficial resemblance in arrangement of parts seems to be 
an adaptation to the sessile habit, developed independently in 
these two groups, and does not indicate any phylogenetic relation- 
Fig. 82.-Diagrams to represent the formation of the rudiments of the coelom and 
of the water-vascular system in an echinoderm as viewed from the dorsal surface. 
Left hydrocoel rudiment in solid black; right stippled. constriction of hydrocoel 
rudiments from coelom; B, left hydrocoel acquires communication with exterior through 
stone canal; C, left hydrocoel encircles stomodaeum and buds off rudiments of five 
radial canals, coelomic pouches increase in size and surround the stomach. (Orig.) 
ship. Radial symmetry of Coelenterata is primitive while that 
of the Echinoderma is only secondarily derived from a bilateral 
condition, as is evidenced by the marked bilaterality of the 
larvae. Details of the transformation from the one type of 
symmetry to the other will be discussed in detail. 
Origin of the Water-vascular System.-During early larval 
development, an echinoderm is distinctly bilateral in form. 
Radial arrangement of the parts characteristic of the adult makes 
its appearance only following the formation of the mesoderm. 
Mesothelial sacs (Fig. 82) are formed as lateral outpocketings 
150 PHYLUM ECHINODERMA 
151 
of the entoderm just as in embryos of many other groups. Both 
the right and the left mesoderm pouches undergo a constriction 
(A) which separates each into an anterior and a posterior sac. 
The posterior sacs continue to increase in size and ultimately 
form (C) the right and left coelomic cavities. The anterior sacs 
are the rudiments of the water-vascular system and are termed 
the hydrocoel sacs. The left hydrocoel acquires communication 
with the body wall through a tubular outgrowth (B) which 
becomes the stone canal of the water-vascular system. The 
right hydrocoel fails to develop but remains vestigial and finally 
disappears. The entire water-vascular system is thus formed 
from the left hydrocoel. As it increases in size, it encircles the 
esophagus of the larva and becomes the ring canal of the vascular 
system. Five radial pouches extend outward from this ring 
canal and as they increase in length they become recognizable 
as the radial vessels one of which passes along each arm of the 
adult. The water-vascular system thus has its origin from one 
of the mesodermal pouches of a bilaterally symmetrical larva 
which in later development assumes a radial arrangement of its 
parts. 
The skeletal system is one of the most characteristic features of 
the echinoderms. Though details of arrangement and extent of 
development are highly variable, certain of the plates are fairly 
constant in their fundamental relations throughout a number of 
the classes. The skeletal plates have their origin in the mesoderm 
and lie near the surface of the body directly beneath the outer 
body covering. Spines, frequently associated with these plates, 
suggest the meaning of the name Echinoderma which is Latin for 
"spiny skin." In some instances (Echinoidea), the plates are 
rigidly articulated to form a continuous shell or test, unchange- 
able in form, within which most of the organs lie. In 
other classes (Asteroidea, Ophiuroidea, and Crinoidea), at least 
some of the plates are movable and permit of some flexibility 
in the parts which they cover. Among the Holothuroidea, the 
skeletal plates are so poorly developed that their presence in the 
soft body wall is restricted to minute discs and anchor-shaped 
bodies scattered through the tissues. 
Orientation.-The main axis of the body extends between the 
oral and aboral poles. This arbitrary morphological orientation is 
frequently not in accord with the natural or physiological orien- 
tation as determined by the natural position of the body of the 152 
INVERTEBRATE ZOOLOGY 
living animal. In Asteroidea, Ophiuroidea, and Echinoidea, 
the oral surface is ventral in position, while in the Crinoidea the 
mouth is directed upward and in the Holothuroidea the chief axis 
is parallel to the surface on which the animal rests. Typically, 
the parts of the body are arranged in radial manner about the main 
axis. In most echinoderms, the ambulacral system occupies 
certain radial regions of the body where the skeletal plates are 
perforated for the tube-feet. These regions are designated the 
ambulacral areas while the plates between the ambulacral areas 
constitute the interambulacral areas. Many of the echinoderms 
have on the oral and aboral surfaces series of plates which seem to 
be constant enough in their appearance in the various groups to be 
considered homologous. 
The crinoids and the extinct blastoids and cystoids are sessile. 
Correlated with this habit, they possess a stalk of attachment 
which is a prominent morphological character in these groups. 
Recognition of this character is expressed in the establishing of 
two subphyla; the Eleutherozoa which lack a stalk and the 
Pelmatozoa which characteristically carry a stalk. 
SUBPHYLUM ELEUTHEROZOA 
The echinoderms which are devoid of a stalk and are 
consequently capable of free locomotion have been assembled 
within the subphylum Eleutherozoa. The common starfishes 
(Asteroidea), the brittle stars and serpent stars (Ophiuroidea), 
the sea urchins and sand dollars (Echinoidea), and the sea 
cucumbers (Holothuroidea) are the classes included within this 
subphylum. 
Class Asteroidea 
The Asteroidea are the common starfishes. In these, the body 
is composed of a central disc from which usually five arms radiate. 
Arms may occur in multiples of five, though a few species fail to 
adhere to the pentagonal form. In the metamorphosis of a six- 
rayed starfish (Leptasterias hexactis'), the sixth ray makes its 
appearance late in development subsequent to the laying down 
of the characteristic five -rayed condition. A sharp differentiation 
of arms and disc is wanting in the cushion stars. The coelomic 
cavity of the disc is continued into the rays and many of the 
viscera thus extend into the rays. The mouth occurs on the PHYLUM ECHINODERMA 
153 
ventral surface, in the center of a membrane called the peristome. 
Along the oral surface of each ray, extend the tube-feet which are 
confined to a depression called the ambulacral groove. 
Arrangement of Plates.-Each skeletal plate of the ambulacral 
grooves is provided with notches in the margins which articulate 
with other ambulacral plates. The notches in adjacent plates 
coincide so the opening for each ambulacrum is between two 
plates. The ambulacral pores thus formed are in parallel rows. 
Distally, each double row of ambulacral plates terminates in a 
single ocular plate. This is so named because of the sense organ 
which it bears. The rigid body of a dried or preserved specimen 
gives little idea of the powers of movement possessed by the 
arms of the living starfish. The plates are articulated and their 
movements are controlled by body muscles in a manner which 
permits of considerable flexibility and freedom of movement 
in the arms. Laterally, yet on the oral surface, the ambulacral 
plates are bordered by a row of interambulacral plates which 
usually bear movable spines. A series of less regularly arranged 
adambulacral plates edges each row of interambulacrals. 
The aboral surface of each arm is made up of a series of plates 
of considerably variable arrangement in different members of the 
group. The disc of the Asteroidea lacks the regularity in the 
arrangement of its plates so characteristic of some other classes of 
echinoderms. The madreporite is the only conspicuous plate 
which is constant in position and even this is variable in some 
species, for in these more than one madreporite occurs. Both the 
disc and the rays are typically covered with scattered spines. The 
areas between these spines are ciliated and bear numerous small 
hollow filaments, the branchiae or gills, which are direct continua- 
tions of the coelom. Through the thin walls of these gills the 
body fluids are able to carry on the respiratory process. 
Pedicellariae.-Surrounding the body spines and scattered 
over the general surface of the body, there are frequently minute 
pincher-like organs called pedicellariae. Each of these is a small 
calcareous organ at the end of a strongly muscular stalk. In 
various species, these differ in form though two types are 
commonly found. In one type, the two jaws of the pinchers are 
articulated at their bases with a separate small calcareous body 
while in another type the two jaws cross each other at their bases 
and continue beyond the crossing in handle-like processes where 
the muscles for operating the jaws are attached. Pedicellariae 154 
INVERTEBRATE ZOOLOGY 
serve to remove small foreign bodies from the skin of the starfish 
and probably also serve as protection against small organisms 
which might attack the body of the starfish. 
Plates of the Disc.-The five genital plates, which are located 
interradially on the dorsal surface of the disc, are perforated by 
the external openings of the gonoducts. The five pairs of gonads 
lie within the coelom at the bases of the arms. One genital plate 
is in most instances enlarged and perforated by numerous pores 
which serve for the entrance of water into the water-vascular 
system. In some instances, this sieve or madreporite occurs 
as two or more plates. 
Symmetry.-In the typical condition of a single madreporite, 
the eccentric position of this organ is the most conspicuous 
external evidence of deviation from the radial type of symmetry. 
Since a plane passing through the madreporite and through the 
arm on the opposite side of the disc bisects the body, the animal 
is in reality bilaterally symmetrical. For convenience of refer- 
ence, the arm opposite the madreporite is called the anterior arm 
or ray. This, with the two adjacent rays, constitutes the trivium 
while the two remaining rays, between which the madreporite 
is located, comprise the bivium. 
The digestive system opens to the exterior through the ventral 
mouth. Small objects are ingested through the mouth, but 
because of the small size of the peristome large objects cannot 
be taken into the body. Mussels and oysters, which serve 
largely as food for the starfish, are digested outside the body 
through the peculiar provision which admits of the starfish 
everting the stomach through the mouth opening. The everted 
stomach surrounds large food masses and, after digesting them, 
is again withdrawn through the mouth opening into the body. 
A rather conspicuous constriction divides the stomach into 
dorsal and ventral chambers. The mouth opens directly into 
the ventral or cardiac chamber of the stomach while dorsal 
to this lies the pyloric chamber. From the cardiac chamber, 
a gastric pouch extends into each ray. A pair of hepatic ceca, 
occupying much of the space within each ray, communicate by a 
common duct with the pyloric chamber near the base of each arm. 
An intestine of minute size leads from the pyloric chamber to the 
aboral surface of the disc where it either opens through an eccen- 
tric anus or ends blindly. Small, branched ceca are given off 
from the intestine in some starfishes. PHYLUM ECHINODERMA 
155 
The water-vascular system is a series of tubes or canals of 
which the main parts comprise a ring canal surrounding the 
esophagus and a series of radial vessels given off from this, one 
to each arm. The ring canal communicates with the exterior 
by way of a stone canal which opens through the madreporite. 
In many Asteroidea, there is but a single madreporite and stone 
canal, but in some two or more of these structures are present. 
This latter condition is frequently associated with the powers of 
asexual reproduction. Small tufts of tubules called Tiedemann's 
bodies, interradial in position, are connected with the ring canal. 
Within these organs the amoeboid lymph cells which occur in 
the water-vascular system are formed. In addition, long-stalked 
vesicles called Polian vesicles join the ring canal interradially 
in some forms of Asteroidea. To these, also, have been ascribed 
the function of lymph glands. 
The longitudinal canal in each arm passes along the median 
line of the ventral surface just outside of (that is, ventral to) 
the ambulacral plates. At the tip of each ray, the ambulacral 
canal ends in a single tactile organ. Most Asteroidea have four 
longitudinal rows of tube-feet in the ambulacral area of each 
arm but in some (Henricia, e.g.) there are but two longitudinal 
rows. By opposite lateral branching, canals are given off along 
the course of the longitudinal canal and communicate with the 
tube-feet. In the typical condition of four longitudinal rows of 
feet, the two lateral branches arising from the same level on the 
longitudinal canal are of unequal length. This condition usually 
alternates in adjacent pairs so on each side of the longitudinal 
canal a long and a short transverse canal alternate. Thus the 
tube-feet fall into two parallel rows on each side of the longitu- 
dinal canal. Each ambulacrum is a muscular tube which, at its 
inner end, bears a muscular sac called an ampulla. By contrac- 
tion of the walls of the ampulla, the fluid in the tube-foot is put 
under compression. Relaxation of the muscles in the foot per- 
mits the tube to elongate greatly. At the same time the hydro- 
static pressure causes a cup-like disc at the end of the foot to 
flatten and in case the disc comes in contact with some object it 
adheres to it. Shortening of the tube is accomplished by allowing 
the fluid to turn back into the ampulla. When several feet 
become attached to an object, by their concerted contraction they 
drag the whole body of the starfish along and in this manner 
locomotion is accomplished. 156 
IN VER TEB RA TE ZOOLOG Y 
Blood-vascular System.-The circulatory system consists of 
a delicate system of vessels running parallel to the water- 
vascular system. Surrounding the blood-vascular system, there 
is a space termed the perihemal system. 
The Sexes and Reproduction.-Though starfishes are of 
separate sexes, there are no external features which distinguish 
Fig. 83.-Development of starfish larvae. A, early larva, oral view; B, the 
same, lateral view; C, more advanced larva, oral view showing extent of ciliated 
bands; D, bipinnaria larva; E, larva in brachiolarian stage. (Redrawn from A. 
Agassiz). 
the sexes. The gonads increase in size with the approach 
of the breeding season. Mature eggs are discharged directly into 
the water. Sperm cells, discharged by sexually mature males, 
are likewise set free in the water where fertilization takes place. 
Total, equal cleavage gives rise to a free-swimming ciliated 
blastula. Following gastrulation, the rounded body changes PHYLUM ECHINODERMA 
157 
form (Fig. 83, A-D) and becomes known as the bipinnaria with a 
number of ciliated lobes. In later development, the bipinnaria 
becomes further modified in form and is known as a brachiolarian 
larva (Fig. 83, E). Each of these larval forms is distinctly 
bilaterally symmetrical yet by an intricate metamorphosis the 
radial symmetry of the adult starfish is superimposed. In some 
asteroids, development has become modified, resulting in the 
elimination or reduction of the free larval stages. In some 
instances {Leptasterias hexactis'), the female carries the eggs and 
embryos in a brood pouch until the fully formed young are 
able to shift for themselves. Powers of regeneration are developed 
in an extreme degree among the Asteroidea. 
The nervous system consists of a circumoral nerve ring with 
a radial nerve extending along each ambulacral area. The 
system is peculiar in that it retains a superficial location in the 
ectoderm. Minute branches are given off to the various organs. 
There are no centralized ganglia in this system but experiments 
indicate that the nerve ring serves as a coordination center. 
The common starfishes belong to the genus Asterias. Cteno- 
discus has a pentagonal body without conspicuous arms. Solas- 
ter has numerous arms. Henricia is a northern genus with only 
two rows of feet on each ray. 
Class Ophiuroidea 
The brittle stars or serpent stars have highly flexible arms 
radiating from a central circular or pentagonal disc. Though 
superficially resembling the asteroids, they differ radically from 
them in details of organization. The ambulacral system is 
much reduced and fails to function in locomotion but serves 
rather as a series of tactile organs. Writhing movements of the 
arms produce locomotion. The ambulacral plates are withdrawn 
into the interior of each ray where they are fused together to form 
a jointed rod-like structure the units of which are called the 
vertebrae. Both internally and externally, each arm is composed 
of a large number of similar segments. In the basket stars, the 
arms become finely branched. The digestive system is confined 
to the disc and lacks an anus. Bursae are thin-walled sacs, 
leading inward from the ventral surface of the disc, which serve 
for respiration and into which the gonads open. 
On the oral surface of the disc, five interradial groups of plates 
project in toward the mouth to form the jaws which are operated 158 
INVERTEBRATE ZOOLOGY 
by muscles for masticating food or for selecting food particles. 
At the base of each jaw there are usually three plates, a large 
oral shield and two smaller adoral shields. One of the oral 
shields becomes modified to form the madreporite. 
Fig. 84.-Ventral view of young ophiopluteus of Ophiotkrix fragilis. (Redrawn 
from MacBride, courtesy of Macmillan Co.) 
Development involves a bilaterally symmetrical larva (Fig. 
84) known as the pluteus or, better, the ophiopluteus. 
Ophiura, Ophiopholus, and Amphioplus are typical genera of 
brittle stars. Astrophyton is the basket star with its finely 
divided arms. 
Class Echinoidea 
The sea urchins and sand dollars are usually globular, hemi- 
spherical, or disc-shaped. The shape, which is unalterable in any 
given species, is determined by the arrangement of the skeletal 
plates. These are immovably united to form a firm shell or test. 
Spines usually cover most of the test except at the oral and aboral 
poles. Surrounding the mouth, there is a circular opening where 
the plates are replaced by a membrane termed the peristome. 
Normally, the anus occurs at the pole opposite the mouth in a 
region called the periproct while in some instances it occurs on 
the margin of the disc. The skeletal plates are arranged in 
meridional bands part of which bear openings through which the 
ambulacral feet protrude and are therefore termed ambulacral 
areas. The non-perforated plates between two adjacent ambu- 
lacral areas are designated as an interambulacral area. 
Each ambulacral area terminates at the periproct in a single 
ocular plate homologous to the ocular plate at the end of each 
arm in the Asteroidea. A series of genital plates alternate with 
the ocular plates around the periproct and each marks the 
termination of an interambulacral area. One of the genital 
plates is modified to serve as a madreporite. PHYLUM ECHINODERMA 
159 
In its fundamental arrangement, the water-vascular system is 
essentially like that described for the asteroids. From the 
madreporite, the stone canal leads into the circumesophageal 
vessel. Interradially, five Polian vesicles communicate with 
the circular canal and a radial vessel passes along each ambulacral 
area on the inner surface of the test. In the ambulacral plates, 
there are two perforations for each tube-foot. Through one of 
these, the lateral branch of the radial canal passes to the tube- 
foot and through the other the foot is in communication with its 
ampulla. In many forms, the ambulacra are aided in the loco- 
motor process by the highly developed mobile spines which are 
articulated with the surface of the test and are operated by 
special muscles. 
One of the most characteristic structures of the echinoid is 
the Aristotle's lantern. From the oral surface of the animal, the 
five teeth with which this organ is supplied are visible in the 
center of the peristome. The main part of the lantern lies within 
the body cavity. The teeth are at the tips of a set of jaws which 
are operated by muscle bundles attached to a calcareous frame- 
work of intricate pattern surrounding the mouth cavity. From 
the aboral surface of the lantern is given off a short esophagus. 
This in turn leads into the stomach, a part of which is greatly 
dilated and flattened and extends almost around the body. The 
intestine bends backward in the opposite direction from that of 
the course of the stomach and in the case of the urchins passes 
to a median dorsal anus while in the sand dollars it passes along 
the posterior interambulacrum to an anal opening on or near the 
margin of the disc. The siphon occurs as a branch from the 
esophagus which parallels the course of the stomach for some 
distance then reunites with it. It seems probable that this 
heavily ciliated tubule may have a respiratory function and may 
also be of service in washing refuse from the intestine. 
When a sea urchin is opened for dissection by removal of the 
aboral wall of the test, the gonads are usually the most conspicu- 
ous structures first encountered. These are five large masses, 
interambulacral in position, connected at the aboral pole by a 
band of tissue termed the genital rachis. From each gonad a 
gonoduct passes to the opening in the adjacent genital plate. 
The larva (Fig. 85) which results from the cleavage and later 
development of the fertilized egg is termed a pluteus. Since the 160 
INVERTEBRATE ZOOLOGY 
term pluteus is also applied to the larva of Ophiuroidea, the 
name echinopluteus is frequently utilized. 
Respiration is performed to a considerable extent by the water- 
vascular system. In some echinoids, only part of the tube-feet 
are ambulatory while the remaining ones lack the sucking disc 
and seem to have chiefly respiratory and tactile functions. A 
pair of branched, filamentous gills occurs on the margin of the 
peristome opposite each interambulacral area of the sea urchin. 
Fig. 85.-Dorsal view 
of young echinopluteus of 
Echinus esculentus. (Red- 
rawn from MacBride, court- 
esy of MacMillan Co.) 
Fig. 86. - Anatomy L of keyhole urchin, 
Mellita pentapora. (After Coe). 
The siphon, as already mentioned, is also thought to aid in 
respiration. 
The nervous system is fundamentally the same type as that 
described for the Asteroidea, comprising a nerve ring from which 
five primary branches are given off. In addition to pedicellariae, 
there are small organs known as sphaeridia scattered over the 
body surface of many species. These are thought to function 
as organs of equilibrium. Specially modified tube-feet on the 
peristome are associated with the sense of taste. 
Arbacia, Strongylocentrotus, and Toxopneustes are genera 
of the common sea urchins and representative of the order 
Cidaridea. The tropical genus Clypeaster includes some of the 
largest urchins. This, with the sand dollar (Echinarachnius) 
and the keyhole urchin (Mellita) (Fig. 86) represents the order 
Clypeastroidea. Members of the order Spatangoidea, of which 
Spatangus is an example, are usually more or less heart shaped. PHYLUM ECHINODERMA 
161 
There are several groups of echinoids which are known only from 
fossil remains. 
Class Holothuroidea 
The sea cucumbers are elongated echinoderms lacking a 
definite skeleton, with a mouth at one extremity surrounded by 
a circle of branched tentacles (Fig. 87), and an anus at the opposite 
extremity. Since the mouth end goes forward in locomotion, 
it is frequently called the anterior extremity in the holothurians. 
Typically, the body is five-sided and on each side bears a double 
row of tube-feet. In some 
species, the three sides 
constituting the ventral 
surface have the tube-feet 
more highly developed than 
they are on the two dorsal 
ambulacral areas. Some 
few forms have feet irregu- 
larly scattered over the 
body surface while some 
burrowing species lack feet. 
The body wall is highly 
muscular. The alternate 
use of longitudinal and 
circular muscles enables the 
cucumber to creep like a 
worm. Though there is no 
continuous skeleton, the 
body wall is rather firm. 
This is due in large meas- 
ure to the presence of 
microscopic calcareous 
plates embedded in the tissues. The form of the plates is 
highly variable in different species and they serve as important 
features in classification. In some species, a calcareous ring of 
ten plates surrounds the esophagus and serves as a support for 
the tentacles. In a few forms, Psolus for example, the body is 
encased in hard scales or plates. 
The ring canal of the ambulacral system is located around 
the esophagus just behind the tentacles. From the ring canal the 
radial canals pass to the posterior extremity of the body. In the 
Fig. 87.-Anatomy of a holothurian, 
Thyone. (After Coe). 162 
INVERTEBRATE ZOOLOGY 
first part of their course, they run anteriorly and give off branches 
to the oral tentacles which are in reality highly modified tube-feet. 
One or more Polian vesicles are frequently present. The stone 
canal, instead of opening to the exterior, bears one or more 
madreporites which open into the coelom. As in other echino- 
derms the water-vascular system is rather closely paralleled by a 
blood-vascular system the vessels of which form extensive 
anastomoses on the alimentary canal. The nervous system is 
of the type previously described for other echinoderms but the 
quick responses given by the holothurians seem to indicate that 
the nervous and sensory organs are more highly developed than 
in other members of the phylum. 
The digestive canal is held in definite position by mesenteries. 
The esophagus passes into a stomach which is followed by a tubu- 
lar intestine. The main course of this tube is, in most species, 
posterior in the median dorsal interradius, then anterior in the 
left ventral interradius, and finally posterior in the right dorsal 
interradius to the cloaca. From the walls of the cloaca, there 
are usually a pair of minutely branched respiratory trees which, 
by the muscular action of the cloaca, are filled with water and 
serve as respiratory organs. 
A single genital pore occurs in the anterior region of the body. 
This is the opening of the single, much branched gonad. In 
development of the embyro, a larval form known as an auricularia 
is produced. Sea cucumbers have marked powers of regenera- 
tion. Individuals may automatically eject much of the internal 
organs and yet be able to regenerate them. 
Synapta (without radial canals and ambulacra), Thyone, 
Cucumaria, and Stychopus are American genera. 
SUBPHYLUM PELMATOZOA 
The Pelmatozoa are echinoderms which, during the whole or 
at least the early stages of their existence, are fixed by a jointed, 
flexible stalk, or are attached by the dorsal or aboral surface of 
the body. The principle organs are enclosed in a cup-shaped or 
spherical test, called the calyx, the walls of which contain a 
system of calcareous plates and ambulacral or food grooves 
leading to the mouth. The crinoids are the only living examples 
of this group which also includes the extinct Cystoidea and 
Blastoidea. 163 
PHYLUM ECHINODERMA 
Class Crinoidea 
The crinoids or sea lilies are usually provided with a long stalk 
or column at one end of which is attached the calyx with its 
movable arms. In numerous forms, lateral projections called cirri 
are borne along the stalk. In those instances where the stalk is 
lacking, the cirri are frequently attached directly to the base of 
the calyx. Occasionally, there are free-swimming crinoids but in 
their development these pass through a fixed stage (Fig. 88), thus 
giving evidence that the free condition is not primitive in members 
of this class. Of present-day forms, most are restricted to the 
greater depths of the ocean. Though 
distinctly local in their distribution, 
they occur in great numbers as must 
have also been the case in past geologi- 
cal times when they were abundant 
enough to form beds of rock of con- 
siderable thickness. The joints of the 
stems are very conspicuous in many 
limestone deposits. 
The calyx is usually a globular or 
cup-shaped capsule which holds the 
more important internal organs. This 
cup is formed of two or more circles of 
plates. The ring of plates next to the 
point of attachment to the stalk or 
column and extending upward to the 
projections of the arms comprises the 
base of the calyx, within which there 
may be either a single circle of plates 
called the basals or two circles. In 
this last instance, the plates next to 
the stalk are termed the infrabasals 
while the others are called the 
basals. A series of plates designated as the radials follows the 
cycle of basal plates. An arm has its origin with each radial plate. 
The arms are formed of a series of plates continuous with the 
radials and may be either simple or branched. In some of the 
more highly organized fossil forms, and in all of the recent cri- 
noids, the arms are furnished with pinnules alternating on oppo- 
site sides. In these, the gonads are borne. Arms and pinnules 
Fig. 88.-Fixed larva of a 
crinoid, Antedon rosacea. (After 
Carpenter). 164 
INVERTEBRATE ZOOLOGY 
are traversed on the ventral surface by an ambulacral groove at 
the bottom of which there is a tubular extension of the coelom. 
That part of the calyx which lies between the bases of the arms 
may be either in the form of a membrane with thin calcareous 
ossicles embedded in it or in the form of a series of plates making a 
continuous disc. In or near the center of this area, the mouth 
opening (Fig. 89) is plainly discernible, while in an eccentric 
interradial position the anus usually occurs. The grooves 
mentioned in the description of the arms continue across the 
oral disc to the mouth opening. In the case of distinctly dicho- 
tomously branched arms, there may 
be but a single groove across the disc 
to represent each pair of arm branches. 
The grooves are ciliated and serve as 
channels along which food particles 
are borne to the mouth opening. 
Ambulacra line the margin of the 
groove along each arm but they lack 
suckers and ampullae and function as 
tactile organs. The nervous and 
circulatory systems follow the course 
of the ambulacral system in much the 
same manner as already described for 
other echinoderms. 
In its normal position, the crinoid 
directs the oral surface and tentacles 
upward. Its position is thus the reverse of that normally 
assumed by the echinoids, asteroids, and ophiuroids. 
The full course of development is known for but a single species, 
Antedon rosacea. The germ cells are dehisced from the pinnules 
of the arms to the outside of which the eggs become attached. 
Fertilization of the heavily yolk-laden eggs is followed by cleavage 
resulting in the formation of a free-swimming ciliated larva, in 
which there is no communication between the mouth and the 
stomach. In this respect, the larva resembles that of some of the 
highly modified larvae of echinoderm groups rather than the typi- 
cal free-living echinoderm pluteus, auricularian, or bipinnarian. 
