DESCRIPTION 
OF A 
SPECIES OF CAL I G US, 
C. AMERICANUS. From the American Journal of Science and Arts, No. 2, Vol. xxxiv. 
DESCRIPTION 
OF A 
SPECIES OF CALIGUS, 
C. AMERICANUS. 
BY CHARLES PICKERING, M. D., AND JAMES D. DANA, 
Members of the Yale Natural History Society. 
The species of the genus Caligus, and of other allied genera, 
are commonly called fish-lice, in allusion to their parasitic mode 
of life. The individuals which are the subject of the following 
remarks, infest the Common Cod* of this part of the American 
coast. 
During the fall of the year, when the shoal fish are brought to 
the New York market,! the Caligi are exceedingly abundant. Oc- 
casionally, forty or more individuals may be taken from a single 
fish. As the season advances, the fish are taken in fewer num- 
bers off Sandy Hook and Long Island, and afford a much smaller 
proportion of parasites. The Caligi are most numerous on the half- 
grown fish; they are found indiscriminately on the head or different 
parts of the body, but never within the gill-covers. A European 
species has been said to live under the scales : we have never ob- 
served this peculiarity in the species on this coast; indeed, the 
closeness of the small scales of the cod, renders it impossible. 
Read before the Yale Nat. Hist. Soc., Feb. 20, 1838. 
* It has not been satisfactorily ascertained whether the cod of this coast is iden- 
tical with the European species, Morrhua vulgaris; this, however, is the common 
opinion. 
t These investigations were made at the city of New York, and occupied the 
latter part of November last, together with the following months, December and 
January. 2 
Description of a Species of Caligus. 
When disturbed, they move with rapidity over the fish, and either 
backward or forward with nearly equal facility. In swimming, 
their motion is equally rapid. They thus travel over the body of 
the fish at will, and, we doubt not, occasionally leave one fish for 
another. 
Both sexes frequently occur on the same fish, though the fe- 
males (during the months of November, December, and January) 
have been far the most abundant. The latter are, in general, 
readily distinguished by the two filiform appendages to the body, 
which are the external ovarian tubes; or, if these are wanting— 
as often happens—by the larger abdomen, whose greater size is 
owing to the eggs it contains. If destitute of eggs, it does not 
present this peculiarity. The sexes differ, moreover, in the form 
of the first and third pair of feet, as will be particularly noticed 
when speaking of these members. 
The sizes of the individuals which have come under our no- 
tice, have been very various. The adult males frequently attain 
a length of five ninths or nearly two thirds of an inch, (fig. 2, 
PI. III.) The females are always smaller than the males, and sel- 
dom exceed a half inch, exclusive of the ovarian tubes, (fig. 3.) 
The smallest individual seen, was one tenth of an inch long. Its 
legs had less slender proportions than usual; otherwise, it did not 
differ from the adults. 
The Caligi live several hours on the body of the cod taken from 
the water; but generally die soon after the death of the fish. 
When taken from the fish and confined, they exhibit a strong in- 
clination to leave the water. We have often observed, after the 
introduction of fifteen or twenty into a glass of salt water, that 
the greater portion of them seek the surface, where they attach 
themselves to the glass; and quite a number leave the water en- 
tirely, crawling up the glass an inch or two above the surface. 
The water they confine under their broad shell, which is closely 
attached at its margin, supports them for a while ; but, unless as- 
sisted again to their element, they remain, without any apparent 
attempt to return, and in a few hours die. 
These animals, like the cod, on which they live, require a low 
temperature, and have been observed to swim, with scarcely di- 
minished activity, in water that was freezing. In some instan- 
ces, when the water had evidently reached a temperature below 
32° F., without congelation, they have been rendered torpid, and Description of a Species of Caligus. 
3 
apparently dead; but on bringing them into a room not above 
45° F., they have soon resumed their usual activity. When the 
temperature has been as high as 60° F., they have generally died 
in the course of a short time. This may be owing in part to the 
deterioration of the air in the water, arising from the decomposi- 
ton of the animal matter contained in it. They die almost im- 
mediately when thrown into fresh water. 
Although the imperfect descriptions of the European Caligi, by 
early authors, have been improved by subsequent investigators, 
still, in consequence of the obscure structure of these animals, 
their characters are yet very inaccurately described. We have 
therefore been unable to satisfy ourselves fully, that the species 
of this coast is distinct from the European. Yet, as many of the 
characters stated respecting the foreign species, do not apply to 
ours, we have ventured to propose it as new, under the name 
given at the head of this article.* The following description, to- 
gether with the accompanying plates, it is hoped, will enable the 
future investigator of the European individuals to decide in regard 
to their identity. 
The results of our investigations have shown, that many of the 
errors of authors are of the most fundamental character. Among 
the principal of them, we find that a front pair of cups, serving 
for the attachment of the animal, have been mistaken for its 
eyes;—the exserted ovarian tubes have of late been considered 
the respiratory apparatus;—and what is still more essential, as it 
affects the late classification of the Crustacea, the mouth is sup- 
posed to be a sucker, whereas it contains large dentated mandi- 
bles, and other manducatory organs, appertaining to the maxillated 
species. This last character has been proved to belong also to 
the Argulus, another of the Siphonostoma, or Crustaces Suceurs, 
in an article on that animal, in this Journal, Yol. xxxi, 1837. 
* We find in a folio volume by M. Duhamel du Monceau, entitled Traitd G6n6- 
rale des Pesches, mdcclxxiii, Paris, at page 294, a description of the Caligus found 
on the Salmon, accompanied by drawings, which, if at all accurate, show that the 
species are not identical. The same conclusion may be drawn from Desmarest’s 
figures in his Gen. Consid. des Crustacds, if they can be relied on. Other figures 
that we have seen are so evidently inaccurate, or so destitute of details, that we 
would not venture to form an opinion from thorn. 4 
Description of a Species of Caligus. 
I. Tegumentary System. 
a. Segments of the Body. 
The body of the Caligus is provided with a flexible, subcorne- 
ous, and perfectly transparent covering. By dissection, we were 
able to distinguish only two coats. The internal is a thin, moist 
membrane, easily separable from the exterior, and often present- 
ing, especially in old individuals, numerous dendritic delineations 
of an ochre-yellow color. Occasionally, they are so abundant as 
to give the animal an ochreous tinge. The exterior coat or shell 
is pellucid, very flexible, and somewhat elastic, and does not ex- 
hibit a fibrous structure. In some portions of the shell, and par- 
ticularly about the eyes, it is divided into areas, as represented in 
fig. 8, PI. IV. The shaded subtransparent area in this figure 
passes longitudinally over the space between the eyes. 
When the animal dies, it assumes, after some time, a rose-red 
tint. Under , the microscope, this color is found to be disposed in 
dendritic delineations, like the yellow color above noticed, and 
apparently in the same membrane with it; and in a few instances, 
we are confident that the dendrites which before were yellow, 
have this reddish hue. We cannot say that this is true of all 
these delineations. 
The body is composed of four distinct segments, (fig. 7,) of 
which the first two include the head and thorax, and the third 
and fourth, the abdomen. The anterior of these segments, which 
we may designate the cep halo-thoracic, is divided into four por- 
tions, by imperfect articulations. Two of these articulations are 
longitudinal, and separate the lateral portions of this segment from 
the central. The other articulation connects the centre of the 
two longitudinal articulations, like the cross-line in the letter H, 
and thus divides the central part of this segment into an anterior 
and posterior portion. The two lateral portions correspond to 
the united epimerse of the higher Crustacea, and may be called 
the epimeral segments; the anterior of the two central, may be 
called the cephalic portion, and the posterior forms the anterior 
portion of the thorax. The anterior or cephalic segment presents 
an imperfect articulation near its front margin, which separates a 
narrow segment; this segment we shall hereafter designate the 
anterior or first cephalic segment, and the remaining portion the 
posterior, or second cephalic segment. Description of a Species of Caligns. 
5 
Viewed as a whole, the cep halo-thoracic segment is slightly con- 
vex, and has an obtuse ovate form, a little broader posteriorly, 
with an emargination in front, (A, fig. 7,) and a deep sinus on 
each side in the posterior margin, (B.) It is bounded, both ante- 
riorly and laterally, by a thin transparent margin, which appears 
transversely striated, when highly magnified. The lateral mar- 
gin is about four times as wide as the anterior. A row of ex- 
tremely minute curved spines project above the junction of the 
membranous margin, as is exhibited in figs. 1 and 19. Similar 
spines are scattered over the back; but a very high magnifying 
power and the most favorable light are required to discover them. 
The articulation of the first with the second cephalic segment, 
though mostly imperfect, approaches a perfect joint towards each 
side, (C, fig. 7,) where there is an osseous process in the two seg- 
ments, with opposite articulating surfaces. The process on the 
first segment is narrow, and transversely oblong. That on the 
second is long and slender, and extends to a point laterally in ad- 
vance of the eyes ; it is much enlarged at the articulation, and at 
that place resembles the process on the anterior segment. 
The articulation of the cephalic with the thoracic portion of 
the cephalo-thoracic segment, of the body is curved parallel with 
the anterior margin of the animal, and terminates on each side, 
near the centre of each lateral half of the cephalo-thoracic seg- 
ment, (D, fig. 7.) Prom this point the articulation of the epime- 
ral with the central segments commences. A thin semi-corneous 
margin extends from the cephalic segment, and covers its articu- 
lation with the adjacent parts. The junction of the epimeral and 
cephalic segments is directed towards the anterior portion of the 
lateral margin, but becomes obliterated before reaching it; the 
junction with the thoracic segment is continuous in a curve, con- 
cave inward, to the posterior margin just outside of the sinuses 
in the latter segment. An osseous articulation, similar to that be- 
tween the two cephalic segments, unites the epimeral and ce- 
phalic segments, (E, fig. 7;) the process on the former is long 
and slender, and curves backward, giving firmness to that portion 
of the shell. 
The thoracic portion of the cephalo-thoracic segment approxi- 
mates to a circular form. The sinuses before referred to, (B, fig. 
7,) are situated in its outer posterior margin. Between each sinus 
and the articulation of the epimeral segment, there is a narrow 6 
Description of a Species of Caligus. 
lobe, which is provided, on its interior margin, with a folded mem- 
brane. The lobe is slightly movable upon a joint at its base, and 
the membrane has a very free motion, and serves to close the 
sinus. 
The posterior thoracic segment (F, fig. 7,) is quite short; its 
breadth nearly equals one third of the greatest breadth of the an- 
terior portion of the body. Laterally it terminates in an angle, 
from the posterior side of which, the legs arise which belong to 
this segment. 
The first abdominal segment (G,) differs in form in the two 
sexes. In both, the length and breadth are nearly equal, though 
in general the former is somewhat greater in the female, and the 
latter in the male. The sides are much curved in the male, (fig. 
7,) and the whole is narrower anteriorly. In the female, (fig. 18,) 
the form approaches a square with rounded angles. The poste- 
rior angles in the male are projecting, and furnished with three 
short hairy setae; the same in the female are provided with the 
same setae, but they scarcely project beyond the adjoining parts. 
These peculiarities only exist in the gravid female. When the 
abdomen is destitute of eggs, it resembles that of the male. 
