Philip Henry Gosse.

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the Lobster, the internal are two-bristled, both bristles
rather short, while the external are very long. In the
Flat-crabs each pair is simple, the inner minute, the outer
long. In the great Eatable Crab each pair is very small,
and they are dissimilar.


Now taking the last-named animal as the representative
of his class, let us examine one of his inner antennae first.
It consists of a jointed stem and a terminating bristle; the
latter furnished with small hairs common to the general
surface of the body, and with long, delicate, membranous
filaments (setce), often improperly called cilia, which are
larger, and much more delicate in structure than the ordi-
nary hairs.

The basal joint is greatly enlarged: if it be carefully
removed from its connection with the head, and broken
open, it will be found to enclose in its cavity a still smaller
chamber, with calcareous walls of a much more delicate
character than the outer walls. This internal shell is con-
sidered by Mr. Spence Bate to be a cochlea, from its
analogy, both in structure and supposed use, to the organ
so named in the internal ear of man
and other vertebrate animals. It is
situated, as has been said, in the
cavity of the basal joint of the in-
ternal antenna, and is attached to
the interior surface of its wall fur-
KAR OF CRAB, FROM BEHIND, fl^st f rom the median line of the
Crab. It has a tendency to a spiral form, but does not
pass beyond the limits of a single convolution.

If this interior cell does indeed represent the cochlea
of more highly-constructed ears to which it bears some
resemblance, both in form and structure then it seems to
identify , beyond dispute, these inner or upper antennae as
the organs of hearing.

Now with this conclusion agrees well the manner in
which the living animal makes use of the organs in ques-
tion. The Crab always carries them erect and elevated;


and is incessantly striking the water with them, with a
very peculiar jerking action, now and then vibrating, and,
as it has been called, "twiddling" them. These antennae,
therefore, appear to be always on the watch: let the ani-
mal be at rest, let it be feeding, no matter, the superior
antennae are ever elevated and on constant guard.

The lengthened and delicate setae with which they are
furnished are, moreover, peculiarly adapted to receive and
convey the most minute vibratory sensations from the me-
dium in which they are suspended; and, on the whole, it
seems to be satisfactorily settled by Mr. Spence Bate (to
whose excellent memoir * I am indebted for these explana-
tory details) that the inner antennae are real ears.

Having thus taken our Crab by the ears, we will en-
deavor next to tweak his nose. But stay, we must find it
first. We turn our horny gentleman up, and in his flat
ancient face we certainly discern little sign of a nasal or-
gan. Our friend Mr. Bate must assist us again. He will
tell us to look at the outer or lower antennae. We will
look accordingly, magnifier in hand, while he makes it
clear to us that these are a pair of noses.

Each of these organs is formed of a stem consisting in
general of five joints, and a filament of many minute
joints. In the Prawn and the Lobster all the five joints
of the stem are distinct; but in the Crab the whole are,
as it were, soldered together into a compact mass, with
difficulty distinguishable into their constituent articula-
tions; while in some species their position can be indi-
cated only by the presence of the olfactory operculum.

This important little organ varies in its construction in

1 "Annals of Nat. Hist," for July, 1855.


the different families of Crustacea. In the Crab it is a
small movable appendage, situated at the point of junc-
tion between the second and third joints ; it is attached to
a long calcareous lever-like tendon, at the extreme limit
of which is placed a set of muscles, by which it is opened
and closed; to assist in which operation, at the angle of
the operculum most distant from the central line of the
animal are fixed two small hinges. When the operculum
is raised, the internal surface is found to be perforated by
a circular opening protected by a thin membrane.

In the Prawn, Shrimp, and Lobster, there is no oper-
culum, but only the orifice covered by a membrane, which
is placed at the extremity of a small protuberance, and
it is not capable of being withdrawn into the cavity of
the antenna, as in the Crab.

In the latter animal, the little door, when it is raised,
exposes the orifice in a direction pointing to the mouth;
and where there is no door, still the direction of the open-
ing is the same, inward and forward, answering to the po-
sition of the nostrils in the higher animals. In each case
it is so situated that it is impossible for any food to be
conveyed into the mouth without passing under this organ;
and there most conveniently the animal is enabled to judge
of the suitability of any substance for food, by raising the
little door, and applying to the matter to be tested the sen-
sitive membrane of the internal orifice.