Calcareous plates begin to make their appearance early in the 
development of the larva. After a few days of free-swimming 
existence, the larva becomes fixed (Fig. 88) and undergoes a 
complicated series of changes which lead to the differentiation 
Fig. 89.-Oral area of crinoid 
(Antedon), showing by dotted 
lines . the course of the intestine 
from mouth (m) to anus (a); g, 
ciliated grooves leading from 
arms to mouth. (After King- 
sley in Hertwig's Manual, court- 
esy of Henry Holt and Co.) PHYLUM ECHINODERMA 
165 
of calyx and stalk. The larva in this condition is said to be in the 
"Pentacrinus stage." In later development, there is consider- 
able resorption of skeletal plates characteristic of the larva. 
Ultimately, the animal becomes detachable from the stalk 
and becomes capable of independent movement. 
Antedon, Rhizocrinus, Pentacrinus, and Metacrinus are 
names among the present day genera. 
References 
(See general references at close of Chapter I) 
Agassiz, A. 1872-1874. Revision of the Echini. Illustr. Cat. Mus. 
Comp. ZooL, Harvard, No. 7. 
. 1877. North American Starfishes. Mem. Mus. Comp. Zool., 
Harvard, Vol. 5, No. 1. 
Clark, H. L. 1901. Synopsis of North American Invertebrates, XV. 
The Holothuroidea. Amer. Nat., 35. 
. 1904. The Echinoderms of the Woods Hole Region. Bull. 
U. S. Fish Comm, for 1902. 
Coe, W. R. 1912. Echinoderms of Connecticut. Conn. Geol. and Nat. 
Hist. Survey, Bull. 19. 
Fisher, W. K. 1906. The Starfishes of the Hawaiian Islands. Bull. 
U. S. Bur. Fish., for 1903: 987-1130. 
Mead, A. D. 1900. The Natural History of the Starfish. Bull. U. S. 
Fish Comm., Vol. 19. CHAPTER XII 
PHYLUM MOLLUSCA 
The phylum Mollusca includes essentially bilaterally symmet- 
rical, unsegmented Metazoa in which the coelom has been 
reduced by invasion of connective tissue and of musculature 
until only the pericardial cavity and the lumen of the gonads 
represent its remains. The typical bilateral symmetry is 
frequently lost or obscured through secondary coiling or torsion 
of parts of the body. Head, visceral mass, foot, mantle, and 
shell are characteristic structures but in many instances one or 
more of these are lacking and all are subject to great variability 
in the various groups. A modified trochophore, termed the 
veliger, is a larval form common to all of the major groups of 
molluscs though in the modified development of some forms it 
has been suppressed. A larval organ, known as the shell gland, 
is typical though not always retained. Reproduction is exclu- 
sively sexual. 
The usual means of locomotion is by crawling on an unpaired 
foot. Powers of locomotion are lost in some sessile forms and 
in still others swimming is made possible through modifications 
of various structures. The mantle, when present, secretes the 
shell and bounds a cavity which may either contain the gills 
or function directly as a lung. The central nervous system is 
typically composed of three pairs of ganglia of which each pair 
is associated with a special sensory organ. The cerebral 
ganglia communicate with the eyes, the pedal ganglia with the 
statocysts, and the visceral ganglia with the osphradia or 
olfactory organs. 
The Mollusca have continued as important and conspicuous 
components through all the geological periods in which animal 
remains have been found. 
In detail of structure, the members of the various classes 
differ so greatly that morphology will be discussed separately 
under the headings for the individual classes. 
166 PHYLUM MOLLUSCA 
167 
Class Amphineura 
The Amphineura seem to represent the most primitive group 
of the Mollusca. All representatives of this class are marine. 
The oval or flattened body, which is bilaterally symmetrical, 
may or may not have a differentiated head. The nervous system 
consists of a pair of cerebral ganglia connected by a circum- 
esophageal ring, and four longitudinal nerve cords which pass, 
two laterally and two ventrally, through the body. There is no 
centralization of the nerve cells in these cords to form ganglia. 
In many instances, the longitudinal cords are connected by 
numerous cross commissures. Typically, the dorsal surface of 
the body is provided, in the chitons, with a series of eight trans- 
verse plates or shells, though in the Aplacophora these are want- 
ing and skeletal structures are restricted to calcareous spicules. 
I. Order Aplacophora 
These worm-like forms without shells occur at fairly great 
depths of the ocean where they burrow in the mud or sand or 
are associated with various colonial coelenterates. The mantle 
covers the body completely and bears calcareous spicules 
instead of producing a definite shell. The foot is lacking and 
in its place there occurs a longitudinal, ventral ciliated groove. 
The evidence seems to indicate that this is a degenerate rather 
than a primitive type of amphineuran. There are only a few 
genera of which Chaetoderma, Neomenia, and Dondersia are 
characteristic. 
II. Order Placophora 
The chitons are the typical representatives of the order 
Placophora. They are strictly marine, living at all depths but 
occurring in special abundance in shallow water where they move 
freely over the rocks by the action of their powerful foot. The 
head region is not sharply set off. There are no tentacles and usu- 
ally no eyes. The eight transverse plates with which the dorsal 
surface is covered overlap like the tiles on a roof. The plates 
are so articulated that the animal may roll into a ball. 
Sometimes, they are completely covered by the mantle but more 
commonly the mantle only partially covers the plates and extends 
beyond them along the sides of the body where it is covered 
with spines. Mantle folds on the ventral surface produce a 168 
INVERTEBRATE ZOOLOGY 
series of small gills or ctenidia. Nerves penetrate the skeletal 
plates and in the outer, less dense layer of each plate there are 
frequently small sense organs called aesthetes and in some 
instances eyes. The mouth and anus are located near the 
anterior and posterior extremities of the body on the ventral 
surface. The internal organs are disposed bilaterally along the 
median plane marked off by the oral-anal axis. Chiton, Cryp- 
tochyton, Amicula, and Trachydermon are characteristic genera. 
Class Acephala 
The Acephala have a shell composed of two lateral valves. 
Typically the hinge of these valves is dorsal though its position 
is modified in many instances. On the valves, are usually found 
conspicuous concentric lines, known as the lines of growth. The 
umbo, a slight prominence on the dorsal surface around which the 
lines of growth are distributed, is the oldest part of the shell. An 
elastic hinge ligament tends to hold the margins opposite the 
hinge gaping open. The valves of the shell are closed only by the 
contraction of adductor muscles which extend from one valve to 
the other through the body. Immediately within the protective 
shell, lie the right and left lobes of the mantle, secretions from 
which form the shell. 
Structure of the Shell.-The shell is covered externally by a 
thin, organic cuticula. The main bulk of the shell is composed 
of calcium carbonate. In some species, the innermost layer of the 
shell is composed of thin layers arranged parallel to the surface. 
These lamellae are minute enough to diffract the light and thereby 
produce an iridescence. The nacre, or mother-of-pearl, as this 
layer is termed, occurs in many fresh-water mussels and is 
especially conspicuous in the pearl oyster (Meleagrina). One 
type of pearls is formed by the deposition of nacre about foreign 
objects which have been introduced between the shell and the 
mantle. Many of the Acephala have a non-iridescent, porcela- 
neous lining of the shells. A short distance from the margin of 
the shell the mantle is joined to it by a line of muscle fibers called 
the mantle muscle. The line on the shell formed by the attach- 
ment of this muscle is termed the pallial line. 
The Siphons.-When the shell is nearly closed, the margins 
of the two mantle lobes are pressed together tightly except in the 
posterior region. Here the mantle lobes remain lightly separated 
to form two openings termed the siphons. Of these, the ventral PHYLUM MOLLUSC A 
169 
or inhalant siphon is for the inflow of fresh water into the mantle 
chamber and over the gills, while the dorsal or exhalant siphon 
discharges water from the mantle cavity and carries the feces 
along with the water. The margins of the mantle adjacent to 
the siphons are, in some instances, fused together thus leaving the 
siphons as permanent openings. The mantle in the region of the 
siphons frequently elongates and produces a siphon tube which 
projects beyond the shell. Both siphons may be united in a 
single tube or there may be two tubes entirely or only partially 
separated. 
The Gills and Palpi.-The mantle lobes overlie the gills which 
are typically paired structures on each side of the body. In the 
most primitive Acephala the gills are feather-like structures 
similar to those found in the Gasteropoda. Gills of all of the 
higher Acephala are derivable from this simple condition. As the 
flattened filaments elongate, the distal ends of the filaments 
become recurved and thus each filament becomes V-shaped. In 
the Filibranchia, the gill filaments may be either independent or 
united by interlocking cilia while in the Eulamellibranchia the 
walls of adjoining filaments become grown together to form a 
continuous sheet. 
Cilia covering these gills bring water currents through the 
siphon into the mantle chamber thereby making respiration 
possible and at the same time bringing into the mantle cavity 
microorganisms and other organic matter which finally enter the 
mouth and serve as food. A pair of small flap-like structures, 
the labial palpi, surround the mouth opening and aid in the 
selection of food. 
The Foot.-The foot is highly characteristic in the Acephala 
though in some instances, as in the oyster, it has become degenerate. 
It is, typically, a hatchet-shaped muscular organ which, by pro- 
trusion through the gaping valves of the shell, becomes fixed in 
the sand or mud, then by contraction of the muscles the animal is 
drawn a slight distance forward. Posterior to the foot, there 
frequently occurs a byssus gland, secretions from which form 
heavy silken threads by means of which permanent or temporary 
attachment is accomplished. 
The Visceral Mass is a softer body, lying dorsal to the foot, 
within which various organs are located. As the name indicates, 
there is no head in the Acephala. The digestive system has its 
beginning in a mouth at the anterior extremity of the body, 170 
INVERTEBRATE ZOOLOGY 
located between the small leaf-like labial palpi. In the visceral 
mass, the digestive tube makes a number of coils and in most 
Acephala terminates in a rectum which passes through the peri- 
cardium and also through the ventricle. Gonads and liver 
occupy most of the visceral mass surrounding the alimentary 
canal. A gelatinous rod, called the crystalline style, frequently 
occurs in the stomach. Recent investigations seem to indicate 
that this organ aids in separating food from foreign particles and 
probably contains a store of enzymes for use in the digestive 
processes. 
Circulatory System.-The heart, which lies in the dorsal part 
of the visceral mass, is surrounded by a pericardium and consists of 
a single ventricle and two auricles. A system of arteries carries 
the blood from the heart to the tissues where they frequently 
terminate in sinuses. The veins which return the blood to the 
heart pass first through the excretory organ and then through the 
gills. Separate vessels supply the mantle with blood and return 
the blood directly to the heart without passing through the 
excretory organ or gills. 
Excretory Organs.-The pair of nephridia characteristic of 
Acephala are frequently called the organs of Bojanus after their 
discoverer. They lie immediately below the pericardium. Each 
nephridium is a wide tube, bent upon itself, one end of which 
opens into the pericardial cavity and the other, non-glandular in 
structure, serves as a urinary bladder with its opening usually on 
a minute papilla into the inner cavity of the gill chamber. 
Another gland of excretory function, called Keber's organ, lies 
anterior to the pericardium into which it discharges. 
Nervous System.-A pair of cerebro-pleural ganglia is located 
one on each side of the mouth near the base of the labial palpus. 
A transverse supraesophageal commissure connects this pair of 
ganglia. Two nerve cords originate in each cerebro-pleural gang- 
lion, and connect this ganglion with the two other nerve centers 
on each side of the body. One of these passes ventrally to 
communicate with the pedal ganglion, embedded in the muscles 
of the foot, and the other passes posteriorly to a region ventral 
to the posterior adductor where the parietal and visceral ganglia of 
the more primitive molluscs have become fused to form a single 
posterior ganglion. 
Sense Organs.-There are but few highly specialized sensory 
organs. Statocysts are frequently found near the pedal ganglia PHYLUM MOLLUSC A 
171 
though they seem to have nerve connections with the cerebro- 
pleural ganglia. Patches of sensory epithelium called osphradia, 
and thought to have olfactory functions, are located near the 
base of the gills. The labial palpi and the edges of the mantle 
are highly sensory. The small projections from that part of the 
mantle which forms the siphons are especially sensitive to touch. 
Eyes (Fig. 90) are distributed over the margins of the mantle in 
some forms such as the scallops. 
Reproduction.-Most of the Acephala are dioecious. In 
many of the marine forms the gonads discharge their germ cells 
Fig. 90.-A section through an eye from the mantle margin of Pecten. (Slightly 
modified from Patten). 
directly into the water where they undergo fertilization and pass 
through cleavage and early larval development to form free- 
swimming larvae. In the fresh-water mussels, however, the eggs 
after discharge from the ovary are passed into brood pouches 
or marsupia located within the gills. Either in the marsupia, 
or enroute to them, sperm cells, brought in with the water 
currents by way of the siphon, fertilize the eggs. Each egg 
undergoes cleavage to form a larva known as a glochidium (Fig. 
91, A-D). This larval form is provided with a thin bivalve shell 
operated by a single adductor muscle. Other soft parts within 172 
INVERTEBRATE ZOOLOGY 
the shell are not clearly organized except for the presence in some 
species of a "larval thread" of uncertain significance and of 
minute sensory hairs. The glochidia, when discharged from the 
gills of the mother, are unable to continue development inde- 
pendently. Almost without exception the larval mussel must 
exist for some time as a parasite (Fig. 91, E) on the gills or fins of a 
fish. During this parasitic existence the glochidia undergo 
a metamorphosis and finally leave the host (Fig. 91, F) as juvenile 
mussels. Glochidia are frequently referred to as "hooked" or 
Fig. 91.-Development of fresh-water mussels. A, axe-head type of hookless 
glochidium of Proptera alata, anterior end view; B, lateral view of same; C, 
hookless glochidium (Ligumia subrostrata) posterior end view; D, lateral view of 
same; E, glochidium of Actinonais carinata encysted in gill of rock-bass; F, 
young mussel, same species as C, one week after close of parasitic life, showing 
lines of growth beyond glochidial shell and protruded foot with its cilia. (.After 
Lefevre and Curtis'). 
11 hookless." In the hooked forms, the ventral margin of each 
shell bears a hinged book. Glochidia of this type usually become 
attached to fins or scales of fishes (as in Lasmigona and Strophi- 
tus). Valves of the "hookless" glochidia are sometimes provided 
with small non-jointed spines (Fig. 91, A) as in the axe-head type 
or may be entirely devoid of spines (Fig. 91, C and Z>). Most of 
the hookless glochidia become attached to the gills of fish hosts. 
The finger-nail shells are hermaphroditic and differ from the 
other fresh-water Eulamellibranchia in that the young are 
born alive and do not pass through a larval stage outside the 
body of the parent. 
In the classification here adopted, the number and arrange- 
ment of the gills and of the adductor muscles serve as the chief PHYLUM MOLLUSCA 
173 
characters for the recognition of orders. Some students of the 
Mollusca and paleontologists more frequently utilize a system of 
classification based largely upon hinge structure. 
I. Order Protobranchia 
In members of this order, the gills are a single pair of plume-like 
structures each bearing two rows of flattened gill filaments. 
The foot has a flattened ventral surface which is used in locomo- 
tion. Yoldia, Nucula, Leda, and Solenomya are genera included 
in this order of marine Acephala. 
II. Order Filibranchia 
The gills of the filibranchs are plate-like and formed of V-shaped 
filaments which are either independent or united by interlocking 
cilia. The anterior adductor is frequently rudimentary or even 
lacking. This order is distinctly marine. Mytilus, Modiola, 
Area, Anomia, and Trigonia are characteristic genera. 
III. Order Pseudolamellibranchia 
Members of this order have but one adductor muscle. The 
gills are plaited in vertical folds with either ciliary or vascular 
junctions between the filaments. The oysters (Ostraeidae) are 
usually permanently attached by the right valve. The pearl 
oysters (Meleagrina) are included here as are also the scallops 
(Pecten). 
IV. Order Eulamellibranchia 
In the Eulamellibranchia, the two adductors are of approxi- 
mately equal development. The lamelliform gills have the fila- 
ments united by vascular j unctions forming a basket-like structure. 
The fresh-water mussels (Unio, Anodonta, Lampsilis, Quadrula, 
Symphonota), comprising scores of genera and hundreds of 
species, belong here. The giant Tridacna of tropical seas has a 
shell which reaches as much as four feet in diameter and weighs 
several hundred pounds. Shaerium, Pisidium, and Musculium 
are minute fresh-water forms, commonly called finger-nail shells, 
in which the young undergo complete development within the 
brood sacs of their hermaphroditic parents. The clams (Mya, 
Venus, Ensatella), the heart shells (Cardium) and the ship 
worms (Teredo) are important marine representatives. The 174 
INVERTEBRATE ZOOLOGY 
shipworms are of especial importance because of the damage 
they cause by their boring habits in the destruction of piling and 
wooden ships. Members of the genus Pholas even burrow into 
rocks. 
V. Order Septibranchia 
In members of this order, the gills are restricted to a horizontal 
partition within the mantle cavity. Slits in this partition offer 
communication between branchial and cloacal chambers. (Sile- 
nia, Cuspidaria.) 
Class Scaphopoda 
An external tubular shell, open at both ends and more or less 
curved, covers the body of the scaphopods. The shape of the 
shell is responsible for the common name "tooth shell" so fre- 
quently applied to members of this class. Jaws and radula are 
present but in the paired liver and general arrangement of the 
nervous system the Scaphopoda resemble the Acephala. Gills 
are wanting and the rudimentary heart possesses only a ventricle. 
The foot which is rather long and conical, extends from the larger 
opening of the shell and bears two lateral lobes. Dentalium is 
a modern genus of this class all members of which are marine. 
Class Gasteropoda 
The snails, limpets, slugs, and sea hares are examples of the 
Gasteropoda. Representatives of this class usually have head, 
visceral mass, foot, and mantle though in some instances one or 
more of these may be wanting. A shell, when present, is composed 
of a single piece. In some snails, either a limy or a horny disc 
called the operculum, serves for closing the opening when the 
animal is withdrawn into the shell but is not considered as 
comparable to a second valve. The foot is usually flattened so 
that it presents a large ventral surface upon which the animal 
crawls. The head, which is located anterior to the foot, bears 
hollow tentacles, eyes either at the base or tips of the tentacles, 
and a mouth. The mantle covers the dorsal surface of the body 
and bounds a spacious mantle cavity which contains the gills in 
the water-breathing forms. A siphon, through which water 
enters and leaves the branchial chamber, is frequently formed 
by an outgrowth from the edge of the mantle. The shape of the PHYLUM MOLLUSC A 
175 
shell is often greatly influenced by the presence of the siphon. 
In the pulmonate snails, the mantle cavity is largely replaced by a 
highly vascular sac or lung into which air is admitted through 
a single opening, the spiracle. 
The visceral mass lies dorsal to the foot. With increase in its 
size, it frequently assumes a spiral form and the mantle, which 
is carried with it, continues to secrete a shell at the free margin. 
As a result, the shell assumes a spiral form. The shell may be 
either simple cone-shaped or anything between this and a highly 
complex spiral. In the spiral type of shell, the coiled chamber 
usually surrounds a calcareous pillar which marks the axis of the 
shell and is termed the columella. In any given species, the 
coiling of the shell about the columella is normally in a fixed 
direction. Most coiled shells when held with the apex point- 
ing upward and the aperture facing the observer have the 
aperture on the right side and are therefore said to be dextral. 
Some snails are characteristically sinistral. 
Fig. 92.-Diagrams to show shifting of organs in the development of the 
streptoneurous condition of gasteropods. All diagrams as though viewed from 
dorsal surface. A, an orthoneurous gasteropod with posterior anus, and gills, 
ganglia, and auricles arranged in perfect bilateral symmetry; B, hypothetical 
intermediate condition in torsion; C, streptoneurous condition with gill, auricle, 
and parietal ganglion of original left side now on right side of body. (After Lans'). 
Since growth takes place through the addition of new shell 
material by the mantle around the aperture, each whorl is 
successively larger than the preceding. When a siphon is 
present, the aperture is drawn into an elongated process for 
containing it. 
The nervous system consists of two cerebral ganglia, the pairs 
of pedal and visceral ganglia, and two or three additional pairs 
all of which are united by commissures. The cerebro-visceral 
commissures may be either parallel (Fig. 92, A) (orthoneurous) 176 
INVERTEBRATE ZOOLOGY 
or crossed (Fig. 92, C) (streptoneurous) as a result of torsion 
within the body of the animal. Alimentary canal, nephridia, 
gills, circulatory and nervous systems are all affected in their 
arrangement by this torsion which is usually toward the right 
side. The anus may open into the mantle cavity either on the 
right side or in extreme instances may occupy a location near 
the head. In this latter instance, gills, nephridia, and other 
organs which belong primitively on the left side of the body 
become shifted in position to the right and vice versa. 
The alimentary canal has, at its anterior extremity, a muscular 
region which forms a conspicuous protrusible proboscis. The 
pharynx contains an organ variously termed the radula, odonto- 
phore, or lingual ribbon, the surface of which bears sharp teeth 
which are used in rasping off food particles. This radula is 
subject to numerous modifications, and differences in it 
are considered of great significance in the classification of 
gasteropods. Esophagus, stomach, and intestine are but slightly 
marked off from each other in the convolutions within the visceral 
mass. As mentioned above, the anus, which finds its normal 
position at the posterior extremity, may occur either on the right 
side or near the anterior margin of the visceral mass. 
Sense Organs.-The tentacles are the most conspicuous 
sensory organs. The eyes may be either at the bases of or at 
the tips of the tentacles but in some forms they are borne at 
the tips of a second pair of tentacles. 
Torsion of Body.-Modifications of the body correlated with 
torsion (Fig. 92) have in some instances resulted in loss of gill, 
nephridium, and osphradium of the primitively left side. As in 
the other Mollusca, the number of auricles is directly correlated 
with the number of respiratory organs, consequently with the 
loss of one gill the heart comprises but a single auricle and a single 
ventricle. When the lungs or gills are located in the front of 
the heart (Prosobranchia and Pulmonata), the auricles are 
anterior to the ventricle but when placed behind the heart 
(Opisthobranchia and Pteropoda), the auricle is posterior to 
the ventricle. 
Reproduction.-The gonad is always single. In some instances 
the sexes are separate while in others the gonad is hermaphro- 
ditic. The course of development is fairly uniform throughout 
the group, involving the presence of a veliger larva. In ter- 
restrial and fresh-water forms, the veliger stage is passed within PHYLUM MOLLUSC A 
177 
the egg shell and not as a free larval stage. Some gasteropods 
are viviparous. 
Subclass Prosobranchia 
In the Prosobranchia or Streptoneura, are included those 
gasteropods which have the cerebro-visceral commissures of 
the nervous system crossed or twisted into the form of a figure 
eight (Fig. 92, C). The sexes are separate. A shell is almost 
always present and is usually provided with an operculum. 
I. Order Aspidobranchia 
The order Aspidobranchia includes prosobranchs with but 
little concentration of the nervous system. The limpets (sub- 
order Docoglossa) include marine forms with a non-spiral shell 
and bearing either a single true ctenidium or a secondarily 
developed pallial gill beneath the mantle margin or with both 
true and secondary gills. Patella and Acmaea are representa- 
tive genera. The abalones (Haliotis) and the genera Fissurella 
and Trochus exemplify the suborder Rhipidoglossa in which 
both limpet-like and spiral shells occur. 
II. Order Ctenobranchiata 
This order includes large numbers of marine, fresh-water, 
and terrestrial Streptoneura with the shell usually coiled in a 
more or less elevated spiral. The heart has but one auricle. 
Due to torsion, the primitively right gill is shifted to the left side 
of the body. Both shelled and naked forms are included within 
this order. The members of the suborder Heteropoda are free- 
swimming and pelagic marine molluscs in which the foot is 
modified to form a vertical fin. Carinaria with its delicate, glassy 
shell and Atlanta are examples. In the suborder Platypoda 
the foot is flattened ventrally. Littorina, Crepidula, Natica, 
Strombus, Pleurocera, Murex, and Terebra are examples of this 
highly diversified suborder. 
Subclass Euthyneura 
The cerebro-visceral commissures are not crossed in the 
members of this subclass but form a simple loop. The indi- 
viduals are hermaphroditic. The shell, which is typically spiral 
or flattened, is frequently vestigial or wanting. In the order 178 
INVERTEBRATE ZOOLOGY 
Opisthobranchia, are included marine forms of which the Tecti- 
branchs (Aplysiidae or sea hares and Pteropods) and Nudi- 
branchs are examples. 
The order Pulmonata comprises chiefly terrestrial and fresh- 
water forms. The walls of the mantle cavity are modified to 
form a lung into which air is taken in respiration. Most of the 
aquatic forms depend upon periodic visits to the surface of 
the water for renewing the oxygen supply in the lung, though 
in some instances the lung has become adapted secondarily for 
water respiration. Physa, Lymnaea, Planorbis, and the limpet- 
shaped Ancylus are fresh-water pulmonates. The slugs (Limax, 
Arion, and Agriolimax) and many land snails (Polygyra, Helix, 
Pupa, Bulimulus) are also pulmonate Euthyneura. 
Class Cephalopoda 
The cephalophods are the most highly organized molluscs. 
In habits, they are exclusively marine. Among present-day 
forms, they include the squids (Fig. 93, A), cuttlefishes, devil- 
fishes (Fig. 93, B), and Nautilus. In most instances (except 
in Nautilus), a head is well defined. The characteristic arms 
which surround the mouth and the funnel-shaped siphon are 
modifications of the foot. The siphon is a highly muscular 
organ through which water is ejected from the mantle cavity. 
The force of the ejected stream of water, directed backward, 
drives the animal ahead and thus aids in locomotion. A fleshy 
mantle encloses a cavity which contains the gills and also serves 
as a covering for the most of the other viscera. When a shell 
is present, it is frequently internal as the cuttlebone of Sepia. 
A pair of eyes, on the sides of the head beneath the bases of 
the tentacles, very closely resemble the eyes of vertebrates. 
In finer structure and origin, the cephalopod and vertebrate 
eyes are widely different for despite their superficial resemblances 
the two seem to have had entirely independent origin. The 
cornea of the cephalopod eye is perforated and thereby allows 
water to enter the anterior chamber while it is not perforated in 
the vertebrate eye. The arrangement of the sensory cells, 
the retina, is just the reverse in the two types of eye. The 
vertebrate retina is said to be inverted, for light passing through 
the eye strikes the sensory cells of the retina on the same end 
that bears attachment to the nerve endings. In contrast with 
this, the direct retina of the cephalopod eye is so organized PHYLUM MOLLUSC A 
179 
that the light passing through the eye falls upon the free ends of 
the sensory cells. In Nautilus, much simpler eyes are found 
for lens, vitreous body, cornea, and iris are wanting. 