The remaining abdominal joint, (H, fig. 7,) has a flattened 
subovate form, and is about two thirds the breadth of the prece- 
ding. Two short leaf-like appendages are obliquely articulated 
with its posterior extremity. These leaflets are furnished with 
three terminal plumose setee or pinnulae, the cilise of which have 
a length equal to three times the breadth of the seta. There are 
two short setae exterior to the pinnulae, and one interior. These 
leaflets are ciliated on their internal margin. 
b. Organs appertaining to the several segments. 
1. Anterior Cephalic Segment.—This segment presents, in its 
front emargination, (A, fig. 1,) two minute rounded papillae, cov- 
ered on their inner surface with very short hairs, which appear to 
correspond to the inner antennae of other Crustacea. Below and 
just behind their insertion we observe a small semicircular process 
convex outward, which projects a short distance beyond the sur- 
rounding surface. 
Toward the lateral extremity of this segment, on its lower sur- 
face, there is a remarkable organ, which the animal employs in 
attaching itself, (I, fig. 1 and fig. 19,) but which has heretofore 
been considered its eyes. It consists of a thin nearly circular Description of a Species of Caligus. 
7 
membrane, attached by its central portions. Its surface is finely 
marked with lines running towards the outer margin ; on the 
inner margin, these lines, though possessing the same general 
direction, freely anastomose. We have often tested the use of 
these organs by applying the blade of a knife to the front margin 
below, while the animal was on its back, when in numerous in- 
stances it has adhered with sufficient force to be lifted from the 
fish and carried some distance. The membrane of the segment 
extends beyond the cup and curves around over the base of the 
antenna adjoining, (fig. 7, PI. IV.) These antennae have no 
connection with the cup. 
About two fifths of the distance from the cup to the centre of 
the front margin, we find, on the back, a single slender naked 
seta. (K, fig. 1.) 
The antennae which terminate laterally this first cephalic seg- 
ment, (L, fig. 1, and fig. 19, PI. V,) are articulated with it by a 
joint passing obliquely upwards and inwards, towards the cup. 
They are two-jointed. The first joint is broad and large at 
base, and somewhat triangular in form. Its anterior and apical 
portions are covered with soft ciliated oblong papillae, (fig. 19,) 
each of which receives a distinct branch of the large nerve that 
passes to this organ. They shrink up and become obliterated on 
drying, and in this respect differ from similar appendages to other 
parts of the body, and even from the naked setae that terminate 
the apical joint of the antennae. This apical joint is nearly cylin- 
drical in form and is about two thirds the length of the basal. 
The terminal setae are of two kinds ; those at the inferior part of 
the apex are slender and acute, and those at the superior part, 
short and somewhat obtuse. A single naked slender seta, usually 
curved or bent, may be observed near the middle of the posterior 
margin of this joint. 
2. Posterior Cephalic Segment.—The mouth, (figs. 1 and 12,) 
is situated in an oblong mass, which lies entirely external, along 
the under surface of the body, near the centre of the posterior 
cephalic segment. This buccal mass is in part a hollow organ, 
(fig. 12, PI. IV,) bounded above and below by distinct membranes, 
a portion of which represent the upper and under lip. It has a 
lunate opening between the approximating lips, (a a and b, fig. 
12,) and contains a pair of strong mandibles and other organs, 
which we shall soon describe. It is articulated with the cephalic 
segment by its broad posterior portion. 8 
Description of a Species of Caligus. 
The lateral and anterior margin of the buccal mass is formed 
by a slender bone, (c b, fig. 12,) which forms a projection posteri- 
orly where it suddenly curves around inward, and runs backward 
a short distance nearly parallel with the margin. These bones 
form the sides to the lower membrane of the cavity of the mouth. 
At the anterior extremity of the buccal mass within, they are 
connected with several small bones which run to the medial line 
of the mouth, and constitute part of its manducatory apparatus, 
(m, 1, and n, fig. 13, r, s, t, fig. 17, an under view ;) these bones 
lie either on or in the lower membrane of the mouth. No portion 
of the upper membrane of the buccal mass is connected with the 
bones of the lateral margin except a small subtriangular piece 
near the anterior angle, (dae, fig. 12.) These pieces leave be- 
tween them and the anterior margin of the buccal mass a semi- 
circular opening ; the edges of this opening are furnished with 
cilias, and constitute the lower margin of the lunar opening, or 
the lower lip. This lower lip is divided at its centre, (b, fig. 12,) 
and the edges thus formed are curved inward, so that in a vertical 
view several cilise are projected together, and have the appearance 
of one branching cilia. 
The whole membrane forming the upper portion of the buccal 
mass may be called the upper lip. It is represented separate in 
fig. 15. It is united with the lower portions, at its posterior ex- 
tremity, (pp, figs. 12 and 15.) It may be viewed as consisting 
of two parts, an anterior movable, and a posterior, apparently 
immovable. The movable portion, which is very much the 
smallest, is an elliptical, nearly circular, membrane, inserted in a 
semicircular concavity (a a) in the anterior margin of the immova- 
ble portion. Its front edge is coarsely subcrenated and furnished 
with cilice. The large immovable portion of the upper lip is 
bounded by a bony edge, on all sides except posteriorly. At f, 
(figs. 12, 15,) there is a curved process elongated outward, serv- 
ing for the attachment of a muscle. 
Through the opening between the lips, (fig. 12,) we may ob- 
serve the two slender bones 1, (fig. 13,) and just within these, there 
are visible, through the membranes, two dentated organs, which, 
when the membranes above are removed, appear as represented 
in fig. 14. These organs are the mandibles. They are long 
slender organs with a falciform termination, curved inward and 
dentated on the interior edge ; the number of teeth is about Description of a Species of Caligns. 
9 
twelve. The outer margin of the dentated portion is provided 
with a narrow corneous transparent edge. These mandibles ex- 
tend backward and pass out of the buccal mass just anterior to the 
lateral projection, c, (figs. 12 and 13,) and behind the process, f. 
Here they are connected with a bony tendon, to which the large 
muscles are attached which move the mandible. The mandibles 
have no appendages, and are very slightly connected at their base 
with the membranes of the buccal mass. When the buccal mass 
is separated from the body by force applied below, the mandibles 
invariably remain attached to their muscles. 
The remaining corneous organs at the extremity of the mouth 
have been already described as connected with the lower mem- 
brane ; the two pairs m, 1, on the surface of this membrane, and 
the remaining, in its texture. The pair 1, have just been referred 
to as seen through the opening between the lips. These bones 
approximate at their apices; at the other extremity they curve 
backward and terminate under the junction of the two lips, (fig. 
13, and a a, fig. 12;) the bones, m, which are situated under 
the mandibles, are very finely pectinated on their outer margin ; 
they terminate at the same place with the preceding pair. 
The remaining bones form a kind of frame work for the lower 
membrane. Three slender bones r, s, t, (fig. 17,) occupy the 
extremity of this membrane, and the bones, o, its inner portion. 
The bones, o, extend backward and enlarge at the posterior part 
of the buccal mass, (g, fig. 17,) where they serve for the attach- 
ment of the muscles elevating the buccal mass. They appear to 
form by their union at their anterior extremity, (figs. 17 and 13,) 
a short, oblong process (k,) which is situated between the apices 
of the pectinated bones, m. The piece, n (figs. 13 and 17,) passes 
directly outward from this process, and is gradually lost in the 
membrane. 
This complicated apparatus, the buccal mass, appears to be 
composed of the upper and lower lips, united with the different 
parts of a pair of maxillae. 
We have often observed through the upper membranes of the 
buccal mass, and just in advance of the bony arch a, a, fig. 12, 
an obscure curved line nearly concentric with the anterior mar- 
gin of the buccal mass, (fig. 12,) which is frequently in motion. 
From the peculiarities of its action we suppose that there is here 
an internal opening to the esophagus. Within this inner mouth, 10 
Description of a Species of Caligus. 
if we may so call it, there are several folds seen below, (fig. 17,) 
which may be the seat of the sense of taste. Above we observe, 
(fig. 12,) four fleshy oblong organs extending from a point deeply 
situated near the base of the esophagus, obliquely upwards to the 
upper part of the buccal mass. At their lower extremity, they 
are connected by a slender ligament with the hone, g. These 
organs appear to close the esophagus. They often open and close 
in consequence of the similar action of the processes, g, with 
which they are connected. 
The articulation of the buccal mass with the surrounding parts 
is formed by means of a bony process situated in it below f, and 
another slender process (h, figs. 12 and 17,) extending backward 
and outward in the adjacent teguments. A curved bony process, 
(i, fig. 13,) connects the projection c (figs. 17, 13,) with the pro- 
cess below f, uniting the two portions of the buccal mass. 
The remaining organs of the cephalic segment consist of four 
pair of feet, corresponding to the second pair of maxillas and the 
three pairs of maxillipeds in the decapodous Crustacea. 
The first pair are three-jointed. The basal joint is broad and 
oblong, and is connected with the body by its long posterior side. 
At its inner extremity, which is directed outward nearly parallel 
with the basal, it curves upward and receives the following joint. 
The two terminal joints are very different in the two sexes. 
In the male, (fig. 1,) the second joint is large and subconical, 
with an obliquely truncated apex. It appears to be composed of 
two joints, but there is no articulation. The terminal joint is 
obliquely articulated with the preceding; it is small and short, 
and terminates in two strong curved spines, occupying like horns 
the lateral portions of the apex. A slender seta is situated on the 
outer margin, and another on the inner surface near the articula- 
tion. The exterior of the apical spines is often brought in con- 
tact with a prominence on the apex of the preceding joint. In 
the female, (fig. 18 and 18 a,) the second joint is large, but scarcely 
longer than its breadth. The terminal joint gradually tapers with 
an irregular curve to a pointed corneous extremity, which is bent 
downward at right angles with the preceding part. The basal 
joint is peculiar in having a stout spine directed backwards, on its 
posterior margin. 
Exterior to the outer portion of the basal joint of this leg, there 
is a large hooked spine, arising from a broad base, and having an Description of a Species of Caligus. 
11 
oblique position. This may be considered an appendage to the 
maxilliped just described. It is similar in the two sexes. 
The second pair of maxillipeds are rudimentary. They are 
situated along side of the buccal mass. The basal joint is a large 
fleshy mass, having a strong spine directed backward on its poste- 
rior side, (fig. 4, a.) Upon this mass near its anterior part, there 
is a very short cylindrical leg, of a single joint, which bears at its 
apex a long slender spine, and three or four small seta. It moves 
in every direction, and the spine is as frequently pointed inward 
or backward, as in the manner given in the figure. 
The legs of the third pair are situated each on a fleshy base, 
just exterior to the spine of the preceding pair. They are long 
and slender, and composed of three joints. The basal joint is 
rather longer than the two terminal joints of the first pair, and 
diminishes very graduafly to its apex. The second joint is one 
third longer than the basal, and about one fourth as large, and is 
of uniform size throughout. At its apex, below, it has a slender 
ensiform extension, which is doubly edged with a finely pectina- 
ted membrane, (fig. 4, b.) The terminal joint, is very similar to 
the process just described, but is much longer. A single short 
spine is situated on the second joint, a short distance from the ar- 
ticulation of the terminal joint. 