Thus it is concluded that this lower or outer pair of
antennae are the proper organs of smell, as the upper and
inner are of hearing. 1

The eyes, though constructed on the same general prin-

1 Op. cit.


ciples as those of Insects, yet present some particulars
worthy of your notice. In the Crabs and Lobsters they
consist of numerous facets, behind each of which is a con-
ical or prismatic lens, the round extremity of which is
fitted into a transparent conical pit, corresponding to a
vitreous body, while the conical extremity of these lenses
is received into a kind of cup, formed by the filaments of
the optic nerve. Each of these filaments, together with its
cup, is surrounded by pigment matter in a sheath -like man-
ner. To see this structure would require anatomical skill;
but you may here examine with a low power portions of
the cornea, or glassy exterior, of the eye of a Crab and of
a Lobster. In the former, you see that the facets into
which the cornea is divided are hexagonal, like those of
most Insects, but in the latter they are square.

But Crustacea have a far greater faculty of circumspec-
tion than insects have; for besides the extensive convex-
ity and numerous facets of their eyes, these organs are
placed at the extremity of shelly foot-stalks, which are
themselves movable on hinges, capable of being projected
at pleasure, and of being moved in different directions,
and of being packed snugly away, when not in active use,
in certain grooves hollowed out expressly for them in the
front margin of the shell.

If ever you should chance to meet with the exotic
Crustacea of the genera Corycczus and Sapphirina, you
would see a form of eye of a quite remarkable and unique
character. It is described by Dana in the following terms:

"A pair of simple eyes consisting of an internal pro-
late lens, situated at the extremity of a vermiform mass of
pigment, and of a large, oblate lens-shaped cornea. The
cornea is connected intimately with the exterior shell of


the front or the tinder side of the head, and the two
cornese are like spectacles adapted to the near-sighted
lenses within; their size is extraordinary, being often
one-third of the greatest breadth of the body in Corycceus.
The lens and the cornea are often very distant from each
other, being separated by a long clear space. The ex-
ternal surface of the cornea is spherical; but the inner is
conoideo -spherical, or parabolic* The texture is firm, and
when dissected it breaks or cuts like a crystalline lens.
The true lens is always prolate, with a regular contour,
excepting behind, where it is partly penetrated by the
pigment. The pigment is slender, vermiform, of a deep
color, either red or blue, but at its anterior extremity
usually lighter, and often orange or yellow."

We might find much more both instructive and amus-
ing in examining microscopically the structure of the higher
Crustacea; but we will now dismiss them in order to dis-
cuss some of the lower forms, many of which are so mi-
nute that their whole bodies may be watched with ease
performing all the functions of life, while confined under
our eye, on the stage of the microscope. I refer to the
tiny active little creatures known as Water-fleas, which are
abundant in both fresh and salt water.

In this jar of fresh water, which has been standing in
the window for weeks, you may see among the green fila-
ments of Chara many little atoms which scuttle hither and
thither with a rapid succession of short leaps. These be-
long to the genus Cyclops, and are Crustacea, belonging
to the order ENTOMOSTRACA.

By the aid of a glass tube which I stop at one end with

1 "Rep. on Crust," p. 1026.


my finger, I will endeavor to catch one. It is no easy
matter, as you see, for the instant the end of the tube is
brought near to one, he takes the alarm and leaps nimbly
away before I can make the water rush in by withdrawing
my finger from the other end. But I have one at length.

Here it is a minim of life not more than a sixteenth
of an inch in length, looking something like a pellucid
egg, furnished with long antennae, with five pairs of
branching feet, and a long tail terminating in bristles.
But its parts and organs must not be dismissed in this
summary way; we must look at them in detail.

And first of all, in the very midst of his forehead, like
that obscene giant 1 after whom our tiny atom is named,
he bears a single eye that glares like a ruby. It would
need no vast beam of olive-wood sharpened and heated in
the fire, and "twirled about' 7 by the united strength of
five heroes, to "grind the pupil out"; for though brilliant
and mobile, it is far too minute to be touched by the tip
of the finest needle. Yet it is elaborately constructed;
for it consists of a number (not very large) of simple eyes
placed beneath a common glassy cornea. Several muscle-
bands are attached to this compound organ of vision, and
are arranged so as to form a cone, of which the eye is the
base ; these give the eye a movement of rotation upon its
centre which may be distinctly seen.