The sexes are separate. The spermatophores of the male are 
frequently stored in arms which become more or less modified. In 
most instances, this modification involves only sufficient change 
to adapt the arms as accessory copulatory organs but in a few 
genera the entire arm bearing the spermatophores becomes severed 
from the body of the male and acquires independent powers 
Fig. 93.-Cephalopods. A, ventral view of a squid, Loligo opalescens; B, dorsal 
view of Polypus bimaculatus. (After Berry). 
of locomotion. When first observed, these worm-like castaway- 
arms were thought to be entire organisms and were described 
under the name Hectocotylus before their relationship to the 
cephalopod body was understood. The term hectocotylization 
is used to indicate this type of spermatophore transfer through the 
agency of dissevered tentacles. Largely on the basis of the 
number of gills, two subclasses of cephalopods are recognized, 
the Tetrabranchia with four gills and the Dibranchia with 
two. 180 
INVERTEBRATE ZOOLOGY 
Subclass Tetrabranchia 
The genus Nautilus contains the only living representatives of 
the Tetrabranchia. A well-developed, chambered shell within 
which the animal lives, the presence of four gills, four auricles, 
and four nephridia, a divided siphon and many tentacles without 
hooks or suckers, characterize the living examples of this subclass. 
In past geological ages, numerous tetrabranchs flourished, the 
fossil shells of which show many interesting evolutionary ten- 
dencies in development. But little is known of the habits and 
development of Nautilus, for though the empty shells are cast 
ashore in quantities in the Pacific and Indian oceans the animal 
is rarely found alive. 
Subclass Dibranchia 
In this subclass, are included those cephalopods having two 
branched gills in the mantle cavity, two nephridia, two auricles, 
highly organized eyes, and eight 
or ten arms bearing suckers or 
hooks. On the basis of the num- 
ber of arms, two orders are recog- 
nized, namely, Octopoda and 
Decapoda. An ink sac (Fig. 94) 
is usually present. Shells may be 
either present or wanting. The 
powerful, muscular arms are used 
both in swimming and in the 
capture of prey. A constriction 
separates the head from the body 
proper and marks the anterior 
boundary of the mantle. At this 
level, on the ventral surface 
occurs the respiratory opening. 
I. Order Decapoda 
As the name implies, the Deca- 
poda have ten appendages, the 
arms, but two of these are frequent- 
ly longer than the rest and are 
modified as tentacles. These tentacles, which bear suckers only 
on the enlarged tips, are capable of retraction within special 
pouches. Along the sides of the usually cylindrical body are 
Fig. 94.-Diagram of median sec- 
tion of Sepia. {From Lang). PHYLUM MOLLUSCA 
181 
borne longitudinal lateral fins. Some sort of a shell is usually 
present, in the form of an internal, coiled, chambered shell as in 
Spirula, as a long horny pen of purely organic matter as in the 
squids (Loligo), or as a highly calcareous plate known as the 
"cuttlebone" of the cuttlefish (Sepia, Fig. 94). The pigment 
sepia is also derived from the cuttlefish for the ink sac which is 
characteristic of the Decapoda stores quantities of this pigment. 
When the animal is disturbed, clouds of this ink are shot from the 
mantle cavity through the siphon and serve as an effective cover 
under which the cuttlefish moves off to safety. 
Two kinds of hearts are present. The systemic heart is of 
typical molluscan type with its single ventricle and two auricles 
corresponding to the number of gills and receiving blood from 
them, but in addition to this heart there is a branchial heart at the 
base of each ctenidium which forces the blood through the gills. 
The heavily yolk-laden telolecithal eggs undergo partial dis- 
coidal cleavage. The organs of the young cephalopod are formed 
from the blastoderm; first as flattened projections, but as these 
grow the young animal becomes recognizable, at first appended 
to the bulky yolk sac by the head end. 
II. Order Octopoda 
The devilfishes (Octopus) and Argonauta are included within 
the order of eight-armed cephalopods. The arms are usually 
more or less webbed together at their bases and equal in size. 
In the Argonauts, the female is provided with a thin, single- 
chambered shell but the male Argonauts and all other repre- 
sentatives of the order typically lack a shell. The body is 
usually sac-shaped, in contrast with the more frequently cylin- 
drical body of the decapods. The coelom is very greatly reduced. 
References 
(See general references cited at the close of Chapter I) 
Baker, F. C. 1898 and 1902. Mollusca of the Chicago Area. Chicago 
Acad. Sci., Nat. Hist. Survey Bull. 3. 
■ . 1916. The Relation of Mollusks to Fish in Oneida Lake. New 
York St. Coll. Forestry, Tech. Publ., 4. 
Berry, S. S. 1912. A Revision of the Cephalopods of Western North 
America. Bull. U. S. Bur. Fish., 30: 267-336. 
Coker, R. E., Shira, A. F., Clark, H. W., and Howard, A. D. 1921. 
Natural History and Propagation of Fresh-water Mussels. Bull. U. S. 
Bureau Fish., 37: 77-181. 182 
INVERTEBRATE ZOOLOGY 
Drew, G. A. 1911 and 1919. Sexual activities of the Squid, Loligo Pealii 
(Les). Jour. Morphol., 22: 327-359 and 32: 379-435. 
Kellogg, J. L. 1900. Observations on the Life History of the common 
clam, Mya arenaria. Bull. U. S. Fish Comm, for 1899: 193-202. 
. 1915. Ciliary mechanisms of Lamellibranchs. Jour. Morphol., 
26: 625-702. 
Lefever, G. and Curtis, W. C. 1912. Studies on the reproduction and 
artificial propagation of fresh-water mussels. Bull. U. S. Bur. Fish., 
30: 105-301. 
Ortmann, A. E. 1911. A Monograph of the Najades of Pennsylvania. 
Mem. Carnegie Museum, 4, No. 6. 
Pillsbury, H. A. 1885- . Manual of Conchology, second series. 
Philadelphia. 
Simpson, G. B. 1901. Anatomy and Physiology of Polygyra albolarbis 
and Limax maximus and embryology of Limax maximus. Bull. 
N. Y. State Museum, 8. 
Tryon, G. W. 1882-1884. Structural and Systematic Conchology. 
Philadelphia. CHAPTER XIII 
PHYLUM ARTHROPODA 
INTRODUCTION AND CLASS CRUSTACEA 
In many respects, the arthropods represent the highest 
development found in the non-chordate animals. In earlier 
chapters, it has been shown that the bodies of the higher seg- 
mented worms are composed of a linear repetition of similar 
rings or somites. The somites of these -worms often bear appen- 
dages, called parapodia, but these in their highest development 
are mere flap-like folds of the body wall and are never jointed. 
The tendency toward specialization of appendages has been 
carried much further in the arthropods for here they have become 
more highly organized and they are definitely articulated. 
Metamerism of the body also finds higher expression in the 
arthropods, for in most arthropods the segments of the body 
have undergone greater specialization and more distinct regional 
differentiation than is encountered in any annelids. Since the 
segments of the worms show so little differentiation the metamer- 
ism of the worms is said to be homonomous while that of the 
arthropods is very distinctly heteronomous. In most instances 
at least head and trunk regions are recognizable and in many 
cases three body divisions are distinctly set off as head, thorax, 
and abdomen. 
The body covering or integument of arthropods is composed 
of a chitinous material overlying the hypodermis. To permit 
of movement of the body, joints occur in this otherwise unwieldy 
exoskeleton. These joints are termed sutures and skeletal 
areas bounded by sutures are designated as sclerites. Inorganic 
substances, especially lime, are frequently added to the chitin 
thus giving the skeleton additional strength and thereby afford- 
ing greater protection to the underlying parts. The skeletal 
plates are more or less telescoped at the more prominent joints 
so the body surface presents an uninterrupted armor. 
With age, the chitinous covering increases in thickness and 
becomes an effective barrier to further growth of the parts 
183 184 
INVERTEBRATE ZOOLOGY 
which it encases. Growth is then rendered possible only by 
periodic shedding of the cuticula in a process known as molting 
or ecdysis. Immediately following the ecdysis, the body cover- 
ing is extremely soft and pliable and during this period there is 
rapid increase in size. Frequency of ecdysis varies greatly in 
different groups of the arthropods but it is of much more frequent 
occurrence in early stages of development. In the insects, 
attainment of the adult form marks the last ecdysis through 
which the individual passes, except in the mayflies which 
molt once after the wings become functional. 
Number of Segments.-In the fundamental structural plan 
of an arthropod, each segment bears a pair of appendages. 
Fusion of segments and degeneration of appendages on some 
segments frequently obscure this plan, but even in such instances 
evidences of the primitive condition are often still observable 
in the embryo for the vestiges of appendages occur here even 
though they may be entirely wanting in the adult. 
Characteristic Organ Systems.-The chief organ of the 
circulatory system (a dorsal vessel or a heart) lies dorsal to the 
digestive canal. The circulatory system is of the open type for 
the body fluid is not restricted to vessels throughout its course 
but is frequently spilled into sinuses or lacunae. These sinuses 
are so prominent that they are frequently mistaken for a coelom 
though they more correctly represent a haemocoel. 
The central nervous system consists of a ladder-like chain of 
ganglia (Fig. 123) and a brain or supraesophageal ganglion of 
which all but the brain is ventral to the digestive tract. 
Various modifications for respiration and excretion are found 
but these will be discussed under the several classes. 
Development is always sexual but modified types of sexual 
development are found in the establishment of parthenogenetic 
and pedogenetic habits in some groups. Polyembryony has 
been demonstrated in some insects. Hermaphroditism is rare. 
The centrolecithal egg undergoes partial, superficial cleavage in 
most instances. So many different larval stages are involved in 
the various groups of arthropods that the developmental cycle 
will be considered separately for the individual classes of the 
phylum. 
Classes.-There is much disagreement regarding the number 
of classes into which the Arthropoda should be divided. Even 
a conservative judgment would demand that the Crustacea, the 
Acerata, the Onycophora, the Diplopoda, the Chilopoda, the PHYLUM ARTHROPODA 
185 
Symphyla, the Myrientomata, and the Insecta be recognized 
as classes. Some writers maintain that in addition to these, 
several other groups such as the Pyconogonida, the Tardigrada, 
the Linguatulida, and the Pauropoda, should be admitted to 
the rank of classes. 
Class Crustacea 
The cuticula of most crustaceans has become hardened 
through the addition of carbonate and phosphate of lime to the 
organic chitinous skeleton. Members of this class are typically 
aquatic though some (the sowbugs and land crabs) have become 
modified for terrestrial existence. A carapace is very frequently 
present. This has its origin as a fold at the head end and grows 
backward as a continuous shield protecting the underlying parts 
and obscuring the external evidences of segmentation of the parts 
covered. In some instances, the entire body is enclosed in this 
shell-like carapace (Fig. 99) which gives to these crustaceans a con- 
fusing external resemblance to molluscs. More frequently, the 
head and all or part of the thorax are covered by the carapace. 
Appendages.-All of the present-day crustaceans have two 
pairs of antennae though the trilobites (Fig. 96), all of which are 
extinct, have but a single pair. The appendages are typically 
Y-shaped and are said to be of the biramous or schizopodal type. 
The stem of the Y, which provides attachment with the body 
wall, is called the protopodite and is composed of two segments, 
namely, a proximal coxopodite and a distal basipodite. Distally, 
the basipodite characteristically bears two branches of which 
the one nearer the median line of the body is termed the endopo- 
dite and the other the exopodite. Many of the crustacean 
appendages have become modified so that the biramous con- 
dition is not observable as, for example, the walking legs of the 
crawfish and lobster in which the protopodite bears but a single 
series of segments to form the leg. A study of the larvae of the 
lobsters (Fig. 95) gives proof that the distal part of the leg is 
the endopodite, for in larval lobsters the thoracic legs are distinctly 
biramous and it is only in later development that the exopodite 
of these appendages disappears. There are no free larval 
stages in the crawfishes for the entire development is com- 
pleted within the egg but since other structures of the crawfish 
homologize so directly with those of the lobster, the leg of the 
crawfish is likewise considered as being composed of a protopodite 
and an endopodite. 186 
INVERTEBRATE ZOOLOGY 
While the schizopodal or biramous appendages are character- 
istic of most crustaceans, there are some in which a more primitive 
type of appendage is found. In the Phyllopoda, there are leaf- 
like feet on the thorax (Fig. 97) though the antennae are bira- 
mous. The phyllopod appendages bear a number of lateral 
processes called endites projecting from a central axis of podo- 
meres or foot segments. This type of appendage seems to be 
more generalized than the biramous type and the latter may 
have originated as a modification of the foliaceous type. 
The number of body segments is highly variable. The 
anterior five somites are fused with the prostomium to form the 
head. This may be united with some or all of the thorax to 
form a cephalo-thorax. The abdominal feomites are highly 
variable in number. 
Respiratory and Excretory Organs.-Respiration is usually 
through gills, though in some instances there are no modified 
structures for respiration because the entire body surface func- 
tions in this capacity directly. The gills are very frequently 
borne within a gill chamber formed by the walls of the carapace 
and the body wall. Special organs of excretion are the green 
glands and the so-called shell glands which open on the bases of 
the second antennae and the second maxillae respectively. 
These two organs occur together in the larvae but in adult 
crustaceans one or the other fails to develop. They agree 
in fundamental structure. Each consists of a terminal vesicle 
which communicates with an external pore by a slender, greatly 
coiled tubule. 
The digestive canal is largely ectodermal and is consequently 
lined with chitin. The stomodaeum includes not only the 
pharynx but also a dilated portion, modified for grinding, which 
is called the stomach. Much of the intestine is proctodaeum. 
The mesenteron is a relatively short region into which a paired 
digestive gland, the hepato-pancreas, empties. 
Sensory Organs.-Both antennae and antennules are tactile 
organs, innervated from the brain, but in some instances the 
antennae are the chief locomotor organs. Otocysts are found 
only in the Malacostraca where they occupy a position in the 
protopodite of the antennules (Decapoda) or on the last abdominal 
appendages (Schizopoda). Paired compound eyes are charac- 
teristic of many crustaceans but there are some blind forms. 
An unpaired X-shaped "naupli'us eye" occurs in the larvae of PHYLUM ARTHROPOD A 
187 
most crustaceans and is retained as the optic organ in the adults 
of the lower Crustacea. 
Development.-The Crustacea typically pass through one or 
more larval stages before reaching adult organization. When 
direct development occurs, it has either resulted from a sup- 
pression of larval stages or these stages have been passed before 
the embryo leaves the egg. The nauplius is one of the most 
important as well as most characteristic larval stages of Crustacea 
for it is almost universal among the lower orders. This larva is 
composed of three segments bearing three pairs of appendages. 
The foremost of these appendages are simple and later develop 
into the antennules while the second and third are biramous and 
in later development form the antennae and mandibles respec- 
tively. There is a single unpaired eye. 
In a characteristic instance, as in the development of Cyclops, 
the nauplius after its first molt becomes a metanauplius bearing 
the rudiments of three pairs of appendages in addition to those 
found in the nauplius. These additional rudiments represent 
the two pairs of maxillae and one pair of maxillipeds. A 
pair of very rudimentary thoracic appendages also make their 
appearance on the metanauplius. In each of three successive 
molts, the Cyclops larva acquires an additional pair of these 
stump-like rudiments of the thoracic legs, so with the close of the 
larval period there are four pairs of the thoracic rudiments, and 
following a final larval molt the larva has practically attained 
the adult form. 
Among the copepods, the nauplius and metanauplius stages 
are passed within the egg membranes and the larva at hatching is 
termed a copepodid because of its fairly close resemblance to the 
general organization of the free-living copepod. 
The zoea (Fig. 93, B) is characteristic of the Malacostraca where 
it is usually the first larval stage but in a few instances it is pre- 
ceded by a nauplius. It consists of a cephalothorax bearing 
biramous appendages and an abdomen without appendages. 
The head bears a pair of lateral compound eyes. Typically, 
the zoea by several molts transforms into a mysis stage (Fig. 
95, A), in which even the posterior thoracic appendages are 
biramous and are of use in swimming. In later development, 
the exopodite of these biramous thoracic appendages disappears 
and leaves the unbranched walking legs of the adult. 188 
INVERTEBRATE ZOOLOGY 
In the Brachyura (crabs), the thoracic appendages of the 
zoea, which are later destined to become the walking legs, develop 
in the free-swimming larva as only bud-like rudiments and never 
acquire the biramous condition characteristic of the mysis larva. 
Thus in the crabs the mysis stage has been eliminated and the 
zoea in passing to the adult condition transforms through a more 
advanced type of larva (Fig. 103) which is termed the megalops. 
Six subclasses of the Crustacea are here considered: the Trilo- 
bita which are represented by fossil remains only, the Phyllopoda, 
the Copepoda, the Ostracoda, the Cirripedia which are exclu- 
sively marine, and the Malacostraca. 
Fig. 95.-Typical stages in the development of the lobster. A, first swimming 
stage or mysis; B, second larval stage, with abdominal appendages; C, fourth 
larval stage showing loss of exopodites from walking legs. (After Herrick). 
Subclass Trilobita 
The trilobites are of importance because they represent a 
primitive type of crustacean which has not persisted to recent 
times but occurred in abundance during Cambrian times when 
the oldest fossil-bearing rocks were formed, reached an extreme 
development in the Silurian and disappeared in the Permian. 
In habits, they were exclusively marine. Their remains are 
among the most popularly known fossils. The body contains 
a variable number of somites (Fig. 96) which are grouped into 
head, thorax, and abdomen. Each segment bears & pair of PHYLUM ARTHROPODA 
189 
jointed appendages. The head bears five pairs all of which are 
biramous except the antennules and they are simple. A pair of 
compound eyes usually occurs on the head. 
A pair of longitudinal grooves separates the body into a central 
axis and two lateral pleural lobes. This "trilobed" condition of 
the body suggests the origin of the group name. 
About 2,000 species have been described. Proetus, Asaphus, 
Dalmanites, Triarthrus (Fig. 96), Harpes, Ctenocephalus, Para- 
doxides, and Homalonotus are examples of the 200 or more 
described genera. 
Fig. 96.-A trilobite, Triarthrus Becki. A, dorsal; B, ventral aspect. (After 
Beecher, from ZitteW). 
Subclass Phyllopoda 
Of the present-day Crustacea, the Phyllopoda are the most 
primitive. The name intimates one of the outstanding characters 
of the group, the presence of leaf-like appendages. Fresh-water, 
marine, and brackish-water forms are included within the con- 
fines of this group the members of which have little more than 
the leaf-like appendages as a common characteristic. In the two 
orders, Branchiopoda and Cladocera, members of the former have 
numerous segments, very distinctly marked, while members of 
the latter order have few somites and these are frequently not 
clearly defined. Even within each order, body form and struc- 
ture are subject to great variation. There is usually a pair of 
conspicuous eyes and frequently in addition there is a small 
median eye. In some instances, the paired eyes, distinct in the 
young, fuse to form a single eye but even then two optic nerves 190 
INVERTEBRATE ZOOLOGY 
are retained so the double nature of the eye is still observable. 
The sexes are distinct though males are much less numerous than 
females. 
Members of the two orders Branchiopoda and Cladocera are 
separable on the basis of the number of trunk appendages. In 
the former, there are ten or more pairs of trunk appendages while 
in the Cladocera the appendages of this region do not exceed 
six pairs. 
I. Order Branchiopoda 
With the exception of one genus, Artemia, the Branchiopoda 
are exclusively fresh-water phyllopods which occur in all parts of 
the world. Eubranchipus (the fairy shrimp, Fig. 97), Apus, 
and Estheria represent three distinctly different types of struc- 
ture within this order. In habits, all of these are peculiar in that 
they are restricted to small pools and especially the temporary 
pools which are formed by spring rains and disappear during the 
Fig. 97.-Eubranchipus vernalis in normal position for swimming. {After 
Packard, from Kingsley's Hertwig, with permission of Henry Holt and Co.) 
summer. In these temporary pools, they appear in very great 
numbers in early spring, then after a few days or weeks they 
disappear entirely. An examination of the mud after the pool 
has dried up reveals large numbers of the eggs which are capable 
of withstanding desiccation. In fact, the eggs of some forms 
refuse to develop if they are returned to water immediately and 
develop normally only after being dried out. From the egg, is 
derived a larva in the nauplius or metanauplius stage. Many 
phyllopods swim upon the back with the ventral surface upper- 
most. This is especially characteristic (Fig. 97) of Eubranchipus. 
Eubranchipus and the brine-shrimp Artemia are examples of 
the suborder Anostraca, the members of which have no carapace 
and bear stalked eyes. The members of suborder Nostraca, PHYLUM ARTHROPOD A 
191 
of which Apus and Lepiclurus are examples, include branchiopods 
with a shield-shaped carapace covering part of the trunk and 
with sessile eyes. In the suborder Conchostraca, are grouped 
forms which have a bivalve carapace enclosing the entire animal. 
Estheria, Limnadia, and Limnetus with their strikingly mollus- 
can appearances are examples of this suborder. 
II. Order Cladocera 
The Cladocera are small phyllopods, rarely more than three 
millimeters in length, with a distinct head and usually with a 
carapace covering the trunk and legs. In a few instances 
(Leptodora and Polyphemus), the carapace is greatly reduced 
and serves only as a brood sac leaving the trunk and legs entirely 
free. 
The head holds a median compound eye which has usually 
resulted from the fusion of two lateral eyes and is capable of 
rotation within a capsule. The antennules are sensory but the 
second antennae are modified as swimming organs and constitute 
the chief organs for locomotion. In addition, the head bears a 
pair of mandibles and a pair of greatly reduced maxillae. 
A dorsal heart, just back of the head, is the only circulatory 
organ, for vessels are wanting. An ostium on each side of the 
heart allows the blood to enter and contraction of the heart 
forces the blood through a single anterior opening. There are 
no specialized organs for respiration. That portion of the body 
within the shell is divisible into two regions, one, the body proper 
which is not sharply segmented but bears six pairs of foliaceous 
appendages, and the other, an unsegmented post-abdomen. 
The trunk appendages function chiefly in creating water currents 
through the space within the shell. 
Most Cladocera react positively to weak light and negatively 
to strong light but these reactions may be reversed by other 
stimuli acting as controlling factors in their behaviour. Thus, 
in cold water, Cladocera may respond positively to a light 
stimulus which would repel the same individuals living at a higher 
temperature. 
Reproduction is largely parthenogenetic. The eggs are stored, 
frequently in considerable numbers, in a brood case formed 
within the dorsal portion of the carapace. Here they undergo 
full development without ever having a free larval stage. Under 
ordinary circumstances, all of the parthenogenetic eggs produce 192 
INVERTEBRATE ZOOLOGY 
females. However, when unfavorable conditions arise, not all 
of the parthenogenetically developed individuals are females but 
some males are hatched from parthenogenetic eggs. In the 
sexual cycle which follows, each female produces only one or 
two large thick-shelled eggs. These are true sexual eggs which 
require fertilization before development is initiated. The fer- 
tilized eggs pass into a brood sac, the walls or portions of the 
walls of which become modified as an enclosing envelope 
(ephippium) within which the resting eggs lie until the return 
of conditions favorable for their development. 
The Cladocera, along with some other crustaceans, have 
great value in that they are important as food for many aquatic 
animals, especially fish, while they in turn utilize the smaller 
algae which are abundant under conditions in which they are 
found. Daphnia and Bosmina are characteristic shelled forms 
while Leptodora and Polyphemus have greatly reduced shells. 
Subclass Copepoda 
Copepods inhabit both fresh- and salt-water. Some forms 
have become so highly modified through adaptation to the para- 
sitic habit that they are only with difficulty recognizable as arthro- 
pods. In the free-living forms, the body is usually elongated and 
distinctly segmented. The appendages are characteristically 
biramous though some have become so modified that they have 
lost their biramous nature. Six pairs of appendages are borne 
on the head and four or five on the anterior region of the trunk 
while the posterior region of the trunk lacks appendages. The 
last abdominal segment is forked. 
The unpaired nauplius eye is characteristic of copepods and 
prompted the application of the name Cyclops to one genus. 
In many instances, the eggs when discharged from the female 
are surrounded by a gelatinous substance and remain attached 
to the body as prominent egg-sacs. Conspicuous egg-sacs are 
highly characteristic of the female Cyclops. Larvae hatching 
from the eggs are in the nauplius stage. 
Copepods occur in such abundance and are important food 
items for such numbers of different animals that they have 
considerable economic importance. Many fishes feed almost 
exclusively upon small crustaceans of which the copepods are 
the most numerous. Even some whales subsist largely upon 
copepod diet. PHYLUM ARTHROPOD A 
193 
Parasitic Copepods.-In the foregoing, only the free-living 
copepods have been considered. There are immense numbers 
of copepods which dwell as parasites upon other animals. Fishes 
are especially prominent hosts of these parasites which are 
popularly known as fish-lice. Both the body and gills are sub- 
ject to attack. Some of these parasitic copepods (e.g., the genus 
Argulus (Fig. 98) of the order Branchiura) have a pair of com- 
pound eyes, fully developed swimming feet, and a modification of 
the first maxillipeds to form a pair of sucking discs for securing 
attachment to the host. These 
Argulids are not permanent 
parasites for especially at the 
breeding season they leave the 
body of the host and swim free 
in the water. In contrast with 
these, stand some other parasitic 
copepods the bodies of which 
have become so greatly reduced 
as an adaptation to the parasitic 
habit that they are more like a 
simple worm in appearance than 
like an arthropod. It is only 
through the unaltered larval 
stages, the nauplius and the metanauplius, that the affinities 
of these degenerate adults are recognizable. 
There are a number of families displaying an extensive array 
of bizarre body forms. In many instances, all of the appendages 
excepting those modified for attachment to the host have degener- 
ated. Lernaeopoda, Ergasilus, Lernaeocera, Calegus, and Pen- 
nella are characteristic genera of parasitic copepods while Cyclops, 
Diaptomus and Pontilla are free-living. 
Subclass Ostracoda 
Ostracoda are abundant in various fresh- and salt-water 
habitats. The entire animal is enclosed within a bivalve shell 
(Fig. 99) but when the valves open the appendages protrude. 
Segmentation of the body is very indistinct or wanting. The 
head region bears two pairs of antennae, both used in swimming, 
the mandibles, and two pairs of maxillae. Ordinarily, the trunk 
region bears but two pairs of legs. 
Fig. 98.-Dorsal view of a female 
parasitic copepod, Argulus versicolor. 
(After Wilson). 194 
INVERTEBRATE ZOOLOGY 
Most ostracods are omnivorous. Both parthenogenetic and 
true sexual reproduction occur. The eggs produce nauplii which 
undergo a number of molts before reaching the mature form. 
Cypris is free-swimming while Candona is of a burrowing habit. 
In the marine forms (Cypridinidae), a heart is present but in 
fresh-water ostracods there is no heart. 
Fig. 99.--General anatomy of an ostracod, Cypris virens Jurine. (After 
Vavra). (Reprinted by permission from Ward and Whipple's Fresh-waterBioloyy, 
published by John Wiley and Sons, Inc.) 
Subclass Cirripedia 
Most Crustacea are free-moving animals except the parasitic 
forms and the members of the entire group known as the 
Cirripedia or barnacles. Not only are the barnacles sessile but 
as a consequence of this habit they show very marked degenera- 
tion of many structures. Adaptation to the sessile life also helps 
to explain the fact that hermaphroditism, which is so uncommon 
in Arthropods, is the usual condition among the Cirripedia. 