The fourth pair, the last of those on the cephalic segment, is 
very dissimilar in the two sexes. In the male (fig. 1,) it consists 
of a very large basal joint, articulated at its extremity with a stout 
terminal claw, which curves inward and is usually brought in con- 
tact with a strong spine near the apex of the basal joint. The 
breadth of the basal joint is nearly one half its length, except at its 
insertion, where it is quite small. Between the strong spine just 
noticed, and the apex of this joint, there is a small fleshy promin- 
ence, and a stout spine. The terminal claw has a small seta near 
its apex. 
In the female, (figs. 18 and 18 b,) the greatest breadth of the 
basal joint is scarcely one fourth its length, and it is destitute of 
the strong spine, near the apex; in some young females we see 
traces of it, and the leg has more bulky proportions than here 
stated. The terminal joint is much longer and larger, and more 
fleshy than in the male; it has a short spine at its apex, and three 
or four short setse. 
The remaining pairs of legs are four in number; three pertain 
to the anterior thoracic segment and one to the posterior. 12 
Description ofi a Species ofi Caligus. 
3. Anterior Thoracic Segment.—The first two pairs of legs on 
this segment are natatory, the third is expanded into a broad apron. 
Preceding the first pair of these legs, there is, on the medial line, a 
broad furcate corneous process, directed backward, and capable of 
being elevated or depressed. 
The articulations of the two pairs of natatories with the body 
are very remarkable. These organs not only move on their respec- 
tive sternums, with which they are articulated, but the sternums 
have a hinge motion on their posterior margin, in which the legs 
participate. For this purpose the basal joints are attached to the 
adjoining parts of the venter by the greater part of their anterior 
side. The whole distance between the apices of the basal joints 
in the first pair of natatories, forms thus a single hinge on 
which the legs revolve ; and in the second pair of natatories, the 
greater portion of the second joint is similarly attached, and for 
the same purpose. 
Several of these joints are provided with long, finely ciliated 
setse, or pinnulas, similar to those terminating the abdomen, which 
renders them well adapted for swimming. The ciliae, though 
very long, are exceedingly slender. The pinnulae appear to be 
mostly hollow. They contain a central longitudinal line, which 
appears on the first view to divide them into two portions; further 
examination has led us to believe that this is not the case. 
The first pair, are composed of three nearly cylindrical joints, 
the first two of which are very similar in size, and the third about 
one half the length of the preceding. The basal joint has a short 
movable hairy seta at its apex, and another on its posterior margin ; 
also a very short jointed appendage on the same margin near 
its extremity. The second joint has a similar seta near its apex 
and is ciliated on its posterior margin. The terminal joint is fur- 
nished on its posterior margin with three long finely ciliated seta?, 
or pinnulae, whose length about equals the preceding joint of the 
leg. The ciliae are very short on the outer side of these pinnula? ; 
but of the usual length on the inner. At the apex there are four 
short obtuse naked setae. The sternum to which this pair of legs 
is articulated, is very narrow and terminates on each side in a 
process lengthened posteriorly for articulation with the legs. 
The second pair of natatories are composed of five stout com- 
pressed joints, with a large tri-articulate appendage to the second 
joint. The basal is very short, and has on its posterior margin a Description of a Species of Caligus. 
13 
curved pinnula, which extends over the median line. The sec- 
ond joint is very large and increases in size from its base to its 
apex. There is a short seta near its posterior margin, and another 
near its apex. On its posterior edge there is a broad membrane, 
exceedingly thin and transparent, and finely striated like the mar- 
gin of the shell. 
The appendage to this joint arises from the inner part of its ex- 
tremity, and curves backward and inward. It is provided with 
seven long pinnulae, which in general, extend with a curve to the 
median line of the body; the first joint has one of these pinnulae, 
the second two, and the third four. These joints are mostly very 
flat. The first joint is short and very similar in form to the basal 
joint of the leg. It is furnished exteriorly with a broad plate, 
which is ciliated at its apex. The second joint of this appendage, 
is smallest at its base, and increases with a curve to its apex, which 
is rounded. Its outer margin is ciliated. A shallow concavity 
receives the apical joint, which is small and nearly semicircular. 
The remaining joints of this leg, are also furnished with seven 
long natatory pinnulae, of which one appertains to the third joint of 
the leg, one to the fourth, and five to the fifth. The third joint is 
broad and oblong, and is ciliated on its inner margin. A large 
stout spine, with a thin corneous expansion on two opposite sides, 
is articulated with the apex of this joint. The fourth joint is short- 
er than broad, and has a small spine at its apex. The terminal 
has nearly equal length and breadth, and is obliquely truncated at 
its extremity. There are two short spines at its apex. The pin- 
nula terminating this joint, is provided with ciliae only on its inner 
margin. The other margin is furnished with a thin membrane, 
which extends from the apex, to an enlargement in the seta near 
its base. 
The sternum uniting the legs of this pair, is quite large; its 
breadth is one third its length. The posterior margin is some- 
what fleshy and thin, and provided with a delicate membrane, 
whose breadth is nearly as great as that of the sternum. The 
anterior articulating margin of this sternum is firm and osseous. 
The adjacent teguments with which it is articulated are similarly 
ossified, and supported both before and behind by two strong os- 
seous processes, situated in the teguments. The anterior processes 
are short, and terminate in a curve between the two sternums. 
The posterior are nearly three times the length of the sternum; 14 
Description of a Species of Caligus. 
they diverge from their insertion and extend to the base of the 
apron, (fig. 18.) Here they are united by a slender osseous process, 
which forms the upper limits of the sternum of the apron. They 
continue on, making at first one or two irregular curves, and form 
also the lateral boundaries of this sternum. By this remarkable 
arrangement, the articulation of the sternum of the large natatory 
legs is rendered sufficiently firm for their powerful action in the 
motions of the animal. 
The third pair of legs has been already stated to be expanded 
into the form of an apron, forming a broad lamellar appendage to 
the cephalo-thoracic portion of the body. This apron is composed 
of the same parts as the natatory last described, and there is an 
almost perfect coincidence in the number and nature of the ap- 
pendages. 
The sternal piece is very wide and lamellar. On its posterior 
margin there is a broad membranous expansion, identical in struc- 
ture and position with that appended to the preceding sternum. 
The portion of the apron corresponding to the small first joint of 
the natatory leg, is very narrow, and has very imperfectly defined 
limits ; we see an indication of its presence in the single pinnula, 
behind, adjacent to the sternum.* The second joint is expanded 
into a broad, irregular trapezoidal figure, with concave sides ex- 
cepting its posterior margin. Like the same joint in the natatory, 
it is furnished with a broad thin membrane posteriorly, and a 
jointed appendage provided with pinnulae. The first joint of this 
appendage is quite small, and bears a single pinnula as in the per- 
fect leg; the remaining portion is circular and is furnished with 
six pinnulae. We find the analogue of the third joint of the 
natatory in a broad nearly circular plate, which is the lateral termi- 
nation of the apron ; it is connected with the basal portion by an 
indistinct suture. The posterior margin of this joint, like the 
same joint above, bears a ciliated leaf-like expansion. The anal- 
ogy of the parts is still farther apparent in the strong articulated 
spine and pinnula attached to this joint, and in the two small termi- 
nal joints, furnished with pinnulae ; the first with a single pinnula 
and a spine at its apex, and the second with four pinnulae, and two 
short apical spines. 
* The corresponding parts of the apron, and the second pair of natatories, are in- 
dicated in the figure by the similar numbers in them. Description of a Species of Caligus. 
15 
4. Posterior Thoracic Segment.—The legs attached to this 
segment arise from the posterior part of the lateral surface. They 
are composed of four joints, which gradually diminish from the 
base to the apex. The basal joint is large, nearly cylindrical, and 
irregularly rounded at each extremity; there is a short hairy seta 
at its apex. The second joint is scarcely half the diameter of the 
preceding. It gradually diminishes to a pointed apex, furnished 
with a curved spine. The third joint is flat, and is articulated, by 
its obliquely truncated base, with the inner side of the preceding 
joint. There are two long setae on its inner apex, which are 
edged on two opposite sides with a pectinated membrane. There 
is a short pectinated appendage, projecting like an epaulette over 
the base of each of these setae, and also over the articulation of 
the following joint. This terminal joint is long, slender, and 
setiform. It has a row of short spines along its inner edge. 
5. Abdominal Segments.—The anterior abdominal segment is 
entirely destitute of any articulated appendages. Posteriorly, on 
each side, there is a broad lamellar sub-triangular organ, which, in 
the male, is much elongated, while in the female, excepting young 
individuals, it is very short. 
The appendages to the terminal joint have already been des- 
cribed. 
Change of Skin.—But few facts have come under our obser- 
vation respecting the change of skin; these few, however, appear 
quite peculiar and worthy of remark. 
When the time for shedding the old skin approaches, the inter- 
nal membrane, which is to form its new envelop, is very vari- 
ously folded into ridges throughout the whole body. In some 
parts, the ridges or folds are situated around the bases of the mus- 
cles, and enclose regular areas. These folds continue increasing 
in size, till the time of moulting. This process produces a sin- 
gular arrangement of the anterior portions of the inner shell, or, 
we may say, inner animal, as it affects the form of the included 
parts. The centre of the front margin of the internal shell is 
drawn inward and backward, as represented in fig. 23, PI. V, in 
which c d represents the outer margin, and c' n d' the correspond- 
ing edge of the inner shell, e f the articulation in the old shell, 
and e'm f' the corresponding articulation in the inner. 
These folds undoubtedly result from an increase of the animal 
within a shell too small to admit of its expansion. The internal 16 
Description of a Species of Caligus. 
members, like those of animals having a soft skin, appear to in- 
crease in actual quantity of matter, as rapidly when enveloped in 
their unyielding corneous covering, as in their new membranous 
envelop. 
There is a remarkable fleshy appendage to the anterior portion 
of the soft internal animal, the importance or functions of which 
we are unable to explain. It is represented at m n, fig. 23, in its 
natural position. It lies wholly external to the inner shell, and is 
attached only at o, its anterior extremity. Figure 24 is a profile 
view of this appendage; similar letters mark the corresponding 
parts in the two figures. After separating the outer skin, it may 
be drawn forward into the position in fig. 25. In one instance, we 
found an animal with this singular appendage, in front, presenting 
very much the appearance in fig. 25. It was, however, composed 
of three of these appendages, m n, placed end to end, and ap- 
peared to have undergone three successive moultings. 
In external appearance, this organ very much resembles a mus- 
cle, as it is striated, like them, though very coarsely. It is proba- 
bly attached, by its large extremity, to the outer shell. 
II. Muscular System. 
The muscles moving the several members, may, in general, be 
distinctly seen and traced to their insertions through the pellucid 
covering of the body. Yet, under a magnifying power of five 
hundred diameters, we have not succeeded in detecting the ulti- 
mate fibres as given by Straus. With a much lower power, how- 
ever, we have observed that all the muscles appear transversely 
striated, and by means of this important character, have been ena- 
bled to distinguish the nerves from the muscles, which, without 
this aid, would in many instances have been difficult or even im- 
possible. These striations are mo§t distinctly seen in the flat, sim- 
ple muscles; those composed of several bundles of fibres, which 
is the case with many of the large muscles on the back, exhibit 
it, but less perfectly. We have examined the muscles of the 
common lobster, (Astacus marinus,) and have found these stria? 
in some instances, though with less distinctness. These stria- 
tions vary much in their fineness. In general, they are from 
vW o t0 a o'o o °f an inch apart. In some muscles, among which 
we may mention those elevating the buccal mass, we found them 
as coarse as j-Joo °f an inch. We have conjectured that they are Description of a Species of Caligus. 
17 
the result of minute folds in the muscular fibres; but we have 
been unable to detect an approximation of these strias, or any al- 
teration in their appearance, during the contraction of the muscle : 
this, however, may be owing to their extreme minuteness.* 
On account of the very peculiar forms and motions of some of 
the organs in this animal, it contains several muscles of very unu- 
sual character. We reserve the description of them till these or- 
gans come under consideration. 
a. Muscles of the Segments of the Body. 
The first cephalic segment is flexed by two short slender mus- 
cles on each side, (R, R',) situated just exterior to the process which 
forms the articulation of this segment, (figs, land 7.) and di- 
rected backward and outward. They unite in a common short 
tendon. They act. in depressing this segment, and assist in at- 
taching its cup and anterior margin. This margin is provided 
with a narrow ridge, which is striated or wrinkled transversely, 
like the cup, and is apparently intended to produce a closer at- 
tachment of this margin. 
For the mutual motions of the cephalic and thoracic segments, 
there are three pairs of muscles situated in the former, two at- 
tached near the median line, and one pair laterally. A pair of 
short muscles (I, I, fig. 7) run nearly parallel with the median line ; 
they produce the slight flexion admitted at this articulation. An- 
other pair of muscles, long and large, (S, fig. 7,) are situated 
on each side of the preceding ; they pass obliquely outward. In 
addition to aiding in flexion, they produce a lateral sliding motion, 
often observed between these segments. A third pair (K, fig. 7,) 
also assist in flexion. The large muscles, (K/,) situated in the 
posterior segment, appear also to pertain to this joint; but we are 
not fully assured that this is really their insertion. 
The extensor muscles of the posterior thoracic segment, and 
of the abdomen, arise adjacent to the median line, near the cen- 
tre of the anterior thoracic segment. Three pairs of muscles are 
attached at this point. The outer (L) pass obliquely outward and 
* Since writing the above, we have found that these striations have been obser- 
ved by Dr. Hodgkin in the muscles of man. He says : “ Innumerable very mi- 
nute but clear and fine parallel lines, or striae, maybe distinctly perceived, trans- 
versely marking the fibrillae.” These observations have led Dr. Hodgkin to doubt 
the globular constitution of the contractile'fibre. We have also observed them 
distinctly in some of the Arachnides. 18 
Description of a Species of Caligus. 
are inserted near the apex of the thoracic joint. The two pairs 
(M, N,) appear to continue through the thorax, to the last joint of 
the abdomen. Another pair of muscles (O) commence in the 
joint, near the median line; they pass obliquely outward 
to a point in the first abdominal segment, just below its centre, 
where they are inserted into the teguments. Another pair of slen- 
der muscles (P) arise near the insertion of the last, and pass to 
the following segment. 
The flexor muscles of these segments, situated along the ven- 
ter, are remarkable for having but two anterior attachments, al- 
though, counting the several insertions in the posterior segments, 
there appear to be six distinct muscles. Two broad muscles arise 
on each side of the medial line opposite the prehensile legs. As 
they pass between the sternums of the natatory legs, they divide 
into three portions as represented in fig 5 a, PL I. The large 
muscle here suddenly contracts in size, and afterwards continues 
on, much diminished in volume ; exterior to this continuation two 
muscles are attached, each by a tendon, to the diminishing portion 
of the main muscle. Though apparently distinct, these three mus- 
cles continue connected, and pass on beyond the sternum of the 
second pair of natatories, where there is a second subdivison of the 
muscle. We observe an oblique constriction of the whole, (fig. 
5, b,) below which, the three muscles are continued of nearly 
their former size, and a fourth is added, exterior to the three. 
Thus divided, the muscle continues into the abdomen, where the 
four parts are separately inserted: the exterior pair diverge and 
are attached near the base of the abdomen; the interior, are in- 
serted below the centre of the abdomen, directly under the inser- 
tions of the extensor muscles of the back; the two remaining 
pairs are continued into the terminal abdominal segment, the outer 
passing beyond the centre of this joint. Another pair of small 
muscles are inserted in the base of this joint, which arise near the 
attachment of the interior pair of abdominal muscles. 
The other set of muscles, consisting of two pair, arise a short 
distance below the sternum of the posterior natatory, exterior to 
the muscles just described. One pair, the outer, is inserted in the 
base of the thoracic joint, and the inner, laterally below the cen- 
tre of the abdomen. 
The lateral motion of these joints is produced by the simultane- 
ous action of the flexor and extensor of the same side. The in- Description of a Species of Caligus. 
19 
sertion of the more powerful of the abdominal muscles below the 
centre of this segment, in preference to an attachment near its 
base, enables the animal to give this segment great flexion. When 
the animal has been attached to the glass out of the water, we 
have often separated the anterior portion of the body from the 
glass, till it formed an angle of 75° or 80° with the abdominal 
portion, and generally the animal has succeeded through the ac- 
tion of these muscles in restoring its head again to the glass. 
The muscle (0) on the back may possibly be attached to the 
muscle (N) and not to the thoracic segment. We have not suc- 
ceeded, in our dissections, in exposing these muscles in order to 
determine this point. 
b. Muscles of the organs appertaining to the several segments. 
1. Anterior Cephalic Segment.—In the following account, we 
shall in general describe only the muscles moving the basal 
joints of each of the legs. More minute particulars may be ob- 
tained by reference to the plates. 
The muscles moving the cup, have not been satisfactorily de- 
termined. A slightly elevated line passes from each side with a 
curve into the membrane of this organ, which may be muscular ; 
if so, they act in flattening the cup preparatory to its attachment. 
The antennas have two extensors and one flexor. The two ex- 
tensors are inserted in a tendon, occupying the anterior margin of 
the base. They extend half way to the eyes; one (a, fig. 7 and 
fig. 1) above the flexor of the anterior cephalic segment, is 
attached to the upper shell; the other, (a7, fig. I,) much the small- 
est, passing under the same muscle, is attached below. The 
flexor (b, figs. 7 and 1,) is inserted near the outer part of the base, 
by means of a short tendon, and is attached near the base of the 
preceding muscles. These organs have but little motion, and are 
seldom observed in action. 
2. Posterior Cephalic Segment.—The elevators of the buccal 
mass are four short narrow muscles, inserted in the bony pro- 
cesses, g, (figs. 12 or 17,) and attached to the teguments below, 
under the anterior extremity of the mouth ; the insertion of one is 
exactly posterior, and of the other, a little lateral, as is represented 
in fig. 17. By means of these muscles the buccal mass may be 
elevated to a right angle with the surrounding parts. On dying, 
the mouth is often left in this elevated position. A muscular 
band passes across the back part of the buccal mass and after 20 
Description of a Species of Caligus. 
attaching itself to the curved process, f (fig. 12,) on each side, 
continues on, and is inserted in the shell. Ate, (fig. 7,) near the 
eyes, we observe the attachment of a pair of muscles which are 
in action when the buccal mass moves; we have not detected 
their insertion, but suppose from their position that they act in 
depressing it. 
The internal parts of the mouth which receive distinct mus- 
cles are as follow : the upper lip, the mandibles, and the parts of 
the inner mouth. The upper lip is provided with two pairs of 
retractors which are attached near the centre of the exterior mem- 
brane of the mouth. The interior pair are very slender; they are 
inserted in a minute process near the extremity of the lip, (fig. 15,) 
and move merely the extremity, giving it the position in fig. 16. 
The exterior pair are four times the width of the interior; they 
are inserted near the middle of the lip and retract this organ 
nearly to the bony arch. 
The mandibles are provided with muscles of extraordinary 
length and power. There are two pairs connected with the same 
slender bony tendon, the one with its extremity, and the other with 
its posterior side. The former, (d, figs. 1 and 7,) pass outward and a 
little downward, and on approaching the apex of the basal joint of 
the third pair of maxillipeds, curve suddenly backward ; they are 
finally inserted in the margin of the shell opposite the articula- 
tion of the head and thorax, after having run over a space equal 
to one half the whole length of the cephalo-thoracic segment. 
The other pair extend obliquely backward and outward under 
the base of the rudimentary feet. Although these organs are 
provided with such remarkable muscles, they are very confined 
in their motions. They occasionally have a vibratory motion 
when the animal is nearly exhausted, and this is the only action 
we have observed. Their position and the form of the adjacent 
parts satisfies us that their extremities cannot be projected out of 
the mouth; and probably they can scarcely reach the opening 
between the lips. 
On account of the thickness of the enveloping membranes, and 
the difficulty of dissecting the internal parts of the buccal mass, 
we have not discovered the muscles moving these parts. We 
can only specify one pair of slender muscles, which are inserted 
in the lateral portions of the process g, (fig. 17.) It is the re- 
tractor of these processes, and through them opens the folds which Description of a Species of Caligus. 
21 
close the esophagus, by means of a tendon inserted in the lower 
extremity of these folds. 
The basal joint of the first pair of maxillipeds has hut little 
motion. There are two short muscles, elevating or depressing 
the extremities of this joint, which we may consider a flexor and 
an extensor. The flexor, which is inserted near the interior ex- 
tremity, is directed backward and a little outward to its attach- 
ment to the lower shell, exterior to the base of the following pair 
of feet. The extensor is inserted at the posterior margin of the 
joint, and extends obliquely inward, approaching the attachment 
of the flexor. In the female these muscles have nearly the same 
position as in the male (fig. 18 a;) the flexor is inserted near the 
spine on this joint. The united action of these muscles draws the 
anterior margin of this joint from the shell. To oppose this mo- 
tion there is a large muscle inserted near this margin and extend- 
ing one side below the eyes, (e, fig. 7,) where it is attached to the 
back shell. 
The extensor of the second joint of this pair of legs is a long 
broad muscle attached to the shell above the large curved spine, 
(f, fig. 7.) There is a small flexor of this joint, attached to the 
posterior apex of the basal joint. 
The rudimentary feet, or second pair, are provided with but 
few small muscles, requiring no remarks. 
The third, pair are remarkable for having as various motions as 
could be afforded by a ball and socket joint. This arises from 
their insertion on a fleshy prominence, composed probably of the 
rudiments of the small basal joints in the corresponding organs of 
the higher Crustacea. To produce these various motions, each 
leg is provided with five muscles radiating from the base, some of 
which are of very peculiar form. Four of these muscles are in- 
serted into the base of the first joint and one along its posterior 
margin. The latter appears to be attached to the back near the 
median line, a short distance behind the eyes, (g, fig. 7.) Of the 
remaining muscles, two pass forward and outward, (h, i fig. 1 
and fig. 7,) one directly outward, and the fourth, (k, fig. 7,) back- 
ward and outward. The most anterior (h) is a slender muscle, 
attached just exterior to the base of the first pair of maxillipeds. 
The second (i, fig. 1 and fig. 7) is composed of two parts in- 
serted into the same tendon. These parts continue together 
through half their length, then separate, and soon after each di- 22 
Description of a Species of Caligus. 
vides into two nearly equal portions, which diverge under the 
large curved spine and pass to their attachment at the margin of 
the shell. 