Though occasionally males are found they are usually small, 
degenerate forms known as complemental males which very 
frequently live within the shell of the female, at times as 
parasites. 
The body is enclosed in a membranous mantle which, in most 
instances, is encased in calcareous plates. It is not surprising PHYLUM ARTHROPODA 
195 
that until less than a century ago barnacles were thought to be 
molluscs. 
In development, the Cirripedia hatch from the egg as a naup- 
lius. Because of its resemblance to the ostracod, Cypris, a 
second larval stage with a bivalve shell, is termed the cypris stage. 
The free-swimming larva comes into contact with some object 
to which it becomes attached. The first antennae, with their 
associated cement glands, aid in the fixation. Calcified plates 
usually make their appearance in the mantle folds and form a 
protective shell surrounding the animal. Within this shell, the 
body of the barnacle is peculiarly oriented for, as is sometimes 
Fig. 100.-Barnacle. 
said, the barnacle stands on its head and kicks food into its mouth. 
Water currents bearing food to the mouth are set up by the 
beating of the usually six pairs of plume-like biramous appendages 
of the trunk. The mouth is surrounded by a pair of mandibles 
and two pairs of maxillae. 
The goose barnacles (Lepas anatiferd) have the region of 
attachment (Fig. 100) drawn out into a characteristically 
elongated stalk. A mediaeval myth maintains that the goose 
develops from these goose barnacles. The neckless barnacles, 
(Balanus for example), which encrust rocks and other submerged 
objects, have a flattened base of attachment from which the 196 
INVERTEBRATE ZOOLOGY 
skeletal plates arise directly. Barnacles are exclusively marine 
and choose stones, wood, animals, plants, in short any object as 
a place for attachment. When they become attached to the 
bodies of other animals, it is a very common thing for them to 
become dependent upon the animal which offers shelter and as a 
consequence there is here shown in great detail the development 
of the parasitic habit. Various species become more or less 
dependent upon whales and sharks while Sacculina becomes 
attached to the abdomen of decapod crabs and undergoes a para- 
sitic degeneration rarely equaled. All traces of appendages 
and of digestive organs are lost, and, as the name implies, 
Sacculina becomes a mere sac-like structure with a root-like 
system penetrating the body of the host for absorbing food. The 
relationships of this degenerate form are shown only through the 
larval stages which are fully characteristic of the barnacles. 
Fig. 101.-Male of the genus Nebalia. (After Claus). 
Subclass Malacostraca 
The Malacostraca are frequently spoken of as the higher 
Crustacea. Lobsters, crawfishes, crabs, shrimps, prawns, sow- 
bugs, scuds, and sandfleas are common names of some represen- 
tatives of this subclass. The body segments, so highly variable 
in number in members of the foregoing groups, are in the Mala- 
costraca almost rigidly limited to twenty or twenty-one. Usually 
the head consists of a prostomium and five additional somites, 
the thorax of eight, and the abdomen of seven of which the 
last (the telson) bears no appendages. PHYLUM ARTHROPOD A 
197 
The antennal glands are the typical excretory organs of adults 
though larvae and some Isopoda have a shell gland communi- 
cating with the second maxillae for excretion. The genital 
opening of the male is characteristically upon the coxopodite of 
the eighth thoracic appendage and of the female on the sixth. 
Within the Malacostraca, a considerable number of orders are 
recognized but for the purposes of this text attention is limited 
to but six of them. 
I. Order Phyllocardia 
A few marine species belonging to the genus Nebalia (Fig. 
101) show unusual combinations of phyllopodan and mala- 
costracan characters. In number of somites and in location of 
the genital openings, they agree with the higher Crustacea yet the 
thoracic appendages are leaf-like. 
II. Order Schizopoda 
These small, mostly marine Malacostraca with compound eyes 
on movable stalks bear a delicate carapace covering the cephalo- 
thorax. The eight appendages of the thorax are biramous 
swimming organs with both exopodite and endopodite and in one 
family (Mysididae) bear gills projecting freely into the water. A 
post-abdominal somite bears appendages which with the telson 
form a caudal fin. The use of this fin causes the animal to swim 
backward as do crawfishes and lobsters. This order includes a 
number of pelagic marine forms and the family Mysididae, some 
species of which inhabit fresh-water. Mysis relicta occurs in the 
Great Lakes at considerable depths and is identical with speci- 
mens found in similar lakes of northern Europe. 
III. Order Stomatopoda 
In the Stomatopoda, the posterior three thoracic segments bear 
complete biramous appendages which are used in swimming while 
of the five anterior to these all but the first bear appendages modi- 
fied as prehensile maxillipeds. The terminal segment of these 
prehensile appendages folds into a groove of the preceding seg- 
ment as a knife blade folds into its handle. 
The carapace is short for at least the posterior four segments 
of the thorax are not included within it. Antennules and stalked 
eyes are borne on two movable, independent somites at the 198 
INVERTEBRATE ZOOLOGY 
anterior extremity of the head. Gills are borne on the abdominal 
appendages. These are chiefly burrowing forms living in mud. 
Squilla (Fig. 102), Gonodactylus, Chloridella, are characteristic 
genera. 
IV. Order Decapoda 
The lobsters, crawfishes, shrimps, and crabs agree with the 
Schizopoda in having a carapace covering the entire cephalo- 
thorax. The head bears stalked eyes, 
antennules, antennae, mandibles, and 
two pairs of maxillae. Of the thoracic 
appendages, the anterior three pairs are 
modified as maxillipeds while the remain- 
ing five pairs are locomotor and lack 
an exopodite. All of the walking legs 
bear either pinchers or claws. The first 
pair of walking legs, which are known 
as the chelipeds, bear extremely strong 
pinchers termed the chelae. One jaw 
of the pinchers is the movable distal 
segment of the endopodite called the 
"finger" while the other jaw is an 
immovable outgrowth from the next to 
the last segment to form the "thumb." 
The Decapoda have marked powers 
of regeneration. If an appendage is 
removed, a new one begins to develop 
beneath the old shell but does not 
become evident externally until the 
shell is shed at the next molt. Injury 
is frequent in nature and consequently 
the claws are rarely of equal size. An injured claw is autoto- 
mously broken off by the animal at a special joint where there is 
but little tissue exposed to heal over. 
Abdominal Appendages.-Most of the abdominal segments 
bear small biramous appendages termed swimmerets except 
in the crabs (Fig. 103) which have a rudimentary abdomen. 
The appendages of the sixth abdominal somite are enlarged 
and with the telson form a powerful tail fin. In the males, 
the first and second abdominal appendages are highly modified 
as a copulatory organ for the transfer of sperm. In the females, 
Fig. 102.-Squilla alba. 
(From Bigelow). PHYLUM ARTHROPODA 
199 
the swimmerets serve as a place for the attachment of the eggs 
during development. 
Respiration.-Gills are borne on the basal joints of certain 
of the maxillipeds and legs and on the body wall. These are 
usually contained within a branchial chamber formed by the 
overhanging lateral folds of the carapace. A process of the 
second maxilla becomes modified as a gill-bailer the action of 
which pumps water through the gill chamber. The gill chamber 
retains water thus keeping the gills moist and permitting decapods 
Fig. 103.-The rock crab, Cancer irroratus. A, adult male, about one fourth 
size; B, larve in zoea stage; C, larva in megalops stage. (After Rathbun). 
to remain out of water for some time without injury to the gills. 
In some crabs, the branchial chamber has become modified 
as a lung in correlation with terrestrial habits. 
The circulatory system is of the open type. The heart 
lies within a pericardial sinus in the dorsal region of the thorax. 
Five arteries arise from the heart. Three of these pass ante- 
riorly to the organs of the head and thorax, one passes along the 
median dorsal line of the abdomen, and the last is directed ven- 
trally either from the posterior boundary of the heart or as a 
branch from the posterior vessel soon after the latter leaves the 
heart. Blood is received into the heart from the pericardial sinus 
through openings in the heart wall called ostia. The capillaries 
of the arteries allow the blood to pass into a large sternal sinus. 
In the thorax, offshoots from the sternal sinus pass into each gill 
as an afferent branchial vessel or vein. Within the branches of 
the gills, each afferent vein communicates with an efferent 
branchial vein from which the blood is returned to the peri- 
cardial sinus. 
Digestive System.-From the mouth opening between the 
mandibles on the ventral surface of the head, the digestive 200 
INVERTEBRATE ZOOLOGY 
system passes as a short tube, the esophagus, into a spacious 
chamber termed the stomach. The latter is divided into two 
regions; a large anterior sac, the cardiac chamber, which bears 
a chitinous organ for grinding food, and a smaller posterior 
pyloric chamber. The intestine, which proceeds from the 
pyloric chamber, receives the ducts from the two lateral hepato- 
pancreases. On the wall of the cardiac chamber, there are 
frequently hard, rounded masses of lime, the gastroliths. 
Excretory Organs.-Within the head, near the base of each 
antenna, lie the excretory organs, the green glands. Each 
discharges to the exterior by a pore located on the base of 
the antenna. 
The central nervous system consists of a ventral chain of gang- 
lia the number and disposition of which are correlated with the 
extent of the development of the abdomen. A brain, near the 
anterior extremity of the body, communicates with the ventral 
chain of ganglia by means of a pair of circumesophageal connec- 
tives. Six thoracic and six abdominal ganglia comprise the chain, 
except in the Brachyura which have a rudimentary abdomen 
and here the entire ventral chain becomes fused to form a single 
ganglionic mass. 
Reproduction.-The gonads frequently consist of two lateral 
and a single median lobe from which a pair of ducts lead to the 
genital openings on the ventral surface of certain walking legs. 
The eggs undergo superficial cleavage and give rise to either a 
larva or to a young adult the larval stages of which are all passed 
through while yet in the egg. Considering the relative uni- 
formity in structure of the adult decapods, there is a surprisingly 
great number of larval forms. Typically, the larva which leaves 
the egg is a zoea. From this develops a mysis stage (Fig. 95, A) 
with even the thoracic appendages biramous. In later develop- 
ment, the exopodites of these biramous thoracic appendages 
disappear (Fig. 95, C) leaving the unbranched walking legs of 
the adult. In the Brachyura (crabs), the thoracic appendages 
of the zoea, which are later destined to become the walking 
legs, develop in the free-swimming larva as rudimentary bud- 
like appendages only (Fig. 103) and never acquire the biramous 
condition characteristic of the mysis. Thus in the crabs the 
mysis stage has been eliminated and the zoea in passing to the 
adult form involves a more advanced type of larva which has 
been called the megalops (C). Some of the prawns have the PHYLUM ARTHROPODA 
201 
zoea preceded by a nauplius and metanauplius and thus show 
unique combinations of larvae of both the lower and the higher 
crustaceans. 
Typical Genera.-Two suborders are the Macroura and the 
Brachyura. The former include the lobsters (Homarus), various 
species and genera of shrimps (Crago, Peneus), the hermit 
crabs (Eupagurus and others which live in snail shells), some 
shrimps which occur in both fresh- and salt-water (Palaemon 
and Palaemonetes) as well as the crawfishes (Cambarus of North 
America and Potamobius of Europe and the Pacific coast of 
North America). Examples of the Brachyura are the edible 
crabs (Cancer, Callinectes, Carcinus), the fiddler crabs (Gelasi- 
mus) with marked asymmetry in size of the chelae, the spider 
crabs (Libinia, Macrocheira) with extremely long legs. 
V. Order Amphipoda 
Almost all of the Amphipoda are aquatic. Characteristically, 
the body is compressed laterally. The common name of beach 
fleas or sand fleas is due to the leaping movements with which 
they spring into the air when out of the water. The head bears 
six pairs of appendages. There are seven segments in the thorax. 
Fig 104.-An amphipod, Orchestia palustris. {After Kunkel). 
The first of these bears a pair of small appendages which function 
as maxillipeds. 
In the anterior region of the thorax, the sides of the body are 
frequently prolonged ventrally by epimeral plates which are 
borne upon the legs and serve to enclose the gills or gill sacs. 
In the same region, scales from the two sides of the body fre- 
quently enclose a brood pouch ventral to the ventral body wall. 
Within this pouch, eggs and the young are carried.' 
The abdominal appendages are of two types. On the anterior 
three somites of the abdomen, are borne biramous feet with 
many joints and numerous hairs. Posterior to these the appen- 202 
INVERTEBRATE ZOOLOGY 
dages are biramous but the branches are not segmented and form 
springing organs. Gammarus and Hyallela are common fresh- 
water genera while Orchestia (Fig. 104) and Caprella are marine. 
VI. Order Isopoda 
A dorso-ventral flattening of the body is characteristic of 
most isopods. The first thoracic segment is coalesced with the 
head. Though the body superficially shows sharp marks of 
segmentation, there is a tendency for 
the abdominal somites to fuse. All of 
the abdominal appendages are similar 
and usually biramous. Gills are borne 
on the ventral side of the abdomen 
and in the terrestrial species are capa- 
ble of utilizing moist air for respiration. 
A brood sac is borne ventral to the 
thorax as in the amphipods. 
Isopods, or sowbugs or pill bugs 
as they are called popularly, are 
found in both fresh-water (Asellus, Fig. 
105) and salt-water (Idotea) habitats 
and a number of genera are terrestrial 
(Porcellio, Oniscus). In these last, the head bears but a single 
pair of antennae. 
Very great degeneracy has accompanied the acquisition of 
the parasitic habit in a number of forms. 
Fig. 105.-A fresh-water 
i s o p o d„ Asellus communis. 
{After Kunkel). 
References 
(See general references cited at close of Chapter I) 
Birge, E. A. 1895. Plankton Studies on Lake Mendota, I. Vertical 
Distribution of the Pelagic Crustacea during July, 1894. Trans. 
Wisconsin Acad. Sci., Arts, and Lett., 10: 421-484. 
Birge, E. A. and Juday, C. 1912. A limnological Study of the Finger 
Lakes of New York. Bull. U. S. Bur. Fish, 32: 527-609. 
Brooks, W. K. and Herrick, F. H. 1892. The Embryology and Meta- 
morphosis of the Macroura. Proc. U. S. Natl. Acad., 5: 325-576. 
Churchill, E. P., Jr. 1919. Life History of the Blue Crab. Bull. U. S. 
Bur. Fish., 36: 93-128. 
Claus, C. 1863. Die freilebended Copepoden. Leipzig. 
- . 1876. Untersuchungen zur Erforschung der genealogischen 
Grundlage des Crustaceensystems. Wien. 
. 1879-1895. Numerous articles in Arb. Zool. Inst. Wien. PHYLUM ARTHROPODA 
203 
Darwin, C. 1851-1853. A monograph of the sub class Cirripedia. Ray 
Soc., London. 
Grenacher, A. 1897. Untersuchung liber das Sehorgan der Arthropoden. 
Gottingen. 
Herrick, F. H. 1911. Natural History of the American Lobster. Bull. 
U. S. Bur. Fish., 29: 149-408. 
Herrick, C. L. and Turner, C. H. 1895. Synopsis of the Entomostraca 
of Minnesota. Geol. and Nat. Hist. Surv. Minnesota, Zool. Series, 2. 
Huxley, T. H. 1901. An Introduction to the Study of Zoology, illus- 
trated by the Crayfish. New York. 
Kunkel, B. W. 1918. The Arthrostraca of Connecticut. Conn. Geol. 
and Nat. Hist. Survey, Bull. 26. 
Marsh, C. D. 1907. Revision of the North American species of Diapto- 
mus. Tran's. Wis. Acad. Sci. Arts and Letters, 15: 381-488. 
. 1910. A Revision of the North American Species of Cyclops. 
Trans. Wis. Acad. Sci. Arts and Letters, 16: 1067-1134. 
Ortmann, A. E. 1906. The Crawfishes of the State of Pennsylvania. 
Mem. Carnegie Mus. Pittsburg, 2: 343-523. 
Packard, A. S. 1883. A monograph of the Phyllopod Crustacea of North 
America, with remarks on the Phyllocardia. Ann. Rep. U. S. Geol. 
Survey. 
Patten, W. 1887. Eyes of Molluscs and Arthropods. Jour. Morph., 
1: 67-92. 
. 1887a. Studies on the eyes of Arthropods. Jour. Morph., 1: 
193-226 and 2: 97-190. 
Rathbun, Mary J. 1917. The Grapsoid Crabs of America. U. S. Natl. 
Mus., Bull. 97. 
Rathbun, R. 1893. Natural History of Economic Crustaceans of the 
United States. Bull. U. S. Fish Comm, for 1889: 763-830. 
Richardson, Harriet. 1905. A Monograph of the Isopods of North 
America. U. S. Natl. Mus., Bull. 54. 
Sharpe, R. W. 1897. Contributions to a knowledge of the North Amer- 
ican fresh water Ostracoda. Bull. Ill. St. Lab. Nat. Hist., 4: 414-484. 
Wilson, C. B. 1902 to date. Numerous articles on parasitic copepods 
in Proceedings U. S. National Museum. CHAPTER XIV 
PHYLUM ARTHROPODA (EXCLUSIVE OF CRUSTACEA 
AND INSECTA) 
Class 2 Acerata 
The horse-shoe crabs, spiders, scorpions, mites, a number of 
degenerate forms, and some important extinct animals which 
are known only from fossil remains, are grouped as a class to which 
the name Acerata is applied. This name signifies the lack of 
antennae common to all of the members of the class. The body 
usually consists of a cephalothorax and abdomen, though in the 
mites the setwo divisions are not separated. Six pairs of appen- 
dages upon the cephalothorax are arranged about the mouth. 
The bases of one or more pairs of these appendages are modified 
to serve as mandibles. The abdomen is composed of a variable 
number of somites which, in the embryo, bear appendages but 
these are lost or highly modified in the adult, except in the 
Xiphosura. 
Many modifications of the respiratory organs are found in 
this group. Gills occur on the abdomen of some but correlated 
with the air-dwelling habit the gills become drawn into the body 
where they are found as lung books which open to the exterior 
through narrow slits. In some instances, the lung books are 
replaced by tracheae which penetrate to all parts of the body as 
in the insects. 
The basal segment of the abdomen bears the genital opening. 
In development, there is no metamorphosis. 
Subclass Gigantostraca 
The extinct eurypterids and the horse-shoe crabs appear 
strikingly like Crustacea. In details of organization and in 
development, however, they show an intimate relationship with 
scorpions and other Acerata and are consequently recognized as 
constituting a subclass to which the name Gigantostraca has 
been applied. 
204 PHYLUM ARTHROPODA 
205 
Of the six pairs of cephalothoracic appendages, the first are 
preoral and the remaining five pairs, which are for walking, have 
their bases modified as masticating organs. In addition to a pair 
of lateral compound eyes, the cephalothorax bears a pair of 
median ocelli. 
Members of the order Eurypterida have a small cephalothorax 
and a large twelve jointed abdomen. These forms flourished 
during past geological ages and in structure seem to be inter- 
mediate between the scorpions and the members of the order 
Xiphosura, of which Limulus, the horse- 
shoe crab, is the only living example. 
In Limulus, the telson is long and 
spike-like. The abdomen bears six 
pairs of appendages of which the first 
forms a broad, flat operculum which 
overlaps the following five pairs of plate- 
like appendages, each of which bears a 
gill. Upon hatching from the egg, the 
young Limulus is said to be in the 
Trilobite stage (Fig. 106) because of its 
resemblance to the organisms of that 
group. 
Subclass Arachnida 
Scorpions, spiders, mites, harvest-men, and some less 
commonly known Acerata are grouped under a common subclass 
Arachnida. Most of these are air-breathing forms in which the 
cephalothorax bears one pair of pedipalps lateral to or imme- 
diately posterior to the mouth and a pair of preoral appendages 
termed the chelicerae. In addition to these appendages, four 
pairs of walking legs are just as characteristic of the arachnids as 
three pairs are for the insects. As an exception to the foregoing, 
it should be noted that as a rule young mites have but three pairs 
of legs (Fig. 108, C) and in some gall mites only two pairs of 
legs are found. 
True jaws are entirely lacking. Few arachnids swallow solid 
objects. The chelicerae and pedipalps are frequently modified 
for crushing the prey and in some instances the bases of some of 
the walking legs serve the same function. Usually, only the body 
juices of the victim are taken into the stomach and this is accom- 
plished by action of a muscular sucking stomach. 
Fig. 106.-Ventral view 
of trilobite larva of Limulus 
polyphemus. (After Kingsley). 206 
INVERTEBRATE ZOOLOGY 
Some species of arachnids are eyeless but more commonly there 
are from two to twelve simple eyes. Body form and struc- 
ture are so highly variable in the members of this subclass 
that they will be discussed under the more important orders 
individually. 
I. Order Scorpionida 
Scorpions are tropical and subtropical in their distribution, 
occurring in the United States only in the southern part. Some 
look much like crustaceans for in addition to the four pairs of 
walking legs there are a pair of 
large chelate pedipalps which are 
easily mistaken for legs. The 
chelicerae also bear chelae. Post- 
erior to the cephalothorax, which 
bears the appendages, is the 
abdomen. This is conspicuously 
divided into two regions. Seven 
broad somites continue backward 
from the cephalothorax as the 
preabdomen and these are followed 
by a series of six somites of smaller 
size which] constitute the post- 
abdomen. The terminal somite 
of the postabdomen bears a sharp 
spine known as the sting with 
which a pair of poison glands are 
associated. This serves as an 
effective organ for killing insects 
upon which the scorpions feed and 
produces wounds painful even to 
man. Lung books occur in the 
last four somites of the preabdomen on the ventral wall 
of which they open as paired spiracles. From three to six pairs of 
eyes are commonly borne on the cephalothorax. A pair of comb- 
like organs of undetermined function (Fig. 107), referred to as 
the pectines, occur on the ventral wall of the second preabdominal 
somite. 
Fig. 107.--A scorpion (Pandinus) 
from ventral surface. (From Ver- 
sluys and Damoil). 
Buthus, Uroplectus, Pandinus (Fig. 107), and Centrums are 
characteristic genera. PHYLUM ARTHROPOD A 
207 
II. Order Araneina 
Spiders are the most popularly known representatives of the 
arachnids and comprise the order Araneina. The body is divided 
by a deep constriction between the cephalothorax and abdomen. 
The four prominent pairs of legs serve not only for walking and 
jumping but the posterior pair also aid in the formation of the 
characteristic silken webs. Except in members of one family, 
the abdomen is sac-like, unsegmented, and joined to the cephalo- 
thorax by a narrow stalk. At the caudal end, the abdomen bears 
a small conical portion which represents a greatly reduced 
postabdomen. 
Spinning organs are located on the ventral surface near the 
caudal extremity of the abdomen and consist, usually, of three 
pairs of spinnerets. These are finger-like in form and are thought 
to represent rudiments of two pairs of abdominal appendages. 
Spinning tubes are distributed over the terminal portion of each 
spinneret and through these the fluid is expelled, which, upon con- 
tact with the air, hardens to form silk. An additional spinning 
organ known as the cribellum occurs in some spiders. This 
consists of a median ventral sieve-like plate anterior to the 
spinnerets bearing very numerous spinning tubes. 
The tarantulas (Eurypelma) and the orb weavers (many 
genera in the subfamily Araneinae) are among the extremely 
numerous representatives of this order. 
III. Order Acarina 
The mites and ticks have a broad, unsegmented abdomen which 
is not constricted at its union with the cephalothorax. As a 
consequence the entire body appears sac-like, though in some 
instances cephalothorax and abdomen are distinguishable. The 
part usually termed the abdomen includes two somites which in 
reality belong to the thorax. Frequently, the segments bearing 
the chelicerae and pedipalps are more or less distinct from the rest 
of the body and are then designated as a beak or rostrum. Though 
the typical number of legs is four pairs, the newly hatched young 
usually have but three pairs (Fig. 108, C) and in one family 
(Eriophyidae) only two pairs are present in the adults. 
The Acarina are of great biological importance. As parasites 
of man and of other animals they have great economic significance, 
and particularly the ticks, as carriers of disease-producing organ- 208 
INVERTEBRATE ZOOLOGY 
isms, have received considerable attention. The southern cattle 
tick, Boophilus annulatus (Fig. 108), is the carrier of the organism 
(a protozoan, Babesia) which causes Texas fever or tick fever in 
cattle. The cattle industry in the South is greatly hampered 
by outbreaks of this disease. Through misunderstanding of 
Fig. 108.-Acarina. Boophilus annulatus, the tick which carries Texas fever 
A, female; B, male; C, young with only three pairs of legs. (After Banks). 
the status of the generic name Boophilus this tick has been 
referred to frequently in literature under the name Magaropus. 
The itch mites (Sarcopies scabei, Fig. 109) burrow within the 
skin of man and produce a disease known as the itch. Mites 
of poultry and of other birds and of mammals 
also belong to the Acarina. The water mites 
(Hydrachnida) usually live as free adults in 
water though the larvae are frequently encoun- 
tered as parasites on molluscs, insects, and 
other animals. 
IV. Other Orders of Arachnida 
There are a considerable number of small, 
though very sharply marked, groups of arach- 
nids members of which fail to come within 
the foregoing descriptions of the larger groups. 
These are frequently recognized as comprising 
several independent orders. Chief of these 
are the Pseudoscorpionida, the Solpugida, the Phalangida, and 
the Linguatulida. 
The Pseudoscorpions resemble the scorpions except that 
there is no differentiation of pre- and post-abdomen and that 
there is no sting. The chelicerae function as spinning organs. 
Chelifer is a common genus. 
Fig. 109.-An itch 
mi.t e, Sarcoptes 
hum an i s . (After 
Banks). PHYLUM ARTHROPOD A 
209 
The Solpugida are comparatively rare. The head bears 
a pair of extremely large chelicerae which serve for crushing the 
prey, and long leg-like pedipalps which seem to function chiefly 
as feelers. The first thoracic segment is fused with the head 
but the remaining three are free. The genus Eremobates is 
represented by several species in the southern United States. 
The Phalangida are commonly called the harvest-men or 
daddy-long-legs. As the latter name implies, the legs are inordi- 
nately long. Respiration is by means of tracheae. They feed 
largely on mites. 
The Linguatulida or Pentastomida are arachnids, the bodies 
of which have been so much modified in adaptation to the parasitic 
habit that the adults are distinctly worm-like. It is only through 
Fig. 110.-A male pycnogonid, Nymphon stroemii, c, chelicerae; o, ovigerous 
legs; p, pedipalpi; r, rostrum. (After Kingsley, from Hertwig's Manual of 
Zoology, by Kingsley, courtesy Henry Holt and Co.) 
the larval stages that arachnidan relationships are evident 
for the larva bears two pairs of legs. All but the hooks of these 
legs become lost in metamorphosis and these are retained near 
the anterior extremity as organs of fixation. Numerous genera 
have been differentiated among these highly modified arachnids. 
Lungs, respiratory passages, and digestive system are usual seats 
of infestation. Pentastoma, Porocephalus, Armillifer, and 
Linguatula are characteristic genera. 