The base of the fourth pair of maxillipeds has a narrow pro- 
longation, which affords attachment to two muscles ; one passes 
posteriorly, and is attached near the articulation of the head and 
thorax, (m, fig. 7,) another extends outward in front, beneath the 
extremity of the adjacent spine. Two other short muscles are 
inserted at the base of the prolongation, and are also attached 
near the spine ; one on the back, and the other below. The last 
of the muscles moving this pair of legs, extends outward and is 
attached to the epimeral articulation, (1, fig. 7.) 
The terminal claw is provided with flexor muscles of great 
strength. A large conical muscle attached along the whole pos- 
terior margin, is inserted in a bony tendon extending from the 
inner portion of the base of the claw. Another large muscle 
arises from the basal portion of the joint and is inserted into the 
preceding muscle a short distance from its insertion. There is 
the same arrangement in the female, (fig. 18 b.) A small extensor 
is inserted in the outer part of the base of the claw and attached 
to the outer posterior margin of the first joint. 
3. Anterior Thoracic Segment.—The two legs of each pair of 
natatories, have been described as simultaneous in their action, 
which consists in their rotation with the included sternum, on 
their anterior margin. 
The principal elevator of the first pair of natatories is a large 
digastric muscle. This muscle occupies the space between the 
basal joint of these legs and the preceding pair. It is composed of 
four muscles which unite in a common tendon ; this tendon passes 
under a curved osseous process, by which it is confined in its 
place, and is then united to another bundle of muscular fibres in- 
serted in the lower surface of the leg. The depression of these 
legs is produced by a long muscle which is inserted in the joint 
near its base ; it is directed forward and outward, passing under 
the digastric muscle beyond the articulation between the head 
and thorax, and is attached to the epimeral articulation (n, lig. 7.) 
This pair of legs, though thus provided with muscles of consider- 
able strength, is seldom used by the animal in effecting its motions. 
We have already seen that the second pair of natatory legs 
are well adapted to form powerful propelling organs; that the Description of a Species of Caligus. 
23 
flabelliform arrangement of their pinnulae, the attachment of 
these pinnulas to two distinct articulated branches, added to the 
flattened form of the joints, give the oars a broad expanded sur- 
face for action on the water in swimming. They are farther 
fitted for this object by the provision of a large number of pow- 
erful muscles, which occupy nearly the whole of the thoracic 
segment. 
Inserted in the anterior part of these legs, there are three large 
muscles attached to the back shell, two of which (o, p, fig. 7) 
arise on the median line—a third (q) at the articulation between 
the head and thorax. Four powerful muscles are inserted in its 
posterior margin ; the three outer (u, t, s,) pass backward, and are 
inserted in the posterior and medial part of the segment above. 
The fourth (r) is attached to the back shell over the anterior part of 
the base of the leg, near the medial line of the body; it first 
passes inward and backward, then curves outward around the base 
of the muscle adjoining, (s,) and finally extends upward to the pos- 
terior margin of the leg. The circular form of this muscle is so 
very extraordinary, that we at first doubted its muscular nature. 
We have however assured ourselves of this fact by frequent dis- 
sections. Two other short muscles with converging fibres, (w, v,) 
arise laterally from a broad base in the epimeral articulation, and 
serve to retract the leg to the shell. These muscles probably co- 
operate with the posterior, in the depression of the leg. 
If these oar-like legs struck the water with the same broad ex- 
panded surface, in their backward motion, as in their forward pro- 
pelling action, the animal would advance but slowly, if at all; 
for the latter would be counteracted by the former. There is a 
provision against such a defect, in the muscles moving the several 
joints of these legs, by the action of which, the terminal portions 
receive a partial revolution, and cut the water, when drawn back- 
ward, by their thin anterior edge. Their special adaptation for this 
purpose is apparent, even in the pinnula terminating the leg, which 
instead of being ciliated on both edges, is furnished anteriorly 
with a thin membranous expansion. 
These legs appear to be the only organs for walking as well as 
swimming. 
The principal extensors of the third pair of legs, or the apron, 
are four in number; two (y, z) arise on the back near the medial 
line, and pass laterally to the outer insertion of the apron. One 24 
Description of a Species of Caligus. 
of the remaining two, (x,) arises just above the posterior sinus, 
and the other from the inner margin of this sinus; both are at- 
tached on the back, and inserted near the articulation of the ster- 
num. The flexor muscles arise below, just outside the apron, and 
occupy the greater part of its interior. A single muscle is attached 
near the articulation of the sternum, and passes into the basal 
portion. 
This apron, appended to the cephalo-thoracic segment, forms 
the anterior portion of the body into a large, broad cup, which is 
perfectly closed, with the exception of a small opening at each of 
the posterior sinuses. These we have already described as pro- 
vided with a folded membrane, furnished with muscles capable of 
drawing it over and completely shutting the opening. The mem- 
branous margin of the animal near the antennae, has also a fold 
by which a small leak, if it be such, is closed. Considering these 
several provisions, it is probable, that the whole of this anterior 
portion of the animal is especially adapted to enable the animal 
to attach itself firmly during the rapid motions of the fish, and 
that the small marginal cups in front are relied on, only while the 
fish is stationary, or but slowly moving. 
The remaining pair of legs are moved by short slender muscles, 
and seem to possess little power. They usually hang loose and 
motionless while the animal is swimming, and when attached to 
the body of the fish, are commonly extended by the side of the 
abdomen. 
III. Nervous System. 
a. The organs of the senses. 
The only organs in the Caligus, which we have been able to 
distinguish as the undoubted residence of special senses, are the 
eyes and the antennae. The latter organs have already been de- 
scribed ; it remains to explain the structure of the eyes. 
The eyes are wholly internal, and are situated near the centre 
of the posterior cephalic segment, directly over the lower part of 
the buccal mass. They are two in number, simple in their struc- 
ture, and placed near one another, on a single reddish-black ground. 
They project from each side of this colored ground, with a spher- 
ical surface, somewhat exceeding a hemisphere. On dissection 
we readily distinguish the following parts. 
1. A cornea, which is thin and transparent and forms the spher- 
ical surface of the eye: Description of a Species of Caligus. 
25 
2. A lens, simple, spherical, and distant from the cornea; its 
diameter is about half that of the cornea :* 
3. A colorless, transparent fluid, which we presume to be the 
aqueous humor, occupying the space just within the cornea: 
4. A deep red, nearly black pigment, which forms the colored 
spot supporting the eyes. 
We have not observed the vitreous humor. In the spherical 
form of the lens, the eyes resemble the same organs in fishes. They 
are not movable, and have no connections except by the optic 
nerve. The adjacent parts are transparent, enabling the animal 
to see in both directions. We have already described the shell 
above, and referred to. its representation in fig. 8, PI. IV. It is 
perfectly flat, without any spherical projection, corresponding to 
that of the cornea within. The translucent elongated space in fig. 
8, lies in the shell, and passes over the space between the eyes. 
b. The nerves. 
The nervous system contains but two ganglions, and these by 
their close approximation appear at first, to compose but one. 
They are situated directly behind the eyes, the one above the 
esophagus, and the other below it, and are so intimately connect- 
ed on each side of this portion of the alimentary canal, that it has 
been found impossible to separate them, (fig. 20.) Indeed, it would 
scarcely convey an incorrect idea of the form, to describe it as a 
single mass, with a longitudinal cavity through the centre, for the 
passage of the esophagus. The size of the united ganglions is 
rather greater than that of the buccal mass. The nerves arising 
from these ganglions are flat, fibrous cords, enclosed within a 
membranous envelope or neurolemma. This neurolemma is often 
one fourth wider than the bundle of nervous fibres contained with- 
in, and these fibres appear to pass through without any attach- 
ment. The neurolemma is sometimes slightly folded, which 
gives a crenated appearance to the margin of the nerve. 
The brain or cephalic ganglion has a broad ovate or sub-cordate 
form. It gives off three pairs of nerves. 
The first pair, (a fig. 20, PI. IV,) leaves the central part of the 
anterior margin and passes directly to the eyes. As the eyes are 
adjacent to the ganglion, these nerves are very short. 
* The lens in the simple eyes of Crustacea is usually described as being in im- 
mediate contact with the cornea: it was very evidently distant from this mem- 
brane, in the Caligus. 26 
Description of a Species of Caligus. 
The second pair, (b fig. 20,) arises from the same margin later-’ 
ally, and extends upward towards the cup, (fig. 18,) passing just 
within the articulating process of the cephalic segments. It gives 
out large branches which are distributed to the surrounding mus- 
cles and teguments. The anterior extremity which goes to the 
cup is scarcely one third the size of the base. 
A small hollow vessel, (fig. 18,) extends from the organs which 
we have considered analogous to the inner antennae in other Crus- 
tacea, along the median line, and appears to terminate in a bulb, 
about half way to the brain. This vessel has been the subject 
of much investigation, without removing all the doubts respecting 
its nature. When separated from the body, it appears to be a 
large neurolemma, containing two small bundles of nervous fibres, 
and this is our final conclusion, though adopted with some hesita- 
tion. It appears probable, from the result of some of our dissec- 
tions, that this bulb receives a nerve from each side, which either 
arises directly from the brain, or, is a branch of the nerve last de- 
scribed. If this is a distinct pair, it corresponds to the third, or 
inner antennary pair in the lobster, as given by Edwards, and the 
pair described as passing to the cup, and surrounding teguments 
and muscles, is the analogue of the fourth pair, which has a simi- 
lar distribution in the lobster. 
The remaining pair of nerves, (c,) arise from the anterior an- 
gles of the brain, and pass to the antennae ; they are one half lar- 
ger than any other in the body. Near the origin, they give off ex- 
teriorly a slender branch, which continues nearly parallel with 
the main nerve, and passes to the muscles of the antennae. With- 
out farther branching they extend in nearly a straight line to the 
base of the antennae, where they subdivide into four large branch- 
es, which are distributed to the fleshy papillae, (fig. 19, PI. V.) 
Two nerves from the posterior branch run along the muscles, and 
are continued into the terminal joints, one to each of the two ter- 
minating sets of setae. The antennas are so abundantly furnished 
with nerves, that they must be the seat of an important sense. 
The sense of touch is the only one for which their peculiar form, 
and their delicate papillae, appear adapted. 
The thoracic ganglion, which is composed of all the thoracic 
and abdominal ganglions united, has a cordate form, and is some- 
what larger than the brain. Its inferior extremity extends rather Description of a Species of Caligus. 
27 
farther beyond the brain, than the brain beyond it in front. This 
ganglion gives off seven pairs of nerves in front and laterally, and 
two pairs behind, besides a central nerve or the spinal cord. 
The first two pairs originate at the centre of the anterior mar- 
gin, (d, e, fig. 20.) The inner is quite slender and appears tQ en- 
ter the mouth each side of the esophagus. The second has twice 
the diameter of the first; it curves more outward, and is supposed 
to go to the mandibles and their muscles. These nerves pass under 
the buccal mass, and cannot be traced while it is in its natural po- 
sition. They invariably appear broken off when the buccal mass 
is removed; and sometimes after detaching it, a nerve equal in 
size to the first, has been seen entering the mouth near the esoph- 
agus, as above stated. These facts have been deemed sufficient 
to authorize the above opinion respecting the destination of these 
nerves. 
The third pair (f, fig. 20) arise from the anterior angle of the 
ganglion. They give out a branch exteriorly to the muscles of 
the first pair of legs, and afterwards continue to these organs, and 
pass into the terminal joints after giving a branch to the basal. 