Class 3 Pycnogonida 
The pycnogonids are small, exclusively marine arthropods 
which cling to sea weeds and hydroids and at times are dredged 
in great numbers from deep waters. The body consists of 
a cephalothorax and vestigial abdomen. The cephalothorax 
usually bears a terminal suctorial proboscis and seven pairs of 210 
INVERTEBRATE ZOOLOGY 
jointed appendages. The appendages next to the proboscis 
bear chelae. Four pairs of appendages are usually used in 
walking. The third pair of appendages are in some species modi- 
fied in the male for holding the eggs and are termed the ovigerous 
appendages. Reproductive organs open on the second segment 
of certain of the legs. 
Though the abdomen is reduced to a mere vestige, without 
appendages and unsegmented, it contains two pairs of ganglia. 
The pycnogonids rather closely resemble the spiders but the 
presence of seven pairs of appendages is a character not encoun- 
tered in the Arachnida. There is considerable question as to 
the proper place to include the members of this aberrant group. 
Nymphon (Fig. 110) is a characteristic genus. 
Class 4 Tardigrada 
The water-bears or tardigrades are microscopic organisms 
living in both fresh- and salt-water. The body is provided with 
four pairs of unsegmented appendages each of which bears 
terminal claws. The number and form of these claws differ 
in the various genera. Neither antennae nor mouth parts are 
found on the head. Thus far neither respiratory nor circulatory 
organs have been demonstrated. 
A single gonad opens into the cloaca. The nervous system 
consists of a brain, a subesophageal ganglion, and a ventral 
chain of four ganglia. 
There is a possibility that the tardigrades may belong to 
the annelid group though the internal organization seems to 
indicate degenerate arthropod relationships. Macrobitus is 
the name of one genus. 
Class 5 Onycophora 
The genus Peripatus, and several other genera closely allied 
to it, present such a mixture of annelidan and arthropodan 
characters that much significance is usually attached to these 
forms as a possible connecting link between the worms and the 
arthropods. The various species, which have been encountered 
in Africa, Australia, New Zealand, Central and South America, 
and the West Indies, comprise several genera which together 
seem to warrant their being grouped as an independent class. PHYLUM ARTHROPODA 
211 
The body is worm-like or caterpillar-like in form, without exter- 
nal marks of segmentation but with numerous paired legs the num- 
ber of which varies in different species. Though these legs have a 
ringed appearance, they are not distinctly jointed, and in this 
respect they seem to be intermediate between the parapodia 
of worms and the jointed legs of arthropods. The metameric 
arrangement of the legs is paralleled by some features of the 
internal organization. Near each pair of legs, the ventral nerve 
cords bear a slight enlargement though ganglia are not differen- 
tiated. The base of each leg carries a nephridial opening. Thus 
the nephridia are also metameric in their arrangement. The 
head bears three pairs of appendages. At the anterior extremity, 
it bears a pair of ringed antennae and behind these a pair of 
oral papillae. Two pairs of hooked plates within the mouth 
cavity have been regarded as mandibles. Spiracles in longitudinal 
rows, or scattered, communicate internally with respiratory tubes 
which are in the form of tracheae. The genital ducts, which 
are modified nephridia, open just anterior to the anus. The 
Onycophora are viviparous. In habits, they are nocturnal, 
living during the day under bark and in decaying wood. 
Class 6 Myrientomata 
The minute arthropods included within the single order 
Protura are recognized by some as comprising an independent 
class to which the name Myrientomata has been applied. The 
representatives of this class are somewhat similar to the Thy- 
sanura in body form but antennae and cerci are both lacking. 
The thorax bears three pairs of legs and on the abdomen 
there are the vestiges of three pairs of appendages. 
Most of the described species live in humus. The group 
has been so recently established and the species have been 
so little studied that relationships to other arthropods have 
not been well established. 
THE MYRIAPODS 
In the older literature, a number of tracheate arthropods with 
numerous legs were considered as a single class under the name 
Myriapoda. These forms present so many differences in struc- 
ture that at least four distinct classes are now recognized 
under the names; Diplopoda (Class 7), Chilopoda (Class 8), 
Symphyla (Class 9), and Pauropoda (Class 10.) 212 
INVERTEBRATE ZOOLOGY 
The Chilopoda show many evidences of close relationship 
with the insects while members of the genus Scolopendrella, 
which represent the Symphyla, show remarkable combinations 
of diplopodan and insectan characters. Except for the eleven 
or twelve pairs of legs, Scolopendrella very 
closely resembles the insects belonging to 
the order Thysanura. The Pauropods are 
minute forms usually under one millimeter 
in length (Fig. Ill) bearing nine pairs of 
functional legs. 
Class 7 Diplopoda 
The thousand-legs or millipeds are terres- 
trial arthropods with the body consisting of 
two regions, head and trunk. The cylindri- 
cal body is composed of numerous segments 
each of which seems to bear two pairs of 
short legs which are inserted near the median 
line of the body. The apparent doubling in 
number of appendages on each somite, which 
is so uncommon in arthropods, seems to be 
explained on the grounds that each seeming 
segment is, in reality, a double somite for on the ventral surface 
the double nature of the skeletal plates is evident. Some few of 
the segments, especially near the anterior extremity, remain simple 
and bear but a single pair of legs each. The distinct head bears 
a pair of short antennae, usually lateral groups of ocelli, and the 
mouth parts. The latter comprise an unpaired upper lip, a pair 
of mandibles, and one or two additional pairs of jaws. 
Most millipeds are harmless and of little importance for they 
feed largely upon decaying vegetable matter. Julus, Glomeris, 
Polyxenus, and Spirobolus are commonly encountered genera. 
Class 8 Chilopoda 
The centipeds are terrestrial tracheate arthropods with only 
two body regions in which each of the flattened trunk somites 
bears but a single pair of legs. On the first trunk somite these 
are modified as poison jaws bearing ducts of poison glands. The 
legs are lateral in position and are not borne near the median 
ventral line as in the diplopods. 
Fig. 111.-Pauropus 
huxleyi. (After Ken- 
yon). PHYLUM ARTHROPOD A 
213 
The antennae are long and many-jointed. The head also 
bears a pair of mandibles and two pairs of maxillae. The common 
house centiped, Scutigera forceps, is a representative of this 
class. Lithobius, Geophilus, and Scolopendra are other genera. 
References 
(See general references at close of Chapter I) 
Banks, N. 1904. A Treatise on the Acarina, or Mites. Proc. U. S. 
Natl. Mus., 28: 1-114. 
. 1908. A Revision of the Ixodidae, or Ticks, of the United 
States. U. S. Dept. Agr., Bureau Entomol., Technical Ser., 15. 
Comstock, J. H. 1912. The Spider Book. Garden City, N. Y., Double- 
day Page. 
Hunter, W. D. and Hooker, W. A. 1907. Information concerning the 
North American Fever Tick. U. S. Dept. Agr., Bureau Entomol., 
Bull. 72. 
Kingsley, J. S. 1892-3. The Embryology of Limulus. Jour. MorphoL, 
7: 35-68; 8: 195-268. 
Montgomery, T. H. Jr. 1909. On the Spinnerets, Cribellum, Colulus, 
Tracheae, and Lung-books of Araneads. Proc. Acad. Nat. Sci., Phila., 
May, 1909: 299-320. 
Nuttall, G. H. F. Numerous papers on the morphology, classification, 
and biology of ticks. Parasitology, Cambridge, England. 
Packard, A. S. 1880. The Anatomy, Histology, and Embryology of 
Limulus polyphemus. Mem. Boston Soc. Nat. Hist., for 1880. 
Patten, W. 1893. On the Morphology and Physiology of the Brain and 
Sense Organs of Limulus. Quart. Jour. Mier. Sci., 35: 1-96. 
. 1912. The evolution of the vertebrates and their kin. Phila- 
delphia, Blakiston. 
Stiles, C. W. 1910. The Taxonomic Value of the Microscopic Structures 
of the Stigmal Plates in the Tick Genus Dermacentor. U. S. Hygienic 
Lab. Bull. 62. CHAPTER XV 
PHYLUM ARTHROPODA (CONCLUDED) 
Class 11 Insecta 
The members of this class are air-breathing arthropods with 
distinct head, thorax, and abdomen, except in some larvae and 
in some modified adults. They have a single pair of antennae, 
three pairs of thoracic legs, and usually one or two pairs of wings 
in the adult stage. The opening of the reproductive organs is 
near the caudal extremity of the body. As chiefly terrestrial 
animals, insects have become adapted to the greatest variety of 
conditions and have been so successful that they outnumber all 
other animals both in species and in individuals. Practically 
every possible relationship of organic beings is found in a list of 
the habits of insects. Both as predators and as parasites upon 
plants and upon other animals and in their relations to the spread 
of disease, they occupy a position of economic importance not 
excelled by any other animal group. 
Types of Metamorphosis.-Most insects in their development 
pass through conspicuous changes in form between the time 
that the individual leaves the egg and when it reaches full 
maturity. In these changes, which are collectively known as 
metamorphosis, the young lacks some structures or organs 
characteristic of the adult. These are attained only in later 
development, and frequently some structures or organs of the 
young are distinctive and become lost during later development. 
Numerous gradations occur between the condition in which the 
young is but slightly different from the adult and that in which it 
bears practically no resemblance to the adult to which it later 
transforms. As a consequence, several different types of meta- 
morphosis are recognizable. Insects which undergo a complete 
metamorphosis, that is, those which involve profound changes 
in form (Fig. 112) and have a pupal stage which is usually inactive, 
are said to be holometabolous or are referred to as the Holo- 
metabola. Egg, larva, pupa, and imago are all distinguishable in 
the Holometabola. The organs of one stage in development are 
214 PHYLUM ARTHROPOD A 
215 
not necessarily carried over directly into the following stage. 
Many of the larval organs disappear as a result of phagocytic 
or chemical action and through histogenesis new organs of the 
adult are formed. In the Holometabola, wings and legs of the 
adult do not develop externally on the larva but develop inter- 
nally as imaginal buds (Fig. 126) which emerge and become free 
only later in development. 
The Heterometabola include those insects which transform 
without a true pupal period. The stages between the egg and 
the imago fairly closely resemble the general bodily structure 
of the imago (Fig. 113) and are termed nymphs. The lack of 
Fig. 112.-Development of a holometabolous insect. A, larva; B, puparium; 
C, imago of fly (Phormia regina). (From Folsom's Entomlogy, courtesy of P. 
Blakiston's Son and Co). 
functional wings usually differentiates the nymph from the adult 
but external wing-pads from which the wings later develop are 
characteristic of many nymphs. 
The Thysanura and Collembola are wingless and throughout 
life essentially retain the forms they have at hatching. There 
are some changes, but these are so inconspicuous that many are 
inclined to refer to the insects of these two orders as the 
Ametabola. 
Appendages of the Head.-The head of an insect bears a single 
pair of antennae, the eyes, and the mouth parts (Fig. 114). 
These last comprise an unpaired labrum or upper lip, a pair of 
mandibles, the hypopharynx, the maxillae, and the labium. 
The labium, or lower lip, is in reality a second pair of maxillae 
of which at least the basal portions are fused along the median 
line. Both the maxillae and the labium are composed of several 216 
INVERTEBRATE ZOOLOGY 
distinct sclerites and bear palpi, designated respectively as 
maxillary and labial palpi. All of the appendages of the head 
are articulated with immovable parts forming the head capsule. 
Modifications of Mouth Parts.-The mouth parts are subject 
to numerous modifications of form and function. As described 
above they are suited for holding and chewing food, but in many 
groups only liquid food is taken and in these groups some of 
the mouth parts are modified to form a sucking tube. The 
most significant of these suctorial modifications are found in the 
Hemiptera, the Lepidoptera, the Diptera, and the Hymenoptera. 
The jointed beak of the Hemiptera consists of a trough-like 
labium partially covered above at its base by the labrum. 
Fig. 113.-Development of a heterometabolous insect. Six successive 
instars of the squash bug, Anasa tristis. A to E, nymphs; F, adult or imago. 
(From Folsom's Entomology Courtesy of P. Blakistons' Son and Co.) 
Within this trough the elongated maxillae and mandibles are 
ensheathed. 
The long, coiled proboscis of the Lepidoptera is formed of 
parts of the maxillae while the labrum, mandibles, and labium are 
greatly reduced or wanting. 
The female mosquito has a piercing type of mouth parts. 
The labrum and epipharynx are fused and with the hypopharynx 
form the food channel. The linear mandibles and maxillae are 
used in puncturing the skin of the victim, while the labium 
forms a sheath for the other mouth parts. 
Suctorial and mandibulate functions are both performed by 
the mouth parts of the honeybee. The mandibles are used for PHYLUM ARTHROPODA 
217 
cutting, crushing, and other purposes as in strictly mandibulate 
types. The maxillae and the labial palpi are folded to form a 
sheath within which an elongated portion of the labium serves 
as a lapping tongue. 
Sclerites and Sutures of the Head.-A considerable number of 
sclerites are fused to form the head capsule. In many instances, 
the sutures separating the sclerites are visible and both sutures 
and sclerites bear definite names. In the generalized insects, 
as, for example, in the Orthoptera, the epicranial suture (Fig. 
Fig. 114. 
Fig. 115. 
Fig. 116. 
Figs. 114-116.-Morphology of an insect head. 114, frontal aspect of cock- 
roach head; 115, lateral aspect; 116,caudal aspect. from Comstock with 
permission). 
114) is one of the best and most constant landmarks. This 
suture originates at the margin of the occipital opening (through 
which the viscera of the head and thorax are continuous) and 
extends as a median suture over the dorsal surface of the head. 
At its ventral extremity, the epicranial suture bifurcates, and 
thus its form is that of an inverted Y. Between the arms of the 
Y, there is an unpaired sclerite, called the front, which in most 
insects bears the median ocellus. An additional unpaired sclerite 
ventral to the front is the clypeus. On its ventral border the 
clypeus is articulated with the labrum or upper lip. 
The several paired sclerites of the head, including the lateral 
surfaces (Fig. 115) and the parts dorsal to the arms of the epi- 
cranial suture, constitute what is termed the epicranium. The 
vertex is just dorsal to the front. It is between the compound 
eyes, and usually bears the paired ocelli. The occiput extends 
between the vertex and the occipital foramen mentioned above. 
The genae and the postgenae form the lateral portions or cheeks 
of the epicranium. A chitinous supporting structure called the 
tentorium (Fig. 116) is found within the head. This usually 218 
INVERTEBRATE ZOOLOGY 
consists of a central plate from which two or three pairs of arms 
pass to the exoskeleton of the head. 
Sensory Organs.-Both simple (Fig. 117) and compound eyes 
are found in the insects. Compound eyes occur on the sides of 
the head in most adult insects except some generalized and some 
parasitic forms. Practically all insects which have a complete 
metamorphosis (the Holometabola) have simple eyes in the 
larval stages. 
Aside from the eyes, other sensory organs in insects show 
remarkable lack of uniformity in location upon the body and in 
localization and organization. 
Even the antennae, which are 
popularly thought of as tactile, 
have in some species of insects 
no less than seven different 
types of microscopic sensory 
organs. Most of these are 
probably tactile, auditory, and 
olfactory though frequently it 
has been necessary to assume 
functions upon the basis of 
structure and position and by 
interpretation of the reactions 
of the insect rather than to 
determine them by actual de- 
monstration. Hairs of gradu- 
ated lengths upon the feathery antennae of mosquitoes and moths 
vibrate in response to sound waves through a range which in the 
mosquito coincides with the pitch of the mosquito hum. Another 
type of auditory organ, of widely different structure and location, 
is the tympanal organ which consists of a drum-like membrane 
for the reception of sound waves. In the common grasshoppers, 
this tympanal organ is located on each side of the first abdominal 
somite while in katydids a similarly constructed organ is found 
on the second joint of each front leg. 
End organs of taste and smell are usually located on the maxil- 
lary palpi, the epipharynx, the hypopharynx, and the labial 
palpi but they are not restricted to any given organ or appendage. 
Tactile organs and sensory structures of undetermined func- 
tions occur as modifications of the body covering over most of 
the surface of an insect. 
F i g . 117 .-Median ocellus of a 
beetle, Acilius. (After Patten). PHYLUM ARTHROPOD A 
219 
Somites of the Head.-As in the other arthropods, paired 
appendages are considered as a criterion for the determination of 
the number of somites in the head. There are evidences that the 
eyes, antennae, mandibles, maxillae, and labium are borne on 
distinct segments. Beyond this, study of early embryological 
stages furnishes evidence that one or two additional pairs of 
appendages start to form in the embryo but never become func- 
tional, or are at most rudimentary in the adult. Traces of these 
embryonic appendages are between the fundaments of the eyes and 
antennae and between the mandibular and maxillary somites. 
Thus in position these correspond to the first antennae and to 
a pair of the maxillae of the crawfish and may be homologized 
with them. It thus seems probable that the head of an insect 
has resulted from the fusion of seven original somites only five 
of which have retained their appendages or their rudiments in 
the adult insect. 
The Thorax.-The thorax is the region which bears the legs and 
wings when they are present in the nymphs and adult insects. It 
is composed of three more or less firmly united segments. In 
order, backward from the head, these are; prothorax, mesothorax, 
and metathorax. In many insects, the last two bear wings. 
Each somite is composed of several sclerite groups which, accord- 
ing to their location, are recognized as comprising the parts of the 
tergum (dorsal wall), sternum (ventral wall), and pleura (lateral 
walls). 
The Legs.-Each leg is articulated to the wall of the thorax 
partly by means of small articular sclerites in the region of articu- 
lation of the sternum and pleuron. Five divisions or regions are 
recognizable in each leg. From articulation outward these are; 
coxa, trochanter, femur, tibia, and tarsus. In most insects, each 
region is but a single segment except the tarsus which commonly 
has five segments. Correlated with highly variable modifications 
in function the legs display numerous modifications in form. 
The Wings.-In many of the winged insects the mesothorax 
and the metathorax each bears a pair of wings but in instances of 
only one pair of wings these are usually borne upon the meso- 
thorax. Rudiments of the second pair of wings are frequently 
present as halteres or balancing organs upon the metathorax. 
Some striking modifications of the primitively membranous 
wings occur. In beetles, the mesothoracic wings are thickened, 
horny structures, the elytra, modified for the protection of the 220 
INVERTEBRA TE ZOOLOGY 
metathoracic wings and the dorsal surface of the body. In some of 
the bugs, the bases of the front wings (hemelytra) are horny while 
the tips are membranous. In Orthoptera, the entire front wings 
are somewhat thickened and are designated as tegmina. 
Structure and Origin of the Wings.-Typically, the wings of 
insects are two pairs of membranous appendages which develop 
as sac-like folds of the body-wall. In fully developed wings, this 
sac-like nature is obscured because the two walls of the sac 
become so closely applied that they appear as a single membrane, 
and a very delicate one at that. A framework upon which this 
membrane is supported is composed of hollow tubes or veins which 
originate as modified tracheae of the respiratory system. In 
Fig 118. 
Figs. 118-120.-Insect wing venation. 118, hypothetical primitive type of 
wing venation with cross veins added; 119, fore wing of Hyptia showing great 
reduction in number of veins; 120, fore wing of a mayfly showing great increase in 
number of veins. (Redrawn from Comstock with permission). 
Fig 119. 
Fig 120. 
the early stages of development, the tracheae are accompanied 
by nerves and blood-filled outgrowths of the body cavity. The 
pattern which this framework assumes is spoken of as the wing 
venation. In most groups, this pattern shows remarkable con- 
stancy among the members of the same species. Between genera 
and the larger groups there is usually considerable variation in the 
details of arrangement of the veins. It seems, however, that the 
wings of all insects are directly homologous and that their principal 
individual veins are likewise homologous. Comparisons of PHYLUM ARTHROPODA 
221 
numerous wings and the study of the arrangement of the tracheae 
before the wings reach their functional form have led to the formu- 
lation of a hypothetical plan of primitive venation (Fig. 118) from 
which all wing patterns are derivable, though all of the homol- 
ogies are not readily observable by the beginner in this study. 
Plan of Wing Venation.-In the plan of primitive venation 
(Fig. 118), eight principal veins are present and most of these may 
be branched. Beginning with the anterior margin when the 
wings are at right angles to the body these veins are: costa, 
subcosta, radius, media, cubitus, first anal, second anal, and third 
anal. Changes from the hypothetical type occur either by the 
addition of new veins or branches (Fig. 120) or through the 
Fig. 121.--Extremity of abdomen of a grasshopper, Melanoplus clifferentialis; 
A, male; B, female. The terga and sterna are numbered, c, cercus; d, dorsal 
valves of ovipositor; e, egg guide; p, podical plate; s, spiracle; s.p., suranal plate; 
v, ventral valves of ovipositor. (From Folsom's Entomology'). « 
reduction in the number of veins (Fig. 119) through atrophy or 
through coalescence of two or more veins. Cross veins fre- 
quently connect two of the longitudinal veins. 
The Abdomen and Its Appendages.-The abdomen bears a 
highly variable number of segments in the various groups of 
insects. A study of insect embryology shows that the abdomen 
consists normally of eleven segments, but in later development 
adjacent segments may coalesce and in some adults they are 
telescoped one within another. The wall of each segment is 
composed of a tergum and a sternum united by a pair of pleural 
membranes. The segments are typically without appendages and 
are approximately similar except near the caudal extremity where 
certain segments are more or less modified. In the Thysanura, 
rudimentary abdominal limbs occur and in the embryos of some 
other insects each segment may bear a rudimentary 
appendages. Those of the first seven abdominal segments are 
usually lost during early embryonic life, while the last two 222 
INVERTEBRATE ZOOLOGY 
or three pairs frequently persist to form the genitalia-the 
genital claspers of the males and the ovipositors of the females 
(Fig. 121). 
A true ovipositor consists of three pairs of valves, called gono- 
pophyses, arranged as a dorsal and a ventral pair surrounding an 
inner pair. The inner valves 
form a channel through which 
the eggs are conveyed. There 
is strong evidence that these 
three pairs of gonopophyses re- 
present the paired appendages 
of the eighth, ninth, and tenth 
abdominal somites and are seri- 
ally homologous with the thoracic 
legs. In the Hymenoptera, the 
ovipositor is modified as a sting- 
ing organ and has poison glands 
associated with it. 
The inner pair of gonopophyses 
of the males are modified as an 
intromittent organ or cirrus while 
the other two pairs of gono- 
pophyses are frequently modified 
as clasping organs which func- 
tion in copulation. Through 
many groups the external geni- 
talia show remarkable constancy 
of form within each species. In 
many instances, the forms 
grouped together as single 
species by early writers are at 
present being separated into 
several clearly distinct species 
chiefly on the basis of characters 
furnished by a study of the male 
genitalia. The last (eleventh) 
abdominal somite in many insects bears a pair of caudal appen- 
dages known as cerci. Both in form and in function these are 
highly variable. 
The Respiratory System.-The internal organization of insects 
is essentially like that of the other arthropods already described. 
Fig. 122.-Tracheal system of an 
insect, a, antenna; b, brain; I, leg; n, 
nerve cord; p, palous; s, spiracle; st, 
spiracular branch; t, main tracheal 
trunk; v, ventral branch; v.s., visceral 
branch. {From Folsom's Entomology, 
after Kolbe). PHYLUM ARTHROPODA 
223 
The respiratory system offers one of the most conspicuous points 
of difference from most other arthropodan groups. This is a 
system of air tubes, called tracheae and tracheoles, which carries 
air to all parts of the body (Fig. 122). This system is in com- 
munication with the outside air through openings in the body 
wall, termed spiracles, of which there are normally ten pairs 
arranged, as a rule, two pairs on the thorax and eight on the 
abdomen. Opening and closing of the spiracles for the admission 
or expulsion of air are under control of the insect. From each 
spiracle, a short trachea commonly leads to a main tracheal trunk 
of which there is one on each side of the body. Branches from 
these two main trunks penetrate between even the minutest parts 
of the body. 
Tracheae arise as invaginations of the body wall and con- 
sequently the infolded chitinous covering of the body continues 
within the tracheae as a chitinous internal lining of the tubes. 
Within the tracheae the chitin is not disposed in a uniform layer 
but assumes the form of a coiled spiral thread lining the inner wall 
of the tracheal vessel. Small tubes lacking the spiral chitinous 
threads form the most minute subdivisions of the tracheal system 
and are designated as tracheoles. Tracheae in some insects may 
become modified as enlarged sacs which serve as air reservoirs. 
Modifications of the Respiratory System.-Of the typical 
respiratory system with spiracles communicating directly 
between the tracheae and the outside air, there are many modifi- 
cations. In the Collembola, and in many aquatic larvae, there 
are no specialized organs for respiration for that function is per- 
formed directly through the skin. Gills occur in many aquatic 
nymphs and larvae. These are of two distinct types, tracheal 
gills and blood gills. In the former, lateral or caudal evagina- 
tions of the body wall are furnished with numerous tracheae which 
are continuous with the vessels of the tracheal system within the 
body and conditions suitable for a respiratory exchange are thus 
provided. Even a portion of the digestive tract may be appro- 
priated as a respiratory organ, as in the rectal tracheal gills of 
the dragonfly nymphs. Blood gills are usually thread-like 
evaginations from the body wall of aquatic insects. The spaces 
within these gills are in direct communication with the fluid- 
filled body cavity and through the delicate walls of the gills a 
respiratory exchange is made possible without requiring the 
presence of tracheal tubes. A few insects, especially larvae, live 224 
INVERTEBRATE ZOOLOGY 
under conditions which exclude the presence of oxygen. This 
is especially true of some larvae living in very deep water. 
Under these conditions, anaerobic respiration is carried on. 
The central nervous system consists of a brain or supraeso- 
phageal ganglion and a longitudinal nerve chain ventral to the 
digestive tract as in all of the Arthropoda. Typically each 
thoracic and abdominal somite is supplied with a ganglion 
(Fig. 123, A). It is, however, a very common thing for the nerve 
Fig. 123.--Different degrees of concentration of the ventral cord of Arthropods 
(from Gegenbauer). A, termite (after Lespes); B, water beetle (after Blanchard); 
C. fly (after Blanchard)-, D, Scorpion-spider (after Blanchard), a, abdomen; 
p2, g3, ganglia of ventral cord; gi, infraesophageal ganglion; gs, supraesophageal 
ganglion; o, eye; p'p", walking feet; tr, lung books; 1, chelicerae; 2, pedipalpi. 
(From Hertwig's Manual of Zoology by Kingsley, courtesy Henry Holt and Co.) 
chain to undergo concentration as a result of which the ganglia 
in adjoining somites fuse (B and C). The extent of such longi- 
tudinal condensation of the nervous system varies greatly in 
different arthropods and even shows remarkable changes during 
the life of a single individual. Thus, in the honeybee, the larva 
has a distinctly metameric nerve chain through the thorax and 
abdomen but when the adult stage is reached the thoracic ganglia 
have been reduced to two and the abdominal to five. 
A sympathetic nervous system may include a ventral trunk 
associated with the central nervous system and some very 
delicate branches near the dorsal part of the body. Branches PHYLUM ARTHROPODA 
225 
from this sympathetic system pass chiefly to the dorsal vessel 
and the tracheal system. 