The fourth pair (g) arise just posterior to the last, and are dis- 
tributed to the outer teguments. They afford a branch near their 
origin, which probably passes to the rudimentary legs : soon after 
they divide into two parts; one branch passes outward and a 
little forward towards the curved spine, and subdivides into four 
branches before reaching it, which are distributed to the neigh- 
boring teguments; the other branch, extends backward to the 
epimeral articulation, just below the articulating processes, where 
it passes to the epimeral segment; it then branches, and is dis- 
tributed to the various parts of the inferior portion of tips segment. 
The fifth pair (h) arise from the lateral margin of the gan- 
glion, some distance behind the preceding. They give off a slen- 
der branch near their origin, and pass along with the branch to 
the third pair of maxillipeds. 
The sixth pair (i) arise near the preceding, and are large nerves. 
They divide immediately, and then subdivide into several branch- 
es, which are distributed to the fourth pair of maxillipeds, and 
their muscles. 
The seventh pair (k) originate near the last, soon divide into 
two branches, which pass to the muscles of the same legs. They 
are slender nerves. 28 
Description of a Species of Caligns. 
These seven pairs appear to correspond to those of the first tho- 
racic ganglions in the macrural Crustacea. The remaining nerves 
pertain to the thoracic legs, and the abdominal portions of the 
body.* 
The outer pair (1) belong to the anterior natatories. They 
continue parallel with the spinal cord till they reach the furcate 
process on the venter; they then curve outward, exterior to the 
ventral muscles, give off three branches in succession from the 
outer side, to the muscles of the first natatory. Before entering 
the basal joints of these legs, they divide into three portions, 
which enter together; the inner branch is quite slender, and pass- 
es to the posterior movable seta, and the jointed appendage ; the 
middle is distributed to the muscles of the basal joint; the outer 
branch gives a slender nerve to the apex of the basal joint, and 
then passes to the two following joints, dividing as it enters them. 
We refer for minuter details to figure 18, Plate Y. 
This pair of nerves give off a slender branch near their origin, 
(r, fig. 20,) which passes to the attachments of the stomach. 
The next pair of nerves (m) are distributed to the second pair 
of natatories. They diverge from the spinal cord—to which they 
are adjacent—below the furcate process, and soon give off a branch 
* The apparent correspondence of these nerves, with those of the first thoracic 
ganglion in the higher Crustacea, together with the great similarity in the last two 
pairs, to those which are distributed to the posterior pair of maxillipeds in the 
decapodous species, (see Edwards’s Hist. Nat. des Crustaces, T. I. p. 137) have 
induced us to adopt the designations heretofore employed in speaking of the organs 
of this segment of the body, in which we consider the three posterior pairs of ce- 
phalic legs as the anologues of the three pairs of maxillipeds in the typical species. 
The only objection which can be urged to this view, arises from there then being 
bat four pairs of thoracic legs. This objection will however be removed if we may 
consider the furcate bone on the venter, just anterior to the first pair of natatories, 
as the rudiment of the sternum of a fifth pair, which view is favored by its posi- 
tion, and its resemblance to the sternums of the following pairs. 
If this conclusion is correct, the twenty-one rings, the normal number consti- 
tuting the body, may be considered as distributed in the following manner : 
The anterior cephalic segment, includes the second and third, as the first—the 
ophthalmic—is wanting. The posterior cephalic segment, contains the following 
six, corresponding to the mandibles, (fourth,) and a pair of maxilla, (fifth,) in the 
buccal mass; and the four pair of feet attached to this segment. The anterior 
thoracic segment will include the tenth, eleventh, twelfth and thirteenth rings, to 
which the furcate process, the two pairs of natatory legs, and the apron, pertain. 
The posterior thoracic segment, is the fourteenth ring. The remaining seven rings, 
the abdominal, constitute the terminal portions of the body; some of these last 
may however be wanting. Description of a Species of Caligus. 
29 
interiorly, which passes down the venter, and appears to be dis- 
tributed to the ventral muscles. As they approach this pair of 
natatories, they give off another branch from the same side, which 
also passes backward, and is supposed to furnish nerves to the pos- 
terior muscles of these legs. On entering these natatories, the 
nerve divides into two branches, the upper of which soon gives 
off a third; the inner nerve, as in the preceding legs, goes to the 
posterior seta, and the articulated appendage ; the middle furnishes 
the basal joint, and sends a branch into the terminal; the outer 
affords a small nerve to the seta at the apex of the basal joint, and 
then passes into the extremity of the leg. 
This pair of nerves give off a branch exteriorly near their ori- 
gin, (s, fig. 20,) which curves outward under the furcate process, 
(s, fig. 18,) beneath the ventral muscles, gives a nerve to these 
muscles, and is then distributed to the anterior muscles of the 
second pair of natatories, and to the adjoining teguments. Its 
branches may be seen at s', s", fig. 18. 
The spinal cord, furnishes the nerves to the remaining mem- 
bers. It appears to be composed of two parts near its origin, but 
there is no division till it has passed beyond the sternum of the 
second pair of natatories. Previous to this division, a short dis- 
tance below the sternum, this spinal cord gives off from each side 
a large nerve which goes to the apron. These nerves are seldom 
exactly opposite in their origin ; as is also the case with the nerves, 
r and s, fig. 20. 
The nerves to the apron, just before entering it, give off a branch 
exteriorly, which is distributed to the outer portions of the apron, 
or more properly its terminal joints. Soon after entering the 
apron the main nerve again divides, and one branch is distributed 
to the basal part, and the other to the muscles of the following 
portion of the apron. 
The spinal cord, after giving off the nerves to the apron, soon 
divides. Thus divided, it gives off a pair of nerves to the re- 
maining thoracic legs, and on entering the abdomen, furnishes a 
pair of nerves which branch in this segment. It thencp con- 
tinues to the last segment, and distributes fibres to the terminal 
portions of the body. 30 
Description of a Species of Caligus. 
IY. Nutritive System. 
a. Organs of digestion. 
The alimentary canal, (fig. 9, PI. V,) is composed of three dis- 
tinct parts, corresponding to the esophagus, the stomach and the 
intestine. 
The esophagus constitutes one sixth the whole length of the 
alimentary canal, and in large individuals is about one sixteenth 
of an inch long. It extends in the form of a long slender tube of 
uniform diameter to the stomach, and passes a short distance into 
its cavity. Its insertion in the buccal mass may be seen in fig. 
17, which is an under view of this organ. The anterior opening 
is closed by two fleshy folds, which have already been described 
when speaking of the organs and muscles of the buccal mass. 
At its commencement, there is an oblong enlargement, (fig. 
10,) longitudinally striated, which may be considered a pharynx. 
The communication with the stomach is closed, but whether by 
a sphincter or valve is undetermined. The peristaltic motion fre- 
quently seen in the stomach and intestine, never extends into the 
esophagus. 
This portion of the alimentary canal is readily separated into 
two membranes. The inner, the mucous coat, is thin and trans- 
parent, and very smooth. The outer is much thicker, and scarcely 
semi-transparent; its muscular fibres are not apparent. When 
highly magnified, its exterior surface appears very uneven. If 
the mouth is detached from the body with care, the esophagus often 
continues attached to it, and presents the appearance exhibited in 
fig. 17. The inner coat is usually entire to its termination in the 
stomach, while the outer which is continuous with the exterior 
membrane of the stomach is invariably torn off, not far from the 
base of the esophagus, as in the figure. 
The stomach has a broad cordate form, and is a little shorter 
than the esophagus, and when expanded is somewhat wider than 
long ; vertically it is quite narrow. The anterior extremity lies 
between the prehensile legs, and posteriorly it extends under the 
furcate process on the venter. The lateral margin is very deeply 
crenated., owing to the peculiar arrangement of its muscles. The 
teguments of the stomach are composed of the same coats as the 
esophagus, and they present the same general character. The 
inner appears uniformly smooth and even. The outer contains Desciiption of a Species of Caligus. 
31 
several muscular bands, which connect the opposite crenations ; in 
their contraction the crenations are rendered more prominent. 
These muscles are connected by other slender muscles irregularly 
arranged, which contract the stomach longitudinally. The lateral 
portions of the stomach are connected on each side with the shell 
adjoining, by ligamentous or cellular attachments, as is represented 
in fig. 9. There is no valve between the stomach and the intes- 
tine, and. when the peristaltic motion is reversed, as often happens, 
the fluids frequently return into the stomach. 
The intestine, at its commencement, is between three and four 
times the diameter of the esophagus, and about one fifth the di- 
ameter of the stomach. It is slightly enlarged below the second 
pair of natatories, where there are two pairs of glands, contracts 
again as it passes below the apron, and thence continues of uni- 
form size to the rectum. Its structure is very similar to that of 
the stomach, both in its inner and outer coat. The arrangement 
of its muscles in regular bands is represented in fig. 11; during 
their action the canal is crenated as in the figure. The intestine 
is attached by distinct ligaments at several places; near the 
glands, d, and the glands, e and f, we have distinctly seen these 
attachments. 
The rectum occupies the terminal half of the last abdominal 
segment, and is about one half the diameter of the intestine. Its 
communication with the intestine, is closed in the natural state of 
the parts. This rectum, if it may be so called, appears to have a 
longitudinal opening below, extending its whole length and its 
walls are usually in close contact. The external opening or anus 
is situated at its extremity. 
This portion of the alimentary canal is opened laterally by 
seven pairs of slender muscles. The first pair at the extremity 
pass directly outward along the margin of the joint; the second 
are inserted near the extremity, and pass upward and a little 
outward. The following three pairs, are attached near the mid- 
dle, and pass outward and a little upward; the remaining two 
pairs, are inserted near the opening to the intestine, and have the 
same direction as the last. The muscles have often been seen in 
action, in expelling the fasces; the two sides move either simulta- 
neously or alternately, according to the necessity of the case, in 
the act of expulsion. 32 
Description of a Species of Caligus. 
The intestinal fluids are usually light yellow; occasionally they 
present a deep wine yellow color, especially below the sternum of 
the second pair of* natatories. Solid vermiform masses of a brown 
color, are often seen floating in the fluids. 
Along the alimentary canal, there are several small glands, 
which have a granulous structure and are in general but slightly 
colored. Their particular functions, are mostly conjectural. 
The central projection between g, g, fig. 12, is the termination 
of a gland of considerable size, which is situated beneath the pos- 
terior extremity of the buccal mass, and is usually detached with 
it, on dissection. It is represented in fig. 9, a, where its size cor- 
responds to the mouth in fig. 9. When separated from the mouth, 
a duct may be seen on each side, entering the mouth near the 
esophagus. Anterior to the mouth, another collection of glands 
is observed, (fig. 9, b, see also fig. 1,) which also communicate 
with the mouth by ducts. These are probably salivary glands. 
The esophagus, especially near its base, is furnished with a 
large number of exceedingly minute, transparent globules, sup- 
ported on short pedicels, (fig. 10, PI. IV.) These appear to be 
glands, and their pedicels ducts. 
Below the stomach in the thorax, there are four pairs of glands. 
One pair of nearly spherical form, are situated at the lower extrem- 
ity of the stomach, (c, fig. 9.) The second pair, larger, of an 
oblong form, (d,) occur just below the sternum of the first pair of 
natatories, and are connected with the intestine by a duct under 
the following sternum. The third and fourth pairs, (e, f,) are sit- 
uated on the enlargement of the intestine, below the sternum of 
the second pair of natatories. The functions of a liver are prob- 
ably performed by some or all of these glands. 