Embryology.-The fertilized egg nucleus of insects undergoes 
several divisions and many of the resulting nuclei migrate from 
the yolk mass to the superficial layer of the cytoplasm. The 
layer of cytoplasm containing these nuclei undergoes cleavage 
and there is thus formed a layer of cells surrounding a central 
yolk mass (Fig. 8). This is essentially a blastula stage in develop- 
ment and the cell layer is frequently termed the blastoderm. 
Those nuclei which remain within 
the yolk become surrounded by 
cytoplasm and are designated as the 
yolk cells. The blastoderm becomes 
thickened in one region and forms 
the germ band from which the 
ventral surface of the embryo later 
develops. The course of this devel- 
opment follows two different paths in 
different insect groups. These are 
known as the overgrown and the 
invaginated types of development. 
The former of these involves much 
the less complicated narration. The 
germ band (Fig. 124) sinks below 
the surface of the surrounding 
blastoderm to form a groove. As this 
groove deepens, the walls of the 
blastoderm fold up over the germ 
band. When the folds of the blastoderm meet, they fuse and the 
double wall thus formed encloses a cavity known as the amniotic 
cavity which lies between the germ band and the before mentioned 
double wall. The outer layer of cells, which comprises the outer 
margin of the double wall, is termed the serosa and is directly con- 
tinuous with the blastoderm surrounding the yolk. The inner cell 
layer, which lines the amniotic cavity, is termed the amnion. The 
cells of the germ band comprise a layer of ectoderm adjacent to 
the amniotic cavity and underlying this ectoderm is a mass of 
cells which represent both the entoderm and the mesoderm of the 
embryo. The surface of this germ band, which represents the 
ventral surface of the developing insect, becomes traversed by a 
series of transverse grooves to form the primitive segments of the 
Fig. 124.-Two diagrammatic 
sagittal sections to show over- 
grown type of insect develop- 
ment. A, amniotic folds start- 
ing to cover the germ band (cross 
lined); B, amniotic folds com- 
pleted. (From Korschelt and 
H eider). 226 
INVERTEBRATE ZOOLOGY 
embryo. Upon these primitive segments, paired outgrowths 
occur. These are the fundaments of the legs and other appen- 
dages. At first these appendages are all similar in appearance 
but as development proceeds some of them become suppressed 
while the remaining ones begin to take on different forms 
depending upon the kind of appendage each is to form in the 
adult animal. Lateral and dorsal parts of the insect body 
result from the growth and extension of the germ band after the 
ventral structures have been laid down. 
A much more complicated condition exists when the germ band 
undergoes invagination. In such an instance, the germ band 
Fig. 125.-Diagrammatic median sections to illustrate development of a 
Libellulid egg. A, development of germ band; B, invagination of germ band; 
C, development of amniotic fold; D, closure of opening into amniotic cavity and 
development of rudiments of appendages; E, migration of embryo from amniotic 
cavity back to surface of egg. (From Korschelt and H eider after Brandt}. 
makes its appearance (Fig. 125, A) on the posterior ventral 
surface of the egg. One end of the germ band sinks into the 
underlying portion of the egg and, submerged within the egg, 
begins to grow forward (B) toward the anterior pole of the egg, 
carrying with it the blastoderm which was attached to the 
posterior margin of the band. Practically all of the germ band 
thus invaginates. The cavity formed within the egg by this 
invagination is the amniotic cavity, one wall of which is composed 
of the cells of the germ band and the other of the blastoderm cells. PHYLUM ARTHROPODA 
227 
The amniotic cavity is then cut off from the exterior. The 
invagination which has brought the germ band to lie within the 
amniotic cavity has seriously altered the orientation of the germ 
band with reference to other parts of the egg. The ventral 
surface of the embryo, which originated on the ventral external 
surface of the egg, has by invagination come to be directed 
toward the dorsal side of the egg. While in this position (O' and 
D), the embryonic appendages make their appearance. Soon 
afterward the embryo is everted from the amniotic cavity and 
again comes to lie (E) on the surface of the egg with its parts 
coinciding with the original orientation of the egg. 
In the embryological development of most of the Orthoptera, 
Trichoptera, Diptera, Lepidoptera, and Hymenoptera, the over- 
Fig. 126.-Development and transformation of imaginal discs in head and 
thorax of A, larva; and B-D, pupa of a fly (Musca). Wing rudiments are omit- 
ted. (From Korschelt and Heider after VanRees, in part after Kovalevsky). 
grown or superficial type of germ band occurs. The invaginated 
or immersed type of germ band is found in some of the Odonata, 
Coleoptera, Thysanoptera, and Hemiptera. Transitional con- 
ditions, which seem to be intermediate between these two types, 
are found in some of the Coleoptera and of the Orthoptera. 
Internal Metamorphosis and the Imaginal Discs.-In both 
types of metamorphosis, an insect undergoes radical changes in 
its internal organization before reaching the adult stage. Even 
organs which are found alike in the larval and adult stages do not 
pass over directly from one stage to the next but through the 
processes of histolysis the tissues of one stage disappear and 
entirely new tissues are formed through the processes known as 
histogenesis. In the Holometabola, where entirely new struc- 
tures such as wings and legs become functional for the first time 
with attainment of the adult form still more profound internal 228 
INVERTEBRATE ZOOLOGY 
changes accompany metamorphosis. During the larval and 
pupal stages, rudiments of the legs, wings, and head appendages 
make their appearance as internal buds. These imaginal discs, 
as they are called, are, throughout their early development, 
enclosed within internal sacs (Fig. 126, A). The hypoderm of the 
body wall invaginates to form these sacs and the rudiment of the 
appendage which each of these sacs contains lies thus within a 
cavity entirely surrounded by hypoderm. Only in later develop- 
ment (Fig. 126, C and D) do the sacs open and allow the develop- 
ing appendages to extend freely beyond the body surface. 
The development and transformation of the imaginal discs of 
the head and thorax of a fly are shown in Figure 126. 
In the classification of Linnaeus, there were but seven orders 
of insects recognized. These have, in recent times, been sub- 
divided and new orders have been added until today there are 
more than twenty well-defined orders of the Insecta. In the 
accompanying table, these orders are listed and some of the 
characters which are usually considered of importance are 
furnished for each. 
The Importance of Insects.-Insects have such significance 
in their relations to human welfare that it is difficult to give even 
a faint idea of their importance without going into extensive 
details for volumes have been written upon this phase of the 
subject. The honeybees, the silk worms, and the various insects 
which pollinate flowers are so widely and so popularly known that 
their importance need not be discussed here. As pests, the 
Hemiptera occupy a place of extreme significance, with the 
chinch bug (Blissus leucopterus) as a menace to grain farming 
and the hundreds of species of scale insects which attack trees 
and shrubs as outstanding examples. Among the Diptera, 
numerous disease-carrying flies and mosquitoes, the Hessian 
fly which destroys growing grain, and warbles and bots which 
attack domestic animals demand attention. The gipsy moth 
in its ravages upon shade trees, the coddling moth in its molesta- 
tion of the apple industry, and the numerous cutworms are 
examples of important Lepidoptera. Extensive damage to 
cotton crops through the boll-weevil and much of the injury 
by wood-boring and leaf-eating insects are attributable to species 
of the Coleoptera. 
Predaceous insects such as the coccinelid beetles, the dragon- 
flies, and the Neuroptera aid materially in checking the 
damage wrought by other insects. Likewise, the Hymenop- PHYLUM ARTHROPOD A 
229 
The Orders of Insects 
A Table Showing Some of the More Important Characters 
Order 
Examples 
Metamorphosis 
Mouthparts 
Wings 
Genera 
Common names 
Mesothoracic 
Metathoracic 
Thysanura 
Lepisma 
silver-fish 
ametabolous 
mandibulate 
wanting 
wanting 
Collembola 
Sminthurus 
snow-fleas 
ametabolous' 
mandibulate 
wanting 
wanting 
Orthoptera 
Blatta Melanoplus 
cockroaches, grass- 
hoppers 
earwig 
heterometabolous 
mandibulate 
tegmina (leathery) 
membranous folded 
Dermaptera.... 
Forficula 
heterometabolous 
mandibulate 
elytra or wanting 
membranous 
Plecoptera 
Pteronarcys 
stonefly 
heterometabolous 
mandibulate 
membranous 
membranous (larger) 
Isoptera 
Termes 
white ants 
heterometabolous 
mandibulate 
membranous 
membranous 
Embioptera.... 
Embia 
embiids 
heterometabolous 
mandibulate 
membranous or 
membranous 
Corrodentia.... 
Psocus 
book-lice 
heterometabolous 
mandibulate 
wanting 
membranous or 
membranous 
Mallophaga.... 
Menopon 
bird-lice 
heterometabolous 
mandibulate 
wanting 
wanting 
wanting 
Odonata 
Anax, Lestes 
dragon flies 
heterometabolous 
mandibulate 
membranous 
membranous 
Ephemerida.... 
Hexagenia 
mayflies 
heterometabolous 
atrophied in adult 
membranous 
membranous (smaller) 
Thysanoptera.. 
Hemiptera 
Thrips 
Anasa, Aphis 
thrips 
squash bug, plant-lice 
heterometabolous 
heterometabolous 
suctorial 
suctorial 
narrow, fringed 
with hair or 
wanting 
hemelytra or mem- 
similar 
membranous or 
Anoplura 
Pediculus 
sucking lice 
heterometabolous 
suctorial 
bran ous or wanting 
wanting 
wanting 
wanting 
Coleoptera 
Phyllophaga 
beetles 
holometabolous 
mandibulate 
elytra 
membranous 
Neuroptera. . . . 
Corydalus 
Dobson fly 
holometabolous 
mandibulate 
membranous 
membranous 
Trichoptera.... 
Molanna 
caddice flies 
holometabolous 
suctorial or rudi- 
membranous 
membranous 
Hymenoptera.. 
Apis, Lasius 
bees, ants 
holometabolous 
mentary in adult 
mandibulate and 
membr nous 
membranous 
Strepsiptera.... 
Xenos 
twisted-wings 
holometabolous 
suctorial 
mandibulate 
rudimentary or 
membranous 
Mecoptera 
Panorpa 
scorpion flies 
holometabolous 
mandibulate, usual- 
wanting 
membranous or 
membranous 
Lepidoptera.... 
Cecropia, Pieris 
Ctenocephalus 
moths and butterflies 
holometabolous 
ly at end of beak 
suctorial 
wanting 
membranous 
membranous 
Siphonaptera... 
fleas 
holometabolous 
suctorial 
wanting 
wanting 
Diptera 
Musca, Anophelese 
flies and mosquitoes 
holometabolous 
suctorial 
membranous 
wanting (halteres) 230 
INVERTEBRATE ZOOLOGY 
tera and the Diptera include many useful species in that they are 
frequently parasitic upon and cause the death of destructive 
insects. The illustrations cited here are only a start in the 
enumeration of the economically significant representatives of 
the class Insecta. 
References 
(See general references at close of Chapter I) 
Comstock, J. H. 1920. An Introduction to Entomology. Second 
Edition. Ithaca, N. Y., Comstock Publ. Co. 
Comstock, J. H. and A. B. 1915. A Manual of the Study of Insects. 
Ithaca, N. Y., Comstock Publ. Co. 
Folsom, J. W. 1922. Entomology with special reference to its Ecological 
Aspects. Third Edition. Philadelphia, Blakiston. 
Herms, W. B. 1915. Medical and Veterinary Entomology. New York, 
Macmillan. 
Herrick, G. W. 1914. Insects Injurious to the Household and Annoying 
to Man. New York, Macmillan. 
Kellogg, V. L. 1908. American Insects. New York, Holt. 
Lutz, F. E. 1921. Field Book of Insects. New York, Putnam. 
MacGillivray, A. D. 1923. External Insect-Anatomy, Urbana, Ill., 
Scarab Co. 
Needham, J. G. and Lloyde, J. F. 1916. The Life of Inland Waters. 
Ithaca, N. Y., Comstock Publ. Co. 
Osborn, H. 1896. Insects Affecting Domestic Animals. U. S. Dept. 
Agr., Div. of Entomol., Bull. 5 (n. s.). 
Pierce, W. D. 1921. Sanitary Entomology. Boston, Gorham Press. 
Riley, W. A. and Johannsen, O. A. 1915. Handbook of Medical Ento- 
mology. Ithaca, N. Y., Comstock Publ. Co. 
Sanderson, E. D. and Peairs, L. M. 1921. Insect Pests of Farm, 
Garden, and Orchard. New York, Wiley and Sons. 
Snodgrass, R. E. 1910. Anatomy of the Honeybee. U. S. Dept. Agr., 
Bur. Entomol., Tech. Series, No. 18. 
Wheeler, W. M. 1893. A Contribution to Insect Embryology. Jour. 
Morphol., 8: 1-160. CHAPTER XVI 
PHYLOGENY 
The phylogenetic relationships of organisms and the origin 
of species have long been topics of more than ordinary interest to 
the scientists. In one of the most generally maintained of the 
early conceptions all relationship was denied because it was 
thought that each species represented an independent, super- 
natural creation. Such was the belief of even so recent a scien- 
tist as Linnaeus whose systematic arrangement and classification 
of plant and animal forms furnish one of the best keys to an 
interpretation of the relationships of many groups. The natural 
origin of species from preexisting forms of life was first given 
strong support through the careful observations and generaliza- 
tions of Charles Darwin whose book on "The Origin of Species" 
(1859) has probably been more discussed than any other scientific 
work. With the establishing of a belief in blood relationship 
between the various animals a new interest in the tracing out 
of that relationship became awakened. 
At the present time there is much ground for lack of agreement 
concerning how species have come into existence but among 
scientists there is no longer any doubt but that species, genera, 
and even phyla have moved in a continuous procession since the 
inception of life upon this planet. Even a cursory survey of the 
evidences from paleontology reminds one of the facts that 
there are no indications of vertebrates existing upon the earth 
prior to the lower Cambrian period of the earth's history and that 
in general the faunas of successive periods and eras show advance- 
ment beyond yet undeniable relationships with the preexisting 
life. It is not the purpose of this chapter to prove the ideas of 
continuity, development, and differentiation of living organisms 
for this is now accepted by those who take the trouble to examine 
the evidences as little less than axiomatic. Nor yet is it the 
intention to speculate upon the methods or processes whereby 
these changes in the fauna and the creation of different types 
have come -about. Even best informed scientists 
there is yet lack of unanimity of upon these subjects. 
231 232 
INVERTEBRATE ZOOLOGY 
Any attempt at classification in some measure endeavors to 
convey a concept of relationships. Groupings or separations are 
based upon an evaluation of the presence or the lack of common 
characteristics. In most instances, the possession of fundamental 
characters of like nature is considered as indicative of common 
origin. As pointed out in Chapter I, the student is apt to con- 
sider phylogenetic relationships from an erroneous point of view, 
assuming that such kinship is traced between the representatives 
of present-day groups when as a matter of fact all of our existing 
organisms have undergone some degree of differentiation since 
their origin. It is only through common ancestry that these 
lines of kinship are traced. 
Sources of Evidences of Relationships.-Obviously, then, 
relationships are not observable directly, but frequently may be 
deduced from evidences gathered from one or more of the fields 
of paleontology, embryology, and comparative anatomy. Even 
these available sources in many instances give incomplete and 
inconclusive evidence of an organism's past history. Present-day 
organisms are but points in the lines of evolutionary progress. 
The source and the termination of any such line are but rarely 
discovered. Through the study of paleontology and embryology 
we gain a glimpse here and there of portions of the path which 
various organisms have followed in reaching their present posi- 
tion in the lines of evolutionary progression. 
Paleontology.-In determining a broad outline of the course 
of phylogeny among the invertebrates the science of paleontology 
offers but fragmentary and woefully incomplete data concerning 
the ancestral forms. The Cambrian period has disclosed repre- 
sentatives of most of the important invertebrate phyla, yet as 
Schuchert has said, more fundamental evolution had taken 
place up to this time than subsequently. Studies of the rocks 
of the preceding periods have revealed rare and imperfect and 
almost indecipherable evidences of a marine fauna including 
Protozoa and marine worms. Careful studies by Walcott and 
others have furnished evidences that practically all of the 
important invertebrate phyla were represented in the fauna of 
pre-Cambrian times. However, the pre-Cambrian rocks have 
undergone metamorphosis under the action of heat and pressure 
until their contained fossil remains have become concealed or 
even destroyed. PHYLOGENY 
233 
Because of the incompleteness of our knowledge of pre-Cam- 
brian life, the evidences of phylogeny of the lower forms must 
be sought elsewhere than in the fossil remains. In individual 
development, there is furnished a clue to racial history. 
Law of Biogenesis or Recapitulation Theory.-Since the days 
of von Baer, it has been commonly observed that individuals 
which are markedly different in appearance as adults have stages 
in their development when the embryos have much the same 
appearance. Structures common in the embryos may have 
entirely different fates in the adults. The works of Haeckel did 
much to popularize this observation. So generally did the princi- 
ple seem possible of application that it became expressed as a 
law-the law of biogenesis-which states that ontogeny is a brief 
recapitulation of phylogeny. 
Thus, since the course of individual development more or less 
faithfully repeats racial history, those characters which make 
their appearance early in the course of embryology represent a 
heritage from distant ancestors of the race. The more distant 
the ancestor the more numerous the offspring and consequently 
the more kinds of animals which would display these ancestral 
characters in the course of their development. Such ancestral 
characters, appearing early in development, have been desig- 
nated as palingenetic. In contrast with them, the coenogenetic 
characters which appear relatively late in development are 
more nearly specific and are found in much smaller groups of 
individuals. 
There are many instances in which individual development has 
become so highly modified that the operation of the biogenetic law 
seems to be invalidated. In some instances the processes of 
ontogeny become so much shortened that whole chapters in the 
racial history are deleted in the course of individual genesis. 
These and other facts have led some investigators to discredit 
the law entirely. It seems probable, however, that in many 
instances valuable light is thrown upon racial history through 
the study of embryology. 
Gastraea Theory.-Since a gastrula stage occurs in the onto- 
geny of practically all of the Metazoa, Haeckel propounded a 
theory which endeavored to establish a blood relationship among 
all Metazoa through an ancestral form to which he applied the 
name of the Gastraea. The Gastraea he considered as a hypo- 
thetical form which in its adult organization displayed the 234 
INVERTEBRATE ZOOLOGY 
characters of the embryonic gastrula. Though our present-day 
Coelenterata are said to stand on a level of the gastrula in funda- 
mental structure, the Gastraea must not be confused with the 
members of this phylum which have undergone great differentia- 
tion and progressive evolution. 
Without discrediting the Gastraea theory in the least, it 
represents but a step in the right direction, for do not the various 
gastrulae in turn originate from a single-celled condition-the 
fertilized egg? Thus all animal forms may be traced along their 
phylogenetic paths to a common ancestry in the one-celled 
organisms. Even Haeckel recognized this fact at the time 
when he propounded his Gastraea theory. The highways 
between the single-celled condition and our highly diversified 
Metazoa have not all been surveyed. Only here and there are 
there indications of the routes which have been taken. Some 
forms seem to have gone on an independent track early in the 
course of evolution. Others seem to have traveled long distances 
together before the ancestral stock became diversified to form 
various ones of our present-day animals. Some groups, such 
as the Vertebrata, the Echinoderma, and the Nematoda, 
seem to have left no conclusive evidence regarding their relation- 
ships to any other animal groups. Speculation plays a large 
part in attempting to decipher the faint lines which are directed 
toward but never lead to any of the usual roads of descent. 
In the following section attention will be directed to some of 
the probable ancestral lines of the various Metazoa. 
Metazoan Tendencies in the Protozoa.-In Chapter II, it has 
been pointed out that within the Protozoa are found many 
indications of tendencies toward metazoan conditions. Chief 
among these are colony formation and isolation of germplasm 
from the soma. Much has been made of the parallel between 
the blastula stage in the embryology of the Metazoa and the 
spherical arrangement of the cells in colonies such as Volvox. 
By one or several paths of descent, the ancestors of our present - 
day Protozoa have probably been the direct or indirect source 
of all the remaining phyla. 
Porifera.-While the Porifera are usually accorded a position 
as the lowest phylum of the Metazoa, there are numerous reasons 
for assuming that the simplicity of sponges is attributable, at 
least in part, to degeneracy or to regressive evolution. The 
collared entoderm cells of the Porifera are the exact counter- PHYLOGENY 
235 
parts of the bodies of Choanoflagellates. Proterospongia is a 
unique form which might resemble the ancestors of the Porifera 
and in an unusual manner stands intermediate between the 
Choanoflagellates and the sponges. This is a colony of Choano- 
flagellates the individual cells of which are embedded in a matrix 
containing some wandering amoeboid cells comparable to the 
mesoderm cells of the Porifera. 
Coelenterata and Ctenophora.-It seems probable that a 
primitive gastrula of the nature of Haeckel's hypothetical 
Gastraea must have had its origin from a spherical protozoan 
colony by more rapid growth at one pole which resulted in an 
invagination to form the gastrula cavity. The planula, some 
modification of which occurs in the larval development of all 
coelenterates and of the ctenophores, is but a slightly modified 
gastrula. As MacBride has so well pointed out, a planula- 
like ancestor probably gave origin to the members of these two 
phyla. 
Plathelminthes.-In an earlier chapter it has been stated 
that Coeloplana and Ctenoplana are modern genera which stand 
intermediate between the flatworms and the ctenophores and 
display an odd combination of characters some of which had 
come to be considered as diagnostic for one or the other of the 
two phyla. It is probable that through some form similar to 
these the flatworms have arisen from a ctenophore-like stock. 
Nemathelminthes.-Regarding the origin and relationships of 
the Nemathelminthes practically nothing is known. They 
stand as peculiarly isolated forms whose history has never been 
deciphered. The method of mesoderm formation seems to 
preclude any close relationships with the annelid worms. There 
has been no well-founded theory advanced as to their origin or 
relationships. The numerous suggestions which have been 
offered are based upon very minor details. 
The Trochophore.-In the Plathelminthes, Coelhelminthes, 
Molluscoidea, Trochelminthes, and Mollusca there occur larvae 
which represent only minor modifications of the trochophore. 
Thus a direct relationship among these forms is traceable through 
the larvae. In fundamental structure the trochophore has been 
likened to the ctenophores. It has already been shown that the 
Plathelminthes are readily derivable from the ctenophore 
organization. The metamerism, so characteristic of the Anne- 
lida; has been explained on various grounds. Beginnings of 236 
INVERTEBRATE ZOOLOGY 
metamerism are readily observable in the flat worms. Regular 
arrangement of the lateral diverticula from the digestive tract 
and orderly disposition of the gonads between these diverticula 
indicate preparations for a segmentation of the body before the 
septa make their appearance as in the annelids. 
Echinoderma.-The echinoderms offer but few clews to an 
explanation of their origin. Their radial symmetry is not 
primitive and, since the larvae are bilaterally symmetrical 
throughout, it seems probable that the members of this group 
have evolved from a bilaterally symmetrical coelomate ancestor. 
MacBride has emphasized the fact that echinoderm larvae 
represent a primitive type not directly related to the trochophore. 
Echinoderm larvae show more resemblance to the tornaria larva 
of Balanoglossus than to any of the lower Metazoa. In some 
instances this fact has been utilized in tracing a possible ancestry 
of the vertebrates through a line which passes back through the 
Enteropneusta (Balanoglossus) to a primitive type from which 
the echinoderms and the Enteropneusta have arisen. 
Arthropoda.-The arthropods have many points in their 
morphology which are likewise shared by the annelids. Even 
the parapodia of the annelids are not so markedly different from 
the foliaceous appendages of the lower Crustacea. The Onyco- 
phora (Peripatus) were for a long time considered as worms but 
closer investigation revealed characters which have been looked 
upon as diagnostic of the Arthropoda. Standing as they do, mid- 
way between the arthropods and the annelids, the Onycophora 
show a transition from one group to the other which may well be 
taken as indicative of a worm-like ancestry for the arthropods. 
In this phylum there arises an interesting conflict between the 
evidences presented by embryology and morphology. On the 
basis of morphological study the arthropods seem to have their 
closest affinities with the elongate annelid-like forms but in 
development all of the lower Crustacea pass through a character- 
istic larval form known as the nauplius. This larval form is 
very short and bears but three pairs of appendages. There is no 
conclusive method of weighing the relative merits of the two 
lines of evidence. It seems, however, that the nauplius might 
be an adaptation to a free-swimming existence and thus may 
have undergone changes a full record of which have not been 
retained in the much shortened history of the race which is pre- 
sented by ontogeny. Some who see in the nauplius a significant PHYLOGENY 
237 
ancestral form have considered the nauplius as a modified troch- 
ophore not fundamentally different in structure from some of our 
present-day rotifers. The swimming appendages of rotifers like 
Pedalion are thought by some to represent incipient arthropod 
appendages. 
Ancestry of the Vertebrates.-Some of the most widespread 
interest in the blood relationships of animals centers around the 
question of the origin of the Chordata. Even though the verte- 
brates seem to have made their appearance at a time when a 
fairly complete picture of the fauna is observable through the 
fossil remains, there is little light thrown upon the origin of 
the ' vertebrates through the study of paleontology. Fossils 
of some of the heavily armored fishes, the Ostracoderms, closely 
resemble the fossil arachnids known as the Merostomata and our 
modern Limulus which some one has called a "living fossil." 
However, the study of comparative anatomy seems to indicate 
that the vertebrates must have had a simpler beginning than 
this would indicate. 
The three subphyla of the Prochordata, represented by Amphio- 
xus (or Branchiostoma), Balanoglossus, and the tunicates, have 
characters which permit them to be classified with the Chordata, 
yet in most instances display a low type of general organization 
which seems to relate them to the non-chordate forms. Each 
of these groups has been considered as a possible ancestral stock 
from which the true vertebrates have had their origin. Since 
these forms lie outside the scope of this textbook their relation- 
ships to the problem of vertebrate phylogeny will not be considered 
in detail but attention will be directed to some of the inverte- 
brate groups through which the ancestry of the vertebrates 
has been derived by various investigators. 
As has been pointed out in an earlier discussion, the Metazoa 
in general, including the Vertebrata, develop from a single 
cell, the zygote. Thus a possible ancestral history of the verte- 
brates leads back to the single-celled organisms. However, 
this relationship is so distant that many attempts have been 
made to find satisfactory evidences of vertebrate geneology 
through the higher groups of the invertebrates. Early metamer- 
ism of the vertebrate embryos indicates that some segmented 
organism gave rise to the vertebrate line but further than this 
the evidences are capable of broadly divergent interpretations. 
In consequence, numerous theories have been advanced. Space 238 
INVERTEBRATE ZOOLOGY 
does not permit a detailed account of all of these but some 
of the more important ones have been chosen for discussion. 
The coelenterates, the nemertines, the annelids, and various arthro- 
pods have been championed as the possible origin of the verte- 
brate series. 