Two other pairs of small glands are situated in the abdomen, 
which we presume to be connected with the intestine; we have 
not however distinguished their ducts, neither have we by dissec- 
tions obtained more than one of them separate from the body. 
They are possibly urinary glands. 
The Caligi have heretofore been supposed to live by sucking 
the blood of the fish on which they are found. It is however 
apparent from the structure of the mouth, that they are wholly 
unfitted for this mode of life. There is no organ which can per- 
form the functions of a sucker. Moreover, we have never detect- Description of a Species of Caligus. 
33 
ed any blood in the stomach of these animals, although we have 
often examined them, immediately on taking them from the fish. 
On the contrary, the fluids always have a light color. 
We have not fully satisfied ourselves of the nature of its food, 
but presume that it lives on the mucus which covers the body of 
the fish. The mucus is one of the natural secretions of the fish, 
and is always abundant. The organs of the mouth are well 
formed for the collection of it, and the free motion in the whole 
buccal mass seems peculiarly fitted for this purpose. 
Several specimens of the Caligus, when confined on their 
backs in but a small portion of water, just sufficient to cover them, 
have been observed to elevate the buccal mass, and take in glob- 
ules of air, which passed down the esophagus into the stomach, 
and thence through the intestine. Occasionally the globules of 
air have been so numerous and taken in such rapid succession, as 
to fill the stomach, and very much inflate it. In their passage 
through the esophagus they usually stop for a short time at the 
entrance to the stomach, indicating the existence of a valve or 
sphincter at this place. 
b. Circulation. 
The blood of the Caligus, as in other Articulata, is a limpid 
fluid, containing suspended in it numerous minute colorless par- 
ticles. These particles are very various in their form and size. 
The smallest scarcely equal T „Vo °f an inch. We have ob- 
served one particle the length of which was about of an inch, 
and its breadth J its length ; another had nearly the same length 
and a breadth equal to £ its length. These particles can accom- 
modate themselves to the size of the passage through which the 
blood is flowing, becoming narrow and elongated if the passage 
is narrow, and again resuming their former proportions when they 
have reached a free open space. 
The circulation in the Caligus is wholly lacunal; it appears to 
consist of broad irregular streams, passing through the spaces left 
among the internal organs, and in no part have we discovered dis- 
tinct vessels. These streams have in general definite directions, 
yet are seldom uniform, continuous currents. They mostly ad- 
vance by successive vibrations, depending on the palpitating action 
of the body. A single centre of circulation, or a heart, this ani- 
mal can scarcely be said to possess. There are two points in the 
medial line where there is a valvular action, and each has its 34 
Description of a Species of CaliguS. 
claims to be considered as performing the functions of this organ, 
though neither is entitled to that name. One of these systems of 
valves, the more perfect of the two, is situated in the apex of the 
posterior thoracic joint, (fig. 6 a, b.) There are at this place three 
distinct valves ; two laterally on the back, situated in the dorsal 
currents which are flowing towards the tail, and one centrally 
below, giving passage to the ventral current flowing from the 
tail. The dorsal and ventral valves open alternately. Their ac- 
tion may be seen in the figures above referred to ; a, represents 
the dorsal valves as shut, and the ventral open, and b, the dorsal 
relaxed or open, and the ventral shut. The action of these valves 
is very regular, and the currents which pass them are more uni- 
form than those in other parts of the body. The number of pal- 
pitations has been found to vary from thirty to forty per minute. 
The blood coming down the back* from the head, and also ill 
two lateral currents from the point of intersection of the head, tho- 
rax and epimeral segments, (fig. 7,) passes the dorsal valves. It 
continues posteriorly; a part into the terminal joint of the body, 
and then up the venter, entering the ventral current at the extrem- 
ity of the intestine; another part, into the same ventral current 
near the centre of the abdomen, and at other varying points. The 
ventral current passes through the ventral valve under the ante- 
rior margin of the apron, and continues up the body—washing, 
at the same time, freely over the intestine and stomach, to the 
thoracic ganglion, where it divides, and passes each side of this 
organ. Each of these branches goes off laterally; one portion 
(A) enters the adjoining prehensile legs, and returns down the 
body, uniting with another current which we shall soon mention; 
a second (B) passes a little forward and outward, gives off blood 
to the third pair of maxillipeds, continues outward, accompanies 
the muscles of the mandible, and runs down the body near its 
margin; a third (C) goes forward outside of the base of the first 
pair of maxillipeds, continues to the antennae, to which it gives a 
portion of its blood, turns inward passing into the anterior cephalic 
segment, and along its articulation to the medial line. At this 
place the currents meeting from the two sides, flow down the me- 
dial line to the mouth. 
* The course is marked by arrows on figs. 1 and 7. Description of a Species of Caligus. 
35 
The second instance of valvular action occurs in this last medial 
current, between the second joints of the first pair of maxillipeds, 
(fig. 1.) There is a single valve, composed of a membrane, 
playing backward and forward, and thus preventing the return of 
the blood that has passed it. Between this valve and the mouth 
there appears to be a large cavity for the reception of the blood, 
from which it is propelled by a palpitating motion or powerful 
muscular action in the buccal mass, and surrounding parts. It acts 
in the following manner: the current enters through the valve 
while the posterior part of the mouth is elevated; the valve then 
closes, and immediately the buccal mass is brought down, and 
forces it out in a current on each side. This very extraordinary 
action is carried on uniformly, and is absolutely necessary for the 
flowing of the blood. Indeed, the blood flows in by the out-cur- 
rents, until the action of the buccal mass throws it out. We pre- 
sume that the depression of this organ is produced by the muscu- 
lar band which has been described as passing across the posterior 
part of the mouth, to an attachment in the shell on each side, 
(fig. 12.) If the mouth be cut off, the blood flows out in a large 
free current, and the animal soon dies from exhaustion. 
A current passes from this cavity each side of the mouth, and 
others on the back. One portion of the side-current unites with the 
current C, before described, of which it forms the greater part, and 
thus soon returns to the buccal cavity. Another portion flows out- 
ward, following the muscle of the mandible, and unites with B ; 
this current, thus much enlarged, passes near the margin to the 
posterior extremity of the cephalo-thoracic segment, returns up by 
the epimeral articulation, crosses the same just above the junction 
of the head and thorax, and then turns suddenly backward; a part 
flows on the back, forming the lateral current on the back before 
referred to; the remaining portion below flows to the base of each 
of the natatory legs and the apron, and enters them, and at the 
same time and place, passes in part on the back; the current 
from the apron flows laterally down the abdomen. 
Another portion of the side-current leaves the buccal cavity just 
along side of the mouth, unites with it, and flows to the base of 
the first pair of natatories. The union of these currents is some- 
what peculiar : the blood vibrates upward on the venter, to a spot 
near the base of the prehensile legs, where a portion remains, al- 
though the main current vibrates back on the venter; at this mo- 36 
Description of a Species of Caligus. 
ment, the current comes from the buccal cavity and carries the 
whole below. 
The irregularity in the circulation in this animal is even greater 
than will be inferred from the above description. These currents 
are merely main directions; the blood flows into them or from 
them, through all their extent. The current coming laterally 
down to the base of the second pair of natatories, besides going 
into the natatory and on the back, is carried up the venter at each 
of the upward vibrations of the ventral current. The current 
from the apron also passes into the same current, in addition to its 
backward course. When it is considered that the currents of blood 
occupy merely the spaces left by the muscles and other internal or- 
gans, it will be readily seen that similar irregularities must occur in 
various parts of the body. These directions are occasionally sub- 
ject to singular deviations. One of the two currents which run 
from each side in front, and unite on the medial line, has been ob- 
served to cross the medial line into the other current, and thus 
continue flowing for some time with considerable force ; soon af- 
ter, each flowed by vibrations towards the centre, but with alter- 
nate motion. This was observed immediately on taking the Cali- 
gus from the water, when it was apparently very lively. As the 
cod, however, had been for several days confined in the harbor 
near the market, all the specimens examined may have lost part 
of the activity usual in the open sea. At times, the blood in some 
parts merely vibrates back and forward, without advancing in 
either direction ; and occasionally the blood flows in a direction 
exactly the contrary to its usual course. 
We have not fully satisfied ourselves of the mode of respira- 
tion in the Caligus. The natatory pinnulae—to which we must 
add those of the tail, as they are identical in their structure—have 
been supposed to supply the place of branchiae. When the ani- 
mal is attached to any object, these legs keep up a very regular 
action, which appears to correspond to the palpitations in the 
body.* We have not, however, observed the blood to flow into 
their setae, and the currents passing into the legs are among the 
least regular. We are disposed to believe that these pinnulae are 
not the special organs for this function, but that aeration takes 
* This action is not so rapid and branchial-like as in the Argulus, but takes 
place at intervals of about one and a half seconds. Description of a Species of Caligus. 
37 
place over the whole surface of the body. It is stated by Straus, 
that on separating the branchiae of a lobster, the body absorbed 
nearly one half the oxygen usual before the removal of these or- 
gans. The thin envelop of the Caligus, and the extent of its 
externa] surface, must render its body a far more perfect substitute 
for branchim than the solid covering of the lobster. The vibra- 
ting action of the natatory legs serves to keep up a constant cur- 
rent of water, and thus affords continually a new portion to 
undergo the respiratory action of the body. It might be re- 
marked that these legs, on account of their breadth, could not act 
so as to produce this current of water, when the whole margin 
around is attached. Probably the animal is not thus attached 
except when it is rendered necessary by the swift motion of the 
fish; under which circumstances, there is a sufficient current, 
without the action of these legs. We may presume that the spe- 
cial object of these marginal cups is to enable the animal to attach 
itself, and still keep the principal part of its body free, so that 
these natatory legs, when the fish is motionless, may have space 
to act, and sustain a continued current. 
Y. Organs of Reproduction. 
On each side of the stomach, there is a large pyriform organ, 
(PI. V, fig. 18,) of a glandular appearance internally, and provi- 
ded with a distinct duct, which at first we unhesitatingly pro- 
nounced the liver. Subsequent observation proved that the duct, 
which we had supposed to enter the intestine, extends through 
the whole length of the thorax into the abdomen, where it is con- 
tinuous, in the male, with organs known to be seminal, and in 
the female, with the egg-bearing vessels. These organs, thus 
shown to be connected with the organs of generation, have been 
since proved to correspond with the spermatic glands in the male 
and the ovaries in the female. 
In the male, they are rather larger than the buccal mass, (PI. 
Y, fig. 21,) and are situated just anterior to the stomach, in part 
beneath the base of the prehensile legs and the spine of the pre- 
ceding pair. Their small posterior extremity is produced into a 
short ligament, by which it adheres above the stomach; the an- 
terior portions are so enveloped in their cellular or membranous 
attachments, that they are separated with great difficulty. In 
general appearance, it resembles a pyriform membranous sac, 38 
Description of a Species of Caligus. 
with an internal granulose structure. The duct, which is at- 
tached on the outer margin, is a slender vessel, of a thin membra- 
nous nature. It continues of a uniform size through the thorax 
to the central parts of the abdomen, where it gradually enlarges 
and undergoes a few convolutions. 