Annelid Theory.-The metameric condition of annelids, the 
relation of their nephridia to the coelom, and the fundamental 
relationships of vascular and nervous systems to the digestive 
tract, closely resemble the conditions found in the lower verte- 
brates and in vertebrate embryos. Semper and other investiga- 
tors have shown that by inverting the position of the body of 
an annelid the fundamental systems are brought into almost 
complete harmony with their arrangement in the vertebrates. 
Such a shift in the orientation of the body is not at all uncommon 
in various animal groups. Many of the crustaceans and molluscs 
move with their ventral surface in a dorsal position. Even the 
notochord, which is distinctive for the Chordata, has a counter- 
part in the bundles of supporting fibers which accompany the 
annelidan nerve chain. Recent publications of Delsman have 
awakened a new interest in the annelid theory of vertebrate 
origin. 
Nemertine Theory.-Hubrecht has maintained that the nemer- 
tines stand in the direct line of ancestry of the vertebrates. 
One of the chief arguments in favor of this theory is the possible 
homology between the proboscis sheath of the nemertine and 
the notochord of the chordate. The lateral nerve cords of 
the nemertine could assume a dorsal position as in the chordates 
without a complete change in the orientation of the body such 
as is necessitated in the annelid theory. 
Arachnid Theory.-Patten has seen in the arachnids, especially 
in forms like the scorpions and Limulus, many points of structure 
in direct harmony with vertebrate organization. Through 
comparisons of these arachnids with fish-like Ostracoderms he 
has built up an elaborate theory showing a possible origin of 
the vertebrates from arachnidan ancestry. 
Conclusion.-Each of the numerous theories, of which the 
foregoing are characteristic examples, is based upon a group 
of facts derived from studies in comparative anatomy and embry- 
ology. Yet no one theory depicts a satisfying genealogy of 
the vertebrate group. At most the various hypotheses furnish 
ground for a belief in kinship between the vertebrate and the PHYLOGENY 
239 
invertebrate. Kinship or common origin would explain most 
of the facts which have been arrayed as a proof or demonstration 
of vertebrate origin from the invertebrates. 
Similarity in structure and development and even homologies 
between the members of two animal groups do not prove that 
one has originated from the other. At most they point to a 
common heritage. Our various theories demonstrate undis- 
puted interrelationships between the chordate phylum and the 
non-chordates but the key to the ancestry of the vertebrates 
lies hidden, possibly lost, in some form of past ages which has 
been an ancestor alike to the vertebrates and the higher inverte- 
brates and through which both of these groups have inherited 
the characters which they hold in common. A search for this 
ancestral form among the highly differentiated animal forms of 
today is little short of hopeless. 
References 
Delsman, H. C. 1922. The ancestry of vertebrates as a means of under- 
standing the principle features of their structure and development. 
Weltevreden (Java). 
Gaskell, W. H. 1908. The Origin of the Vertebrates. London, Long- 
mans-Green. 
Hubrecht, A. A. W. 1883. On the ancestral form of the Chordata. 
Quart. Jour. Mier. Sci., N. S., 23: 349-368. 
Lull, R. S. 1917. Organic Evolution. New York, Macmillan. 
MacBride, E. W. 1914. Text-book of Embryology. Vol. 1, Inverte- 
brata. London, Macmillan. 
Osborn, H. F. 1917. The Origin and Evolution of Life. New York, 
Scribners. 
Patten, W. 1912. The Evolution of the Vertebrates and their Kin. 
Philadelphia, Blakiston. 
Semper, C. 1875. Die Stammesverwandtschaft der Wirbelthiere und 
Wirbellosen. Arbeit. Zool. Zoot. Inst. Wurzburg, 2: 25-76. 
Walcott, C. D. 1899. Pre-Cambrian Fossiliferous Formations. Bull. 
Geol. Soc. Amer., Apr. 6, 1899: 199-244. 
Wilder, H. H. 1909. History of the Human Body. New York, Holt. INDEX 
A 
Abalone, 177 
Abdomen, 183, 198, 201, 207, 209, 
214, 221 
Abdominal appendages of Insecta, 
221 
Abothrium, 111 
Acantharia, 39 
Acanthobdella, 140 
Acanthocephala, 121 
Acanthocephalus, 122 
Acanthocystis, 37 
Acanthometra, 39 
Acanthosphaera, 39 
Acarina, 207 
Acephala, 168 
Acephaline gregarines, 41 
Acerata, 184, 204 
Acetabulum, 104 
Acineta, 53 
Acmaea, 177 
Acoela, 101 
Acontia, 92 
Actinaria, 93, 94 
Actinomyxidia, 46, 47 
Actinophrys, 37 
Actinosphaerium, 37 
Actinotrocha, 148 
Adductor muscles of Acephala, 168 
Brachiopoda, 147 
Adhesive cells, 95 
Adinida, 29 
Adjustor muscles, 147 
Adoral shields, 158 
Adradii, 89 
Aeolosoma, 138 
Aeolosomatidae, 138 
Aerobic respiration, 69 
Agriolimax, 178 
Alcyonacea, 94 
Alcyonaria, 92, 94 ? 
Alcyonium, 94 
Alternation of generations, 9 
Coelenterata, 83 
Protozoa, 24 
Alveolar layer of cytoplasm, 49 
Ambulacral area, 152, 158 
Ametabola, 215 
Armenia, 168 
Amnion, 225 
Amoeba proteus, 35 
terricola, 35 
verrucosa, 35 
Amoebina, 35 
Amphilina, 108 
Amphineura, 167 
Amphioplus, 158 
Amphipoda, 201 
Amphisolenia, 29 
Ampullae, 78, 155 
Anaerobic respiration, 69, 224 
Anal veins, 221 
Anaplocephala, 111 
Ancestry of vertebrates, 237 
Ancylostoma, 119 
Ancylus, 178 
Ancyrocephalus, 103 
Angiostoma nigrovenosum, 119 
Anisotropic, 63 
Annelida, 131 
Annelidan cross, 15, 134 
Annelid Theory, 238 
Annulations of leeches, 139 
Annulus, 29 
Anodonta, 173 
Anomia, 173 
Anopheles, 44 
Anostraca, 190 
Antedon, 165 
Antedon rosacea, 164 
life cycle, 164 
Antennae, 185, 186 
Antennal gland, 197 
241 242 
INDEX 
Antennules, 186, 191 
Anthophysa, 30 
Anthozoa, 91 
Antipatharia, 94 
Antipathes, 94 
Aphrodita, 137 
Apical plate, 136 
Aplacophora, 167 
Aplysiidae, 178 
Apopyle, 78 
Appendages, 185 
biramous, 185, 192, 197, 198 
crustacean, 185 
foliaceous, 186 
insectan, 214, 219, 221 
phyllopodan, 186, 189, 197 
polychaet, 135 
rotifer, 127 
schizopodal, 185 
Apus, 190, 191 
Arachnid theory, 238 
Arachnida, 205 
Araneina, 207 
Araneinae, 207 
Arbacia, 160 
Area. 173 
Arcella, 34, 38 
Archenteron, 16, 67 
Archiannelida, 141 
Archigetes, 107 
Arenicola, 137 
Argonauta, 181 
Argulids, 193 
Argulus, 193 
Arion, 178 
Aristotle's lantern, 159 
Armillifer, 209 
Arrow-worms, 131 
Artemia, 190 
Arthropoda, 183, 184 
classes of, 184 
phylogeny of, 236 
Articular sclerites, 219 
Articulata, 147 
Articulation, 183 
Asaphus, 189 
Ascaris, 119 
germ cells, 10 
lumbricoides, 119 
Ascon, 77 
Asellus, 202 
Asexual reproduction, 7 
Aspidobranchia, 177 
Asplanchna, 126, 127, 128 
Assimilation, 65 
Asterias, 157 
Asteroidea, 151, 152 
Astrangia, 94 
Astrophy ton, 158 
Atlanta, 177 
Atoke, 135 
Aurelia, life history, 89 
Auricles, 69, 170 
Autoinfection, 44 
Autolytus, 137 
Autophytic, 22, 28 
Autosomes, 12 
Autotomy, 198 
Avicularia, 145 
Axon, 63 
Axostyles, 26 
B 
Babesia, 208 
bovis, 44 
Balantidium, 51 
Barnacles, 194, 195 
de Bary, 21 
Basal granules, 26, 49 
Basals, 163 
Basipodite, 185 
Basis for classification, 2 
Basket stars, 158 
Beach fleas, 201 
Beak, 207 
Beroida, 96 
Biloculina, 38 
Biogenesis, law of, 233 
Bipinnaria, 157 
Biramous appendages, 185 
Bivium, 154 
Bladderworm, 109 
Blastoderm, 181, 225 
Blastoidea, 152, 162 
Blastomeres, 14 
Blastula, 16 
Blepharoplast, 26 INDEX 
243 
Blood gills, 70, 223 
Blood-suckers, 140 
Bojanus, organs of, 170 
Boophilus annulatus, 208 
Bosmina, 192 
Bothrimonus, 111 
Bothriocephalus, 111 
Brachiolaria, 157 
Brachiopoda, 144, 146 
Brachyura, 188 
Bracts, 88 
Brain, 74 
Branchiae, 153 
Branchial chamber, 174, 199 
Branchial vessel, 199 
Branchiopoda, 189, 190 
Branchiura, 193 
Breeding habits, 18 
Bristle jaws, 132 
Brittle stars, 157 
Brood sac, 191, 202 
Bryozoa, 144 
Bucephalus, 106 
Budding, 24 
internal, 8 
lateral, 137 
Bugula, 145 
Bulimulus, 178 
Bursae, 158 
Bursaria, 51 
Buthus, 206 
Byssus gland, 169 
C 
Calcarea, 80 
Calegus, 193 
Callinectes, 201 
Calonympha, 32 
Calonymphidae, 32 
Calyx, 162, 163 
Cambarus, 201 
Campanella, 87 
Campanulariae-Leptomedusae, 86 
Cancer, 201 
Candona, 194 
Capillitium, 40 
Caprella, 202 
Capsular membrane, 38 
Carapace, 185, 198 
Carchesium, 52 
Carcinus, 201 
Cardiac chamber, 154, 200 
Cardium, 173 
Carterius, 80 
Cartilage, 61, 62 
Caryophyllaeus, 108 
Cattle tick, 208 
Cell as an individual, 20 
Cell-constancy, 15 
Cell-lineage, 15 
Cell organs, 21 
Centipeds, 212 
Central axis, 189 
Central nervous system, 63, 74, 101, 
114, 117, 121, 128, 132, 133, 
141, 157, 160, 166, 170, 175, 
177, 200, 211, 224 
Centrolecithal, 15, 184 
Centrosome, 14 
Centrums, 206 
Cephaline gregarines, 45 
Cephalopoda, 178 
Cephalothorax, 186, 198, 209 
Ceratium, 29 
Ceratosa, 80 
Cercariae, 105 
Cerebral ganglia, 166 
Cerebro-pleural ganglion, 170 
Cestida, 96 
Cestoda, 98, 106 
individual, 107 
monozoic, 106, 107 
orientation, 106 
polyzoic, 108 
Cestodaria, 106, 107 
Cestus, 96 
Chaetoderma, 167 
Chaetogaster, 138 
Chaetognathi, 131 
Chaetonotus, 129, 130 
Chaetopoda, 133 
Chalk, 37, 38 
Charybdea, 90 
Chelae, 198, 206, 210 
Chelicerae, 205, 206, 207, 208, 209 
Chelifer, 208 
Cheliped, 198 
Chills in malaria, 44 244 
INDEX 
Chilodon, 50 
Chilomastix, 32 
Chilomonas, 28 
Chilopoda, 184, 212 
Chitons, 167, 168 
Chlamydomonas, 30 
Chlamydophrys, 38 
Chloridella, 198 
Chloromyxum, 47 
Chlorophyll, 27 
Choanoflagellates, 30, 77, 235 
Chordata, 1 
Chordodes, 121 
Chromatophores, 27, 28 
Chromosomes, 12 
autosomes, 12 
X-, 12 
Y-, 13 
Chromulina, 28 
Chronology, geologic, 6 
Chrysidella, 28 
Chrysomonadina, 28 
Chrysopyxis, 28 
Cidaridea, 160 
Cilia, 48, 73 
modifications, 50 
movement, 50 
structure, 50 
Ciliata, 48 
Ciliated grooves, 134 
Cilioflagellata, 29 
Ciliophora, 48 
Circulatory system, 68 
closed, 69 
open, 69, 184, 199 
Circumferential 
Cirri of ciliates, 50 
Cirri of crinoids, 163 
Cirripathes, 94 
Cirripedia, 188, 194 
Cirrus, 73, 128, 136, 222 
Cladocera, 189, 190, 191, 192 
Clams, 173 
Clam worm, 137 
Clasping organs, 73 
Classification, basis of, 2 
linear arrangement, 4 
system of, 2 
tree-like arrangement, 5 
Clathrulina, 37 
Clava, 86 
Cleavage, determinate, 15 
indeterminate, 15 
partial, 14, 15 
patterns, 14 
superficial, 15 
total, 14 
Clitellata, 140 
Clitellum, 138 
Cloaca, 68, 78, 162 
Clonorchis, 106 
Clypeaster, 160 
Clypeastroidea, 160 
Cnidosporidia, 46 
Coccidia, 41 
Coccidiomorpha, 41 
Cocoon, 138 
Coelenterata, 82 
phylogeny of, 235 
Coelenteric cavity, 83 
Coelhelminthes, 131 
Coelomoducts, 134 
Coeloplana, 99 
Coeloria, 94 
Coelosporidium, 48 
Coenogenetic, 233 
Colacium, 30 
Coleps, 50 
Collared flagellate cells, 77 
Collembola, 215 
Colony, 20 
formation, 25 
Colpoda, 50 
Columella, 175 
Combs, 95 
Commission on Nomenclature, 3 
Comparative anatomy, 232 
Complemental males, 194 
Compound eyes, 75, 186, 187, 191, 
218 
Conchostraca, 191 
Congress, International Zoological, 3 
Conjugation, 24 
in Peritricha, 52 
Connective tissue, 61 
Constancy, cell-, 15, 128 
nuclear-, 15, 128 
Contractile layer of cytoplasm, 49 INDEX 
245 
Contractile vacuole, 27 
Copepoda, 187, 188 
parasitic, 193 
Copepodid, 187 
Copulation, 18 
reciprocal, 18 
Copulatory organs, 73 
Corallium rubrum, 94 
Corallobothrium, 111 
Corals, 91, 93, 94, 
Cordylophora, 84 
Coryne, 86 
Costa, 221 
Costia, 32 
Coxa, 219 
Coxopodite, 185 
Crabs, 196, 198 
Crago, 201 
Crania, 147 
Craspedotella, 34 
Crawfish, 185, 196, 198 
Crepidula, 177 
Cribellum, 207 
Cribrillina, 145 
Crinoidea, 151 
Crinoids, 162 
Cristatella, 145 
Crustacea, 183, 184 
Cryptochiton, 168 
Cryptomonadina, 28 
Cryptomonas, 28 
Crystalline style, 170 
Ctenidia, 168 
Ctenobranchiata, 177 
Ctenocephalus, 189 
Ctenodiscus, 157 
Ctenophora, 95 
phylogeny of, 235 
Ctenoplana, 99 
Cubitus, 221 
Cubomedusae, 90 
Cucumaria, 162 
Cultures, 23 
Cunina, 87 
Cunocantha, 87 
Cuspidaria, 174 
Cuticula, 59, 98, 116, 121 
Cuttlebone, 178 
Cuttlefish, 178, 181 
Cuvier, 82, 150 
Cyanea arctica, 90 
Cyathocephalus, 111 
Cyathomonas, 28 
Cyclammina, 38 
Cyclochaeta, 52 
Cyclophyllidea, 111 
Cycloposthium, 51 
Cyclops, 187, 192, 193 
Cyclospora, 41 
Cydippida, 96 
Cypridinidae, 194 
Cypris, 194 
Cypris stage of Cirripedia, 195 
Cysticercoid, 109 
Cysticercus, 109 
Cystoflagellata, 34 
Cystoidea, 152, 162 
Cytopharynx, 21, 48 
Cytoplasm, 22 
regional differentiation, 49 
Cytopyge, 21, 48 
Cytostome, 21, 48 
D 
Dactylogyrus, 103 
Dactylozooids, 88 
Daddy-long-legs, 209 
Dalmanites, 189 
Daphnia, 192 
Darwin, 231 
Davainea, 111 
Decapoda, 180 
Degeneracy, 3 
Delamination, 16 
Delsman, 238 
Demospongia, 80 
Dendrites, 63 
Dendromonas, 30 
Dendrosoma, 53 
Dentalium, 174 
Dermomuscular sac, 73 
Dero, 138 
Desor's larva, 113 
Determinate cleavage, 15 
Deutomerite, 45 
Deutoplasm, 9 
Devilfishes, 178, 181 246 
INDEX 
Diaptomus, 193 
Dibothriocephalus, 109 
Dibranchia, 179, 180 
Dicyemidae, 57 
Didinium, 22, 50 
Dienympha, 32 
Differentiation, histological, 20, 56, 
57 
Differentiation, regional, 183 
Difflugia, 38 
Digenea, 104 
Digenetic trematodes, 104 
Digestion, 65 
Digestive parenchyma, 101 
Digestive system, 65 
Dileptus, 50 
Dimorphism in germ cells, 14 
Dinifera, 29 
Dinobryon, 28 
Dinoflagellata, 29 
Dinophilus, 141 
Diplobothrium, 103 
Diplocardia, 139 
Diplodiscus, 104 
Diplopoda, 184, 212 
Dipylidium, 111 
Directives, 92 
Disc, 153 
Discina, 147 
Discoidal cleavage, 181 
Discomedusae, 90 
Distomatina, 33 
Divaricator muscles, 147 
Docoglossa, 177 
Doflein, 25 
Dondersia, 167 
Dorsal line, 117 
Dorsal vessel, 133, 184 
Dorylaimus, 120 
Dujardin, 21 
Dysentery, 34 
amoebic, 35 
E 
Ecdysis, 184 
Echinarachnius, 160 
Echinococcus, 111 
Echinoderma, 150, 236 
orientation, 151 
Echinoidea, 151, 158 
Echinopluteus, 160 
Echinorhynchus, 123 
Ectoderm, 16 
Ectoplasm, 22, 27 
Ectoprocta, 145 
Edwardsia, 92, 93 
Egg, 72, 79, 101, 103, 104, 107, 113, 
119, 128, 130, 132, 156, 171, 181 
184, 214 
Egg masses of Acanthocephala, 123 
Egg sacs, 192 
Eimeria, 41 
schubergii, 41 
stiedae, 41 
Elastic tissue, 62 
Eleutherozoa, 152 
Elytra, 219 
Embryo, 17 
Embryology, 232 
Enchelys, 50 
Endamoeba blattae, 35 
Endites, 186 
Endomixis, 25 
Endoplasm, 22, 27, 49 
Endopodite, 185 
Endoprocta, 145, 146 
Energy release in respiration, 69 
Ensatella, 173 
Entamoeba buccalis, 35 
coli, 35 
histolytica, 35, 36 
Enteropneusta, 1, 236 
Entoderm, 16 
Entodinium, 51 
Enzymes, 170 
Ephelota, 53 
Ephippium, 192 
Ephydatia, 80 
Ephyra, 89 
Epicranial suture, 217 
Epicranium, 217 
Epimeral plates, 201 
Epimerite, 45 
Epistylis, 52 
Epitheliomuscular cells, 62, 116 
Epithelium, 58 
ciliated, 58 
germinal, 60 INDEX 
247 
Epithelium, glandular, 59 
protective, 59 
sensory, 60 
Epitoke, 135 
Ergasilus, 193 
Eremobates, 209 
Eriophyidae, 207 
Errantia, 137 
Esophagus, 68, 83, 91, 113, 118, 176, 
200 
Estheria, 190, 191 
Eubranchipus, 190 
Eudorina, 30 
elegans, 9 
Euglena, 30 
Euglenoidina, 29 
Euglypha, 38 
Eulamellibranchia, 172, 173 
Eunice viridis, 135 
Eupagurus, 201 
Euplectella, 80 
Euplotes, 52 
neuro motor system, 51 
Eurypelma, 207 
Eurypterida, 205 
Eurypterids, 204 
Euthyneura, 177 
Excretory layer of cytoplasm, 49 
Excretory system, 71, 98, 109, 114, 
117, 127, 134, 140, 170, 186 
Excretory vesicle, 71 
Exopodite, 185 
Exoskeleton, 73, 183 
Extracapsulum, 38 
Exuvaella, 29 
Eyes, 101, 128, 132, 135, 166, 168, 
171, 176, 187, 218 
Eyespots, 28 
F 
Fairy shrimp, 190 
Fasciola hepatica, 104, 106 
Fat tissue, 61 
Favia, 94 
Femur, 219 
Fertilization, 11 
cross, 138 
reciprocal, 138 
self, 18 
Fibrous connective tissue, 62 
Fiddler crabs, 201 
Filariae, 118 
blood inhabiting, 120 
Filibranchia, 173 
Finger, 198 
Finger-nail shells, 173 
Fish-lice, 193 
Fission, 7, 24, 101 
binary, 7, 23 
multiple, 7, 23 
simple, 7, 23 
Fissurella, 177 
Flagella, 26, 73 
pulsella, 27 
trachella, 27 
trailing, 27 
Flagellated chambers, 77 
Flame cells, 71, 98 
Floscularia, 126, 127 
Follicle cells, 72 
Foot of Mollusca, 166, 174 
Foraminifera, 35, 37 
Free-living nematodes, 120 
Front, 217 
Funiculus, 145 
Funnel of Ctenophora, 96 
Funnel vessel, 96 
G 
Gall mites, 205 
Gametes, specialization of, 9 
Gametoblast, 44 
Gametogenesis, 11 
Gammarus, 202 
Ganglia, 63, 74 
Gasteropoda, 174 . 
Gasterostomata, 106 
Gastraea theory, 233 
Gastral tentacles, 89 
Gastroliths, 200 
Gastrotricha, 125, 129 
Gastrovascular system, 67, 68, 83 
Gastrozooids, 87 
Gastrula, 16 
Gelasimus, 201 
Gemmules, 8 
Porifera, 79 248 
INDEX 
Genae, 217 
Generations, alteration of, 9 
in Coelenterata, 83 
in Protozoa, 24 
Genital claspers, 222 
Genital plates, 154, 158 
Genital pore, 108, 162 
Genital rachis, 159 
Genitalia, 222 
Geologic chronology, 6 
Geophilus, 213 
Gephyrea, 142 
Germ band, 225 
Germ cells, 8, 9, 10 
Ascaris, 10 
chromatin diminution, 11 
isolation of, 10 
Miastor, 11 
primordial, 11 
Germ plasm, 11 
Germinal epithelium, 60 
Giant nuclei, 121 
Giardia, 33, 34 
Gigantostraca, 204 
Gill bailer, 199 
Gill hearts, 69 
Gill sacs, 201 
Gills, 70, 136, 138, 156, 160, 169, 
204, 223 
Glands, 68 
Glandular epithelium, 59 
Glaridacris, 108 
Glass rope sponge, 80 
Globigerina, 38 
Glochidium, 171 
axehead type, 172 
hooked, 172 
hookless, 172. 