A short distance below the convoluted portion, there is a small 
oval gland, with well-defined limits, contained within a distinct 
sac. It is apparently composed of several concentric parts, of 
which three are very apparent; there are two less distinct. Its 
interior is a transparent globule j the outer coats are less trans- 
parent, and the one adjacent to the interior, the least so. The 
central part of this gland is connected with a small sub-corneous 
tube, which gradually enlarges, and passes into the anterior ex- 
tremity of the above convolutions. On one occasion, when we 
had separated this gland and its duct from the abdomen, a fluid, 
containing particles similar in appearance to those in the blood, 
rapidly poured out. The convoluted vessel appears therefore to 
receive the secretions of two seminal glands, and probably corres- 
ponds to the vas deferens. Though much time has been em- 
ployed in searching for the exit of the vas deferens, we are yet 
uncertain on this point. It is presumed, from the appearance of 
the parts, that it terminates either on the outer surface of the lap- 
pet at the extremity of the abdomen, or beneath this organ. 
The ovaries in the female have the same situation and attach- 
ments as the spermatic gland in the male. (PL Y, fig. 18.) They 
are however much larger, and extend above the stomach nearly to 
its centre. They may be distinctly seen through the back shell. 
They appear to contain a long convoluted vessel, which gradually 
diminishes in size, from its anterior to its posterior extremity; but 
whether this be truly its nature, cannot be determined. The duct 
arising from its margin, extends without any variation in its size, 
till it reaches the posterior joint of the thorax, where it enlarges 
gradually, and continues to increase as it enters the abdomen. In 
the gravid female, it passes through the abdomen, with a few 
convolutions, and extends out at the vulva, in the form of a long, 
whitish, nearly cylindrical membranous tube. This external por- 
tion of the oviduct is often a little longer than the animal. 
The vessel in the ovary does not appear to contain divisions in- 
dicating the presence of eggs; but the oviduct usually contains 
eggs through its whole extent. Where exserted, it is very dis- Description of a Species of Caligns. 
39 
tinctly divided by membranous partitions into narrow compart- 
ments, each containing an egg, though not quite filled with it. 
The eggs in the anterior slender portion of the oviduct are oblong 
and uniformly transparent. As they increase in size, they present 
a clouded appearance, and become divided into two parts, corres- 
ponding to the white and the yolk. The latter appears clouded 
and composed of albuminous globules. The several portions are 
represented in the exserted portion of the oviduct, (fig. 18.) The 
eggs have the form of short cylinders with rounded edges. 
In the advanced eggs, at the extremity of the ovary, we ob- 
served in one instance, that there were two distinct eyes at their 
outer extremity; they were approximate, but not situated on the 
same black ground. In these eggs, the yolk occupied nearly the 
whole space. 
In addition to the ovaries above described, there is a pair of or- 
gans in the abdomen, connected with the system of generation. 
They are straight, flat-cylindrical organs, usually as broad as the 
external oviduct, and lie along the central portions of the abdo- 
men. At the lower extremity, they are connected with the ovi- 
duct a short distance above the vulva, and at the upper, they termi- 
nate in a cul-de-sac. They contain a single series of transparent, 
flattened globules, (fig. 18,) occupying, like beads, their central 
line, and in width about one half the width of the ovary. These 
false ovaries, when torn or cut, do not emit an albuminous fluid, 
like the true oviducts, but appear to have a gelatinous consist- 
ence. They are as much developed in the young, as in the old 
females. 
The eggs in females of the same size present very different de- 
grees of development. We have seen full grown individuals with 
no eggs in the abdomen, and consequently, instead of the swollen 
appearance usual in the adult female, their abdomens could 
scarcely be distinguished from those of the male sex. Occasion- 
ally, very young individuals have had external ovaries; the small- 
est observed was scarcely one sixth of an inch long. May we 
not infer from this, that a single coition is sufficient to impregnate 
the individuals of at least one succeeding generation ? 
A few instances have come under our notice, of a very extra- 
ordinary irregularity in these organs. The extremity of the 
false ovary has been seen hanging externally in the place of the 
regular external ovaries, and no eggs, nor the internal oviduct, 40 
Description of a Species of Caligus. 
were discoverable in the interior on that side. Moreover, the 
corresponding ovary near the stomach was discovered with diffi- 
culty, and appeared like a folded empty sac. At the same time 
the ovary and the ovarian tube on the other side presented their 
usual appearance. This singular derangement was observed in 
a full grown female, which was perfect in all its other organs. 
An additional peculiarity as yet inexplicable, has been observed 
in some females. The lappets at the extremity of the abdomen, 
each side of the tail, have been already described as very short in 
the female. On their lower surface there is an irregular osseous 
process, from which a slender corneous organ, which we suppose 
to be a duct, runs forward and a little inward, gradually diminish- 
ing, and terminates with a few irregular curves, (fig. 18, PI. V.) 
The peculiarity we refer to, is an appendage to this lappet, arising 
from the termination of the internal duct, (fig. 22.) It is a long 
corneous duct, wholly external, terminating in an oval sac of similar 
texture, and usually filled with a whitish fluid. These appendages 
have been observed in a few instances, hanging each side of the 
terminal joint of the body, (fig. 22.) In one instance the ducts 
were crossed over the adjacent articulation, and each attached by 
its sac to the lappet of the opposite extremity. These are the only 
facts that have been discovered respecting these singular organs. 
They were found attached to very few individuals, and in these 
the eggs were scarcely developed. 
On account of the many similarities between this animal and 
the Argulus, it may be interesting to trace a few of its ana- 
logies. 
The number of legs or organs for locomotion is the same, be- 
ing eight in each. Of the four pairs of natatories in the Argulus, 
two are similar in their use in the Caligus, while a third is expand- 
ed into an apron, and a fourth is attached to a distinct joint, and 
has but little strength. The anterior pair of maxillipeds in the 
Argulus very much resemble in general form the same organs in 
the female Caligus. The fourth pair is large and prehensile in 
each, though very different in form. There is a distinct suture 
in the former, near the anterior margin of the animal, which cor- 
responds to the articulation between the two cephalic segments 
in the latter; but this segment, which in the Caligus is furnished 
with antennae, is wholly without even rudiments of these organs; 
we may hence infer that the Argulus is destitute of antennae, as is Description of a Species of Caligus. 
41 
also evident from the nature of the organs that follow. The space 
contained within the U suture in the Argulus is the analogue of 
the much larger and more distinctly separated segment, which we 
have called the cephalic in the above description. The anterior 
abdominal joint of the Caligus is wholly wanting in the Argulus; 
and the valves in the circulation which occupy the posterior tho- 
racic joint, far from the extremity of the body, have an analo- 
gous situation in the Argulus, close to the last joint of the body. 
This joint being small in the Argulus forms a very distinct and reg- 
ular heart, and serves to keep up a much more active circulation 
than in the Caligus, where the corresponding part is large and 
less energetic in its action. It is remarkable that the circulation 
in the two should be the reverse in almost every particular; the 
ventral current instead of being upward in the Argulus runs to- 
wards the terminal joint of the body; instead of meeting from 
the two sides in front and returning down the medial line, it goes 
out in two currents near the medial line and returns in the wings 
of the shell. This however will not appear so extraordinary 
when we consider that the animals are the reverse of one another 
in some particulars. The cephalic segment in the Caligus is very 
large and broad, and there is therefore space for the current fur- 
nished to the antennae and cephalic organs, to flow along the sides, 
and return along its centre; but in the Argulus, this portion is so 
small that there is only room for the out-current, and the blood is 
compelled to turn outward into the wings of the shell or thoracic 
portion, which is very much larger than in the Caligus. The cur- 
rents are much more definite in their limits in the Argulus, and 
more uniform in their velocity and course j the particles of the 
blood are also less variable in size and form, being about ggos °f 
an inch, in length. The organs of the mouth are also similar in 
position and in the form of the mandibles. This analogy might 
be traced much farther; but we reserve further remarks for a fu- 
ture occasion. 
EXPLANATION OF THE PLATES. 
Fig. 1. Under view of a male, exhibiting the various organs, and the muscles 
that move them. A, minute papillae, supposed to correspond to inner antennae; I, 
a cup, for the attachment of the animal; L, antennae ; d, one of the muscles mov- 
ing the mandibles. The arrows point out the course of the blood. 
Fig. 2. Back view of male, natural size of one of the largest individuals. 
Plate III. 42 
Description of a Species of Caligus. 
Fig. 3. A female, natural size. 
Fig. 4. a. View of the rudimentary or second pair of legs, or maxillipeds; b, 
termination of third pair. 
Fig. 5. a. Ventral muscle, exhibiting its subdivision between the sternums of 
the two pairs of natatory legs; b, a second subdivision in the same muscle, below 
the posterior of the above sternums. 
Fig. 6. a and b. View of the posterior thoracic joint, (see fig. 7,) with the valves 
in the circulation; the two lateral valves on the back and the central on the venter. 
Plate IV. 
Fig. 7. Back view of a male, with the muscles seen in this view. Those mark- 
ed with capital letters, move the segments of the body; those with small letters, 
move the several organs below. F, the posterior thoracic segment; G, the anterior 
abdominal; H, the posterior abdominal. 
Fig. 8. A portion of the shell about the eyes, shewing the areolets exhibited by 
it; the dotted line marks the limits of the dark ground on which the eyes are sit- 
uated, and the dotted circles the eyes themselves. 
Fig. 9. Alimentary canal, exhibiting the esophagus, the stomach and the intes- 
tine, with its glands, and the muscles of the rectum. The mouth at the upper ex- 
tremity is represented as turned back, so as to show its under surface. 
Fig. 10. Anterior extremity of the esophagus. 
Fig. 11. A portion of the intestine. 
Fig. 12. View of the buccal mass, in its natural position. Between the line a a, 
and a b a, is the opening to the mouth, d, the outer extremity of the mandibles, 
with the tendon and its muscles attached. 
Fig. 13. The same with a portion of the upper membrane and the mandibles re- 
moved. 
Fig. 14. The mandibles, together with some of the organs adjacent to their in- 
ner extremities, showing their relative position. 
Fig. 15. The upper lips, with its two pairs of muscles. 
Fig. 16. The same, with the extremity retracted by the inner pair of muscles. 
Fig. 17. Under view of the buccal mass, with the esophagus attached, h h are 
processes lying in the teguments of the body, with which the buccal mass forms an 
articulation at their anterior extremity, g the processes in which the elevators of 
the buccal mass are inserted. 
Plate V. 
Fig. 18. Under view of female, exhibiting the nervous system, and the ovaries, 
and ovarian tubes. Fig. 18, a, the first pair of maxillipeds in the female; 18, b, the 
fourth pair in the same. 
Fig. 19. A view of the cup and an antenna, together with a portion of the lat- 
eral margin of the animal, exhibiting its spines. The dotted lines in the antenna 
represent the nerve with which this organ is largely supplied. 
Fig. 20. The cephalic and thoracic ganglions, exhibiting their close union, and 
the nerves they give out; the outlined organ in front, represents the eyes, attached 
to the ophthalmic nerves. 
Fig. 21. Genital system in the male. 
Fig. 22. Part of the abdomen of a female with an appendage to the same. 
Figs. 23,24,25. Illustrate somo facts connected with the change of skin. fl. ill. 
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