Glomeris, 212 
Glossina, 31 
Glossiphonia, 141 
Glugea, 47 
Gnathobdellida, 141 
Gonads, 60, 72, 170 
Gonangia, 87 
Gonium, 30 
Gonodactylus, 198 
Gonoducts, 134, 159 
Gonophore reduction, 84 
Gonopophyses, 222 
Gonotheca, 86 
Gordiacea, 120 
Gordius, 121 
Gorgonacea, 94 
Gorgonia, 94 
Grantia, 78, 80 
Green gland, 186, 200 
Gregarina, 41, 45 
Gromia, 38 
Group concepts, mutability of, 3 
Gunda, 101 
Gymnodinium, 29 
Gyrocotyle, 107 
Gyrodactylus, 103 
H 
Haeckel, 233 
Haematococcus, 30 
Haemoceol, 184 
Haemogregarines, 44 
Haemosporidia, 41 
Hair snakes, 121 
Haliotis, 177 
Halteres, 219 
Halteria, 51 
Halycystus, 90 
Haplobothrium, 111 
Haplodinium, 29 
Haplosporidia, 48 
Harpes, 189 
Harvest men, 205 
Head, 131, 135, 138, 166, 174, 180, 
183, 219 
Head capsule, 216, 217 
Heart, 133, 170, 184, 191 
Heart shells, 173 
Hectocotylization, 179 
Hectocotylus, 179 
Heliozoa, 36 
Helix, 178 
Helodrilus, 139 
Hemelytra, 220 
Henneguya, 47 
Henricia, 155, 157 
Hepatopancreas, 200 
Hermaphroditic gonad, 10, 176 
Hermaphroditism, 10, 184, 194 INDEX 
249 
Hermit crab, 201 
Heterocoela, 80 
Heterocyemidae, 57 
Heterometabola, 214, 215 
Hetero nemertini, 114 
Heteronereis, 135 
Heteropoda, 177 
Heterotricha, 51 
Hexactinellida, 80 
Hexacoralla, 92, 93 
Hexamitus, 34 
Hinge ligament, 168 
Hirudinea, 139 
Hirudo medicinalis, 141 
Histogenesis, 215, 227 
Histological differentiation, 20, 56, 
57, 85 
Histolysis, 227 
Holometabola, 214, 215 
Holothuroidea, 151, 161 
Holotricha, 50 
Homalonotus, 189 
Homarus, 201 
Homocoela, 80 
Homogeneous connective tissue, 61 
Homolecithal, 14 
Homonomous metamerism, 135 
Hookworms, 118, 119 
Ancylostoma, 119 
development, 119 
Necator, 119 
Hormiphora, 96 
Horse-shoe crabs, 204, 205 
Hubrecht, 238 
Hyallela, 202 
Hyalonema, 80 
Hydatina senta, 126 
nuclear-constancy in, 128 
Hydra, 84 
ciliates commensal on, 52 
Hydrachnida, 208 
Hydrocoel sacs, 151 
Hydrocorallina, 86, 87 
Hydroid, 82 
Hydroides, 138 
Hydromedusa, 84 
Hydropolyp, 84 
Hydrorhiza, 87 
Hydrotheca, 86 
Hydro zo a, 84 
Hymenolepis, 111 
Hypermastigina, 33 
Hypodermis, 59, 116, 183 
Hypopharynx, 216 
Hypotricha, 51 
I 
Ichthyophthirius, 48, 50 
Ichthyosporidium, 48 
Idotea, 202 
Imaginal buds, 215, 227,'228 
Imago, 214 
Implantation cone, 64 
Inarticulata, 147 
Incurrent pores, 77, 78 
Incus, 127 
Indeterminate cleavage, 15 
Individual, the cell as an, 20 
Infrabasals, 163 
Ink sac, 180 
Insecta, 184, 214 
importance of, 228 
orders of, 229 
Integument, 183 
Interambulacrum, 153, 158 
Internal buds, 8 
International commission on nomen- 
clature, 3 
Interradial vessels, 96 
Interradii, 89 
Intestine, 68 
Intromittent organ, 73 
Invaginated development, 225, 226, 
227 
Invertebrates, 1 
lodinophilous vacuole, 46 
Iota, 120 
Irritability, 63 
Isogametes, 9 
Isopoda, 202 
Isospora, 41 
Isotropic, 63 
Itch, 208 
mites, 208 
J 
Julus, 212 250 
INDEX 
K 
Karyosomes, 24 
Keber's organ, 170 
Kerona, 51 
Keyhole urchin, 160 
Kinetic energy, 69 
Krohnia, 133 
L 
Labial palpi, 169, 216 
Labium, 215 
Labrum, 215 
Lacrymaria, 50 
Lacunae, 184 
Lamp shells, 147 
Lampsilis, 173 
Land crabs, 185 
Land snails, 178 
Larva, 17, 214 
Lasmigona, 172 
Lateral lines, 117 
Laurer's canal, 104 
Laverania falciparum, 41, 44 
Law of Priority, 3 
Leda, 173 
Leeches, 139 
Leidyana, 45 
Lemnisci, 121 
Lepas anatifera, 195 
Lepidoderma, 130 
Lepidonotus, 137 
Lepidurus, 191 
Leptasterias hexactis, 152 
Leptocardii, 1 
Leptodora, 191 
Leptolinae, 86 
Leptotheca, 47 
Lernaeocera, 193 
Lernaeopoda, 193 
Leucilia, 78 
Leuckart, 82 
Leucon type, 78, 80 
Leucortis, 80 
Leucosolenia, 78, 80 
Libinia, 201 
Lieberkuhnia, 38 
Ligula, 107, 111 
Limax, 178 
Limnadia, 191 
Limnetus, 191 
Limpets, 174, 177 
Limulus, 205 
Linear arrangement in classifica- 
tion, 5 
Lines of growth, 168 
Lingual ribbon, 176 
Linguatula, 209 
Linguatulida, 185, 208, 209 
Lingula, 147 
Linnaeus, 4, 131, 231 
Liriope, 87 
Lithobius, 213 
Littorina, 177 
Liver, 170 
Liver-rot, 104 
Lobata, 96 
Lobster, 185, 196, 198, 201 
Locomotor skeleton, 73 
Locomotor system, 73 
Loligo, 181 
Lophomonas, 33 
Lophophore, 144, 147, 148 
Loxosoma, 146 
Lucernaria, 90 
Lumbricus, 139 
Lung, 199 
Lung-books, 70, 204, 206 
Lung sacs, 70 
Lymnaea, 178 
M 
Macracanthorhynchus hirudinaceus, 
123 
Macrobdella, 141 
Macrobitus, 210 
Macrocheira, 201 
Macroconjugant, 52 
Macrogametes, 9, 21 
Macronucleus, 48, 49 
Macroura, 201 
Madrepora, 94 
Madreporaria, 94 
Madreporite, 153, 154, 158 
Magellania, 147 
Malacostraca, 187, 188, 196 INDEX 
251 
Malaria, benign, 44 
malignant, 44 
pernicious, 44 
quartan, 44 
spring tertian, 44 
Malarial organism, 24, 42 
Mallei, 127 
Malpighian tubes, 71 
Mandibles, 191, 215 
Mantle of Brachiopoda, 146 
Cirripedia, 194 
Mollusca, 166, 168, 174 
Manubrium, 85 
Manyunckia, 137 
Margaropus, 208 
Marsipometra, 111 
Marsupia, 171 
Martini, 128 
Mastax, 127 
Mastigamoeba, 36 
Mastigella vitrea, 36 
Mastigophora, 26 
Maxillae, 191, 215 
Maxillary papi, 216 
Media, 221 
Medusoid, 82 
Megalops, 188, 200 
Mehlis' gland, 104 
Meleagrina, 168, 173 
Melicerta, 126 
Mellita, 160 
Membranelie, 50, 51 
Meridional vessels, 96 
Mermis, 118, 120 
Merozoites, 42 
Mesectoderm, 77 
Mesenchytraeus, 138 
Mesenterial filaments, 92 
Mesenteries, 83, 162 
Mesenteron, 113, 186 
Mesoderm, 16 
bands, 137 
Mesoglea, 83 
Mesonemertini, 114 
Mesothorax, 219 
Mesozoa, 56, 57 
Metacrinus, 165 
Metagenesis, 83, 86 
Metamerism, 133, 183 
Metamerism, heteronomous, 183 
homonomous, 135, 183 
Metamorphosis, 99, 113, 136, 157, 
172, 187, 200, 214 
complete, 214 
incomplete, 214 
Metanauplius, 187, 190, 193 
Metanemertini, 114 
Metanephridial system, 71, 134, 140 
Metathorax, 219 
Metazoa, 20, 56 
Metazoan tendencies, 20 
Metridium, 94 
Miastor, primordial cells in, 11 
paedogenesis in, 17 
Michaelson, 140 
Microcon jugant, 52 
Microcotyle, 103 
Microcyst, 40 
Microdrili, 138 
Microgametes, 10, 21 
Microgametoblast, 44 
Micronucleus, 48, 49 
Microporella, 145 
Microscolex, 139 
Microsporidia, 46, 47 
Microstomum, 101 
Miescher's corpuscles, 48 
Millepora, 88 
Milliped, 212 
Miricidium, 104 
Mites, 204, 205, 207, 208 
Modiola, 173 
Mollusca, 166 
Molluscan cross, 15 
Molluscoidea, 144 
Molting, 184 
Mother-of-pearl, 168 
Monas, 30 
Monaxonida, 80 
Moniliformis, 123 
Moniligaster, 139 
Monocystis, 45 
Monogenea, 102 
Monogenetic trematodes, 102 
Monopisthocotylea, 103 
Monostomes, 104 
Monothalmia, 37 
Mouth parts of insects, 215 252 
INDEX 
Mouth parts of insects, Hemiptera, 
216 
honeybee, 216 
Lepidoptera, 216 
mandibulate or biting, 215, 216 
mosquito, 216 
Muller's larva, 99, 102 
Murex, 177 
Muscle tissue, 62 
smooth, 62 
striated, 62 
Musculium, 173 
Mussels, fresh-water, 168 
development, 171 
Mya, 173 
Mycetozoa, 35, 39 
Myonemes, 45, 49, 62, 73 
Myriapods, 212 
Myrientomata, 184, 211 
Mysididae, 197 
Mysis, 187, 200 
relicta, 197 
Mytilus, 173 
Myxamoeba, 40 
Myxicola, 137 
Myxidium, 47 
Myxobolus, 47 
Myxoflagellate, 40 
Myxospongia, 80 
Myxosporidia, 46 
N 
Nacre, 168 
Naididae, 138 
Nais, 138 
Narcomedusae, 87 
Natica, 177 
Nauplius, 187, 190, 193, 195, 201 
eye, 186, 192 
Nautilus, 178, 180 
Nebalia, 197 
Necator, 119 
Neck, 107 
Nectocalyces, 88 
Nemas, 116 
Nemathelminthes, 116, 235 
Nematocysts, 82 
Nematodes, 116 
development, 188, ff 
free-living, 120 
Nemertinea, 111 
Nemertine theory, 238. 
Nemertines, 111 
Neoechinorhynchus, 123 
emydis, 122 
Neomenia, 167 
Neosporidia, 45 
Nephridiopore, 134 
Nephrostome, 71, 134 
Nereis, 135, 137 
Nerve ring, 74, 157 
Nervous system, 73, 74, 95, 101, 107, 
114, 117, 121, 128, 133, 157, 160, 
166, 167, 170, 175, 200, 224 
condensation of, 224 
sympathetic, 74, 224 
Nervous tissue, 63 
Nettling cells, 82 
Neurite, 63 
Neuroglia, 64, 74 
Neuromotor system in Euplotes, 51 
Neuron, 63 
Neuropodium, 135 
Nitzschia, 103 
Noctiluca, 34 
Nomenclature, international com- 
mission on, 3 
Non-chordate, 1 
phyla, 1 
Nosema, 47 
apis, 47 
bombycis, 47 
Nostraca, 190 
Notochord, 1 
Notopodium, 136 
Nuclear-constancy, 15, 128 
Nuclearia, 37 
Nucleus, giant, 121 
in differentiation, 65 
Nuda, 96 
Nudibranchs, 178 
Nurse cells, 72 
Nyctotherus, 51 
Nymph, 215 
Nymphon, 209, 210 
O 
Obelia, life history, 86 
Occiput, 217 INDEX 
253 
Ocellus, 76, 205, 218 
Octocoralla, 92, 94 
Octopoda, 180, 181 
Octopus, 181 
Ocular plates, 153, 158 
Odontophore, 176 
Olfactory organs, 75, 132, 166, 171, 
218 
Oligochaeta, 138 
Oligotricha, 51 
Olynthus, 78 
Ommatidia, 75 
Onchosphere, 109 
Oniscus, 202 
Onycophora, 184, 210, 236 
Oocapt, 109 
Oogenesis, 11 
Ookinete, 44 
Ootype, 104 
Opalina, 48, 50 
Opercularia, 52 
Operculum, 145, 174 
Ophiopholus, 158 
Ophiopluteus, 158 
Ophiura, 158 
Ophiuroidea, 151, 157 
Ophryodendron, 53 
Opisthobranchia, 176, 178 
Oral shields, 158 
Orb weavers, 207 
Orchestia, 201, 202 
Organ systems,'65 
Organellae, 21 
Organogenesis, 107 
Organs, 65 
Orientation of cestode body, 106 
echinoderm classes, 151 
Origin of species, 231 
Orthonectidae, 57 
Oscarella, 78 
Osculum, 77 
Osphradia, 166 
Ostium, 191, 199 
Ostracoda, 188 
Ostraeidae, 173 
Otocyst (see statocyst), 76, 95 
of Schizopoda, 186 
Otolith, 60 
Ovaries, 72, 109 
Overgrown development, 225, 227 
Oviducts, 72 
Ovigerous legs, 210 
Oviparous, 18 
Ovipositors, 73, 222 
Ovoviviparous, 19 
Oysters, 173 
P 
Paedogenesis, 17, 184 
Palaemon, 201 
Palaemonetes, 201 
Paleontology, 232 
Palingenetic, 233 
Pallial line, 168 
Palola worm, 135 
Pandinus, 206 
Pandorina, 30 
Parabasal bodies, 32, 33 
Paradoxides, 189 
Paragastric canals, 96 
Paragonimus, 106 
Paragordius, 121 
Paramecium, 49, 50 
Parapodia, 133, 135 
Parasitic copepods, 193 
Parasitic habits, 23 
Parenchyma, 98 
digestive, 101 
Parent, 24 
Parietal ganglion, 170 
Parthenogenesis, 17, 184 
Patella, 177 
Pauropoda, 185, 212 
Pearls, 168 
Pecten, 173 
Pectinatella, 145 
Pectines, 206 
Pedal ganglia, 166, 170 
Pedalion, 126, 127 
Pedicellariae, 153 
Pedicellina, 146 
Pedipalpi, 205, 206, 207, 209 
Peduncle, 146, 147 
Pellicle, 26, 4.8 
Peneus, 201 
Pennatula, 95 
Pennatulacea, 95 254 
INDEX 
Pennella, 193 
Pentacrinus, 165 
stage of crinoids, 165 
Pentastoma, 209 
Pentastomida, 209 
Peranema, 30 
Pericardial sinus, 199 
Pericardium, 170 
Perichaeta, 139 
Pericolpa, 90 
Peridinium, 29 
Peridium, 40 
Perihemal system, 156 
Peripatus, 210, 236 
Peripheral nervous system, 74, 224 
Periphylla, 90 
Periproct, 158 
Perisarc, 86 
Peristome, 154, 158 
Peritricha, 52 
Peromedusae, 90 
Perradial vessels, 96 
Perradii, 89 
Phacellae, 89 
Phacus, 30 
Phalangidae, 208, 209 
Phascolosoma, 142 
Philodina, 128 
Pholas, 174 
Phoronida*, 144, 148 
Phoronis, 148 
Phyllocardia, 197 
Phyllopoda, 188, 189 
Phylogenetic relationships, 2, 4, 231 
Phylogeny, 231 
Phylopaedogenesis, 125 
Physa, 178 
Physalia, 88 
Phytomastigina, 27 
Phytomonadina, 30 
Pilidium, 99, 113 
Pill bugs, 202 
Pinnules, 163 
Piroplasma hominis, 44 
Piscicola, 141 
Pisidium, 173 
Placobdella, 141 
Placophora, 167 
Planorbis, 178 
Planula, 87, 89 
Plasmic products, 57, 61 
Plasmodiophora brassicae, 40 
Plasmodium, 40 
malariae, 41, 44 
vivax, 41, 44 
Plasmodroma, 26 
Plathelminthes, 98 
development, 98 
interrelationships, 98 
phylogeny, 235 
relationships to other phyla, 99 
Platydorina, 30 
Platypoda, 177 
Pleodorina, 30 
Plerocercoid, 109 
Pleura, 219, 221 
Pleural lobes, 189 
Pleurobrachia, 96 
Pleurocera, 177 
Plumatelia, 145 
Pluteus, 158, 159, 160 
Pneumatophore, 88 
Podomeres, 186 
Podophrya, 53 
Poison glands, 206, 222 
Polar capsules, 46, 47 
Polar filament, 46 
Polar plates, 95 
Polian vesicle, 155, 159, 162 
Polychaeta, 133, 135 
Polycladidea, 100 
Polyembryony, 17, 184 
Polygordius, 141 
development, 136 
Polygyra, 178 
Polymastigina, 32 
Polymorphism, 88 
Polyphemus, 191 
Polyopisthocotylea, 103 
Polystoma, 103 
Polystomella, 38 
Polythalmia, 37, 38 
Polyxenus, 212 
Polyzoa, 144 
Pontilla, 193 
Porcellio, 202 
Porifera, 77 
morphological types, 77 INDEX 
255 
Porifera, phylogeny, 234 
Porites, 94 
Porocepbahis, 209 
Portuguese man-of-war, 82, 88 
Post-abdomen, 191, 206, 207, 208 
Posterior ganglion, 170 
Postgenae, 217 
Potamobius, 201 
Potential energy, 69 
Prawns, 196 
Preabdomen, 206 
Precambrian fossils, 232 
Predaceous habit, 22 
Primitive segments, 137, 225 
Primitive vs. degenerate, 3 
Priority, law of, 3 
Pristina, 138 
Proboscis, Acanthocephala, 121 
Gasteropoda, 176 
Nemertinea, 112 
Proboscis receptacle, 121 
Proboscis sheath, 112 
Prochordates, 1 
Proctodaeum, 67, 186 
Proetus, 189 
Proglottids, 106 
Proliferation, 111 
Promitosis, 24 
Pronucleus, 14 
Prorocentrum, 29 
Prosobranchia, 176, 177 
Prosopyles, 78 
Prostomata, 106 
Prostomium, 133, 186, 196 
Protandrous, 79 
Proteocephalus, 111 
Proterospongia, 235] 
Prothorax, 219 
Protobranchia, 173 
Protodrilus, 141 
Protohydra, 84 
Protomerite, 45 
Proto monadina, 30 
Protonemertini, 114 
Protonephridial system, 71, 98 
Protozoa, 20 
colony formation, 25 
classification, 25 
cultures, 23 
Protozoa, definition, 21 
food habits, 22 
habitats, 22 
interrelationships, 52 
metazoan tendencies, 20 
organization, 21 
reproduction, 23 
Provisional setae, 137 
Psammonyx vulcanis, 38 
Pseudocoel, 116 
Pseudolamellibranchia, 173 
Pseudophyllidea, 111 
Psuedopodia, 34, 73 
Pseudoscolex, 107 
Pseudoscorpionida, 208 
Psolus, 161 
Pteropoda, 176 ,178 
Pulmonata, 176, 178 
Pulsella, 27 
Pupa, Insecta, 214 
Mollusca, 178 
Pycnogonida, 185, 209 
Pyloric chamber, 154, 200 
Pyrocystes, 29 
Q 
Quadrula, 173 
R 
Radial chambers, 78 
Radials, 163 
Radial symmetry, 82, 150 
Radial vessels, 151 j 
Radiata, 82, 150 
Radiolaria, 35, 38] 
Radius, 221 
Radula, 176 
Raphidiophrys, 37, 
Reactions of Cladocera, 191 
Recapitulation theory, 56, 233 
Receptaculum seminis, 72, 104, 109, 
138 
Reciprocal fertilization, 138 
Rectal gills, 223 
Rectum, 170 
Rediae, 105 256 
INDEX 
Regeneration, in Crustacea, 198 
in Holothuroidea, 162 
in Porifera, 79 
Relapsing fever, 31 
Relationships, phylogenetic, 2, 4 
Renilla, 95 
Reproduction, 7 
asexual, 7 
sexual, 7, 8 
Reproductive system, 71 
Respiration, 69 
Respiratory system, 69 
Respiratory trees, 162 
Retina, 75 
direct, 178 
inverted, 178 
Rhabditis, 120 
Rhabdocoelida, 101 
Rhabdomes, 75 
Rhipidoglossa, 177 
Rhizocrinus, 165 
Rhizomastigina, 36 
Rhizostomae, 90 
Rhopalura, 57 
Rhyncheta, 53 
Rhynchobdellida, 141 
Ring canal, 151 
Rocky Mountain fever, 44 
Rostellum, 111 
Rostrum, 207 
Rotalia, 38 
Rotifera, 125 
S 
Sacculina, 196 
Sagartia, 94 
Sagitta, 132, 133 
Sand dollars, 158, 160 
Sandfleas, 196, 201 
Saprophytic habit, 23, 28 
Sarcocystis, 47 
Sarcolemma, 63 
Sarcoplasm, 62 
Sarcoptes scabei, 208 
Sarcosporidia, 47 
Scallops, 171, 173 
Scaphopoda, 174 
Schistosoma japonicum, 106 
Schistosomes, 102 
Schizogony, 9, 24, 41 
Schizonts, 24 
Schizopoda, 197 
Sclerites, 163, 217 
Scleroblasts, 80 
Scolex, 106, 110 
Scolopendra, 213 
Scolopendrella, 213 
Scorpionida, 206 
Scorpions, 204, 206 
Scuds, 196 
Scutigera forceps, 213 
Scyphopolyp, 89 
Scyphostoma, 88, 89 
Scyphozoa, 88 
Sea anemones, 91, 93 
Sea cucumbers, 161 
Sea fans, 94 
Sea hares, 174, 178 
Sea lilies, 163 
Sea mouse, 137 
Sea pansies, 91 
Sea pens, 91 
Sea urchins, 158 
Sedentaria, 137 
Selective apparatus, 123 
Self fertilization, 18 
Seminal receptacle, 18, 109 
Semostomae, 90 
Sepia, 178, 181 
Septa, 83 
primary, 91 
secondary, 92 
tertiary, 92 
Septibranchia, 174 
Serosa, 225 
Serpent stars, 157 
Setae, 133 
provisional, 137 
Sex-chromosomes, 12 
Sexual reproduction, 7 
Sheep-liver fluke, 104 
Shell of Brachiopoda, 146 
Mollusca, 166 
Shell gland, 104, 166, 186, 197 
Shipworms, 173 
Shrimps, 196, 198, 201 
von Siebold, 21 INDEX 
257 
Silenia, 174 
Silk, 207 
Sinuses, 184 
Siphogenerina, 38 
Siphon, 168, 178 
exhalant, 169 
inhalant, 169 
tube, 169 
Siphon of Echinoidea, 159 
Siphonoglyphes, 91 
Siphonophora, 88 
Sipunculus, 142 
Sleeping sickness, 31 
Slime animals, 39 
Slime moulds, 39 
Slugs, 174, 178 
Snails, 174 
Solaster, 157 
Solenomya, 173 
Solpugida, 208, 209 
Sow bugs, 185, 196, 202 
Spadella, 133 
Spadix, 85 
Sparganum, 111 
Spatangoidea, 160 
Spatangus, 160 
Spermaries, 72 
Spermatophores, 18, 179 
Spermiducal pore, 138 
Sphaerium, 173 
Sphaerocapsa, 39 
Spider crabs, 201 
Spiders, 204, 205, 207 
Spinnerets, 207 
Spinning organs, 207, 208 
Spinning tubes, 207 
Spiracle, 175, 206, 211 
Spirobolus, 212 
Spirochaets, 31 
Spirorbis, 137, 138 
Spiroschaudinnia, 31 
Spirostomum, 51 
Spirula, 181 
Spondylomorum, 30 
Spongilla, 80 
Spongillidae, 80 
Spongin, 80 
Spongioblasts, 80 
Spontaneous generation, 7 
Sporangia, 40 
Sporoblasts, 44 
Sporocyst, 105 
Sporogony, 9, 24, 41 
Sporonts, 24 
Sporophores, 40 
Sporoplasm, 46 
Sporosac, 85 
Sporozoa, 40 
Sporozoite, 41 
Springing organ, 202 
Spumellaria, 39 
Squid, 178, 181 
Squilla, 198 
Staghorn coral, 88 
Starfishes, 152 
Statoblasts, 8, 146 
Statocyst (see otocyst), 60, 76, 84, 
166, 170 
Statolith, 60 
Stauromedusae, 90 
Stenophora, 45 
Stentor, 51 
Stephanoceros, 127 
Sternal sinus, 199 
Sternum, 219, 221 
Stigma, 30 
Sting, 206, 222 
Stomach, 68 
Stomatopoda, 197 
Stomodaeum, 67 
Crustacea, 186 
Ctenophora, 96 
Stone canal, 151, 159, 162 
Streptoneura, 177 
Streptoneurous, 176, 177 
Strobila, 89 
Strobilization, 107 
Strombus, 177 
Strongylocentrotus, 160 
Strophitus, 172 
Stychopus, 162 
Stylaster, 88 
Stylet, 112 
Stylonichia, 51 
Subcosta, 221 
Subcuticula, 116, 121 
Suckers, 102, 140 
Sucking discs, 103 258 
INDEX 
Sue tori a, 53 
Sulcus, 29 
Sun animalcules, 36 
Supporting tissue, 61 
Sutures, 183, 217 
Swimmerets, 198 
Sycon, 78, 80 
Sycon type, 78 
Syllis, 137 
Symbiotic, 28, 38 
Sympathetic nervous system, 74, 224 
Symphonota, 173 
Symphyla, 184, 212 
Synapta, 162 
Synura, 28 
Syphilis, 31 
System of classification, 2 
Systema Naturae, 4 
Syzygy, 45 
T 
Tactile organs, 60, 75, 218 
Taenia, 111 
saginata, 110 
Taeniolae, 88 
Tail fin, 198 
Tapeworm, 110 
Tarantulas, 207 
Tardigrada, 185, 210 
Tarsus, 219 
Taste organs, 75 
Tectibranchs, 178 
Tegmina, 220 ■ 
Teloblasts, 137 
Telolecithal, 14 
Telosporidia, 41 
Telson, 196, 198 
Tentacles, 138, 176 
Tentaculata, 96 
Tentorium, 217 
Terebra, 177 
Terebratula, 148 
Terebratulina, 148 
Teredo, 173 
Tergum, 219, 221 
Tesseridae, 90 
Test, 158, 162 
Tetrabranchia, 179, 180 
Tetraphyllidea, 111 
Tetrarhynchus, 111 
Tetraxonida, 80 
Texas fever, 44, 208 
Thalamophora, 37 
Thalassicolla, 39 
Thelohania, 47 
Thorax, 219 
Thousand-leg, 212 
Thumb, 198 
Thyone, 162 
Thysanura, 215 
Tibia, 219 
Tick fever, 208 
Ticks, 207 
disease carriers, 44 
Tiedemann's bodies, 155 
Tigroid bodies, 64 
Tissues, 57 
classification of, 57 
Tokophrya, 53 
Tooth shells, 174 
Torsion in gasteropods, 176 
Toxopneustes, 160 
Tracheae, 70, 204, 211, 223 
origin of, 223 
Tracheal gills, 70, 223 
Trachelius, 49, 50 
Trachella, 27 
Trachelomonas, 30 
Tracheoles, 223 
Trachydermon, 168 
Trachylinae, 85, 87 
Trachymedusae, 87 
Trachynema, 87 
Trailing flagella, 27 
Trematoda, 98, 102 
Treponema pallidum, 31 
Triarthrus, 189 
Trichinella, 118, 120 
Trichocyst layer, 49, 50 
Trichodina, 52 
Trichomonas, 32, 33 
Trichonympha, 33 
Tricladidea, 101 
Tridacna, 173 
Trigonia, 173 
Trilobita, 188 
Trilobite larva of Limulus, 205 INDEX 
259 
Trilobites, 185 
Trivium, 154 
Trochal disc, 126, 127 
Trochanter, 219 
Trochelminthes, 125 
Trochophore, 125, 134, 136, 148, 235 
Trochosphaera, 127 
Trochus, 177 
Trophozoite, 46 
Trunk, 183 
Trypanorhyncha, 111 
Trypanosoma brucei, 31 
gambiense, 31 
lewisi, 30 
Trypanosomes, 27 
Trypanosomidae, 30 
Tse-tse fly, 31 
Tubifex, 138 
Tubularia, 86 
Tubulariae-Anthomedusae, 86 
Tunicata, 1 
Turbellaria, 98, 99 
Tylenchus, 120 
Tympanum, 76 
U 
Umbo, 168 
Undulating membrane, 27, 50 
Unio, 173 
Urnatella, 146 
Urocentrum, 50 
Uroplectus, 206 
Urostyla, 51 
Uterus, 72, 104, 123 
V 
Vacuole, contractile, 27 
Vagina, 108 
Valves, brachiopod, 146 
mollusc, 168 
Vampyrella, 37 
Vasa deferentia, 72 
Veliger, 166, 176 
Velum, 84 
Venation, 220 
hypothetical, 221 
Ventral line, 117 
Ventral sucker, 104 
Ventricles, 69, 170 
Venus, 173 
Venus, flowerbasket, 80 
girdle, 96 
Vertebrae, 158 
Vertebrata, 1 
Vertex, 217 
Vestibule, 146 
Vibracularia, 145 
Visceral ganglion, 166, 170 
Visceral mass, 166, 169, 174, 175 
Vitellaria, 72, 104, 109, 128 
Vitelline duct, 109 
Vitelline receptacle, 104 
Viviparous, 19 
Volvox, 30 
Vorticella, 52 
W 
Wagnerella, 37 
Waldeheimia, 148 
Water-bears, 210 
Water-vascular system, 150 
Wheel animalcules, 125 
Wing pads, 215 
Wings, 214, 219 
origin of, 220 
venation of, 220 
Winter eggs, 128 
Worms, arrow, 131 
flat, 98 
round, 116 
segmented, 131 
X 
X-chromosomes, 12 
Xiphosura, 204, 205 
Y 
Y-chromosomes, 13 
Yoldia, 173 
Yolk cells, 225 
Z 
Zoantharia, 92, 93 
Zoea, 187, 200 
Zoological Congress, International, 3 
Zoomastigina, 27, 30 
Zoothamnium, 52 
Zygote, 9 
cleavage of, 14 
composition of, 14