All the limbs, including both pairs of antennae, two
pairs of foot-jaws, five pairs of feet, and a pair of tail-
lobes, are furnished at each of their many joints with tufts
of long hairs; these appear to act the part of paddles, as
the active little animal strikes the water vigorously with

i <<



all its limbs, for the purpose of progression, and also for
the creation of currents in the fluid, which currents sub-
serve a double object the bringing constant supplies of
water to be respired, and floating atoms of food to be

In this individual, which is a female, the antennae are
nearly equal in size throughout their length; but in the
male, the middle joints of the upper pair are remarkably
enlarged, forming a large swelling, followed by a sudden
contraction, the first part of which is hinged. All of the
true feet, and the second pair of foot-jaws, are divided to
the base into two equal branches, so that the animal seems
to possess no fewer than twenty-six limbs, each of which
being many-jointed, and each joint, as I have observed,
being set with delicately plumose hairs, the whole effect
is most elegantly light and feathery.

On each side of the slender tail (more correctly, the
abdomen) you see an oval bag connected with the body
by an excessively slender thread of communication, and
filled tensely with pellucid globose bodies. Like John
Gilpin, of equestrian fame, when

"He hung a bottle on each side
To keep his balance true,"

our little natatory harlequin "carries weight." But these
bags are filled with eggs, a temporary provision for their
due and proper exposure to the water, while yet they are
protected from enemies. They are developed only at cer-
tain seasons, when the eggs, having attained a given amount
of maturity in the ovary, are transferred through the ex-
ceedingly slender tube into these sacs, and are there car-
ried about by the mother until the young are hatched,


when the curious receptacles, being no longer needed, are
thrown off, and speedily decay.

Here is a second form. It is named Lynceus, and is
nearly as common as the Cyclops in our stagnant pools.
Essentially its structure is the same, but it has this pecu-
liarity, that its body is enclosed within a transparent shell,
which is thin and flattened sidewise, and through whose
walls all the movements and functions of its parts are dis-
tinctly visible. The shell is broadly ovate in outline,
comes to a sharp edge above, but is open all along the
lower half of its circumference as if two watch-glasses
had been soldered together, edge to edge, and then a por-
tion of the semicircumference had been ground away, so
as to leave a thin but long entrance. Through this narrow
orifice the limbs are protruded for locomotion, and through
it the surrounding water finds its way in currents, bringing
oxygen to be respired and food to be devoured.

The translucent shell descends in front into a sharp
long beak, below which are seen the organs of the mouth,
.two pairs of foot- jaws, beset with fine bristles. At the
origin of the beak is the eye, consisting, as we saw in the
Cyclops, of several lenses, enveloped in a common cornea,
the whole forming a movable organ of a blue-black hue.
Just behind this, at the very highest part of the shell, is
a little colorless bladder-like vesicle, which constantly
maintains a rapidly alternate contraction and dilatation.
This is the heart, and this motion circulates the blood.

Below this, there is seen a great translucent irregular
mass of flesh, evidently comprising many viscera, which
winds along from one end of the shell to the other, nearly
occupying its entire area, but not in connection with it at
the hinder part, as we see by its free movements there,


where it curves round, and bending beneath terminates in
a blunt tail, armed with two strong hooks, which can at
pleasure be thrust down through the narrow orifice of the
shell, and become partially straightened by being forcibly
thrown backward. This great central mass is mainly occu-
pied by the alimentary canal, in which food in various stages
of. assimilation may at all times be seen, and in which the
interesting function of digestion can be witnessed through-
out, from the first seizure of the atom and its mastication
by the jaws to the discharge of the effete remains.

The individual before us does not carry at this time

eggs in the process of development; but the deficiency is
supplied by a Daphnia which is playing about in the same
drop of water. Here you perceive, between the arched
outline of the shell and the sinuous outline of the free
soft body, an open space of some size, which constitutes
a receptacle, in which the eggs are deposited as they are
laid, and in which they remain not only until the little
animals are hatched, but until they have acquired a suffi-
cient maturity to swim about and get their independent

This receptacle in which you may see five or six eggs


is freely open to the surrounding water, which enters
the slit edge of the shell, behind the tail. Perhaps you
wonder why the eggs are not washed out by the respiratory
currents; they are in fact maintained in their position only
by a slender tongue-like projection from the back of the
parent, which appears to have that special object. When,
however, the young are ready for freedom, the mother
has but to depress her body a little more than ordinary,
when the door is opened, and the young easily slip from
the receptacle into the open water.

These tiny odd -looking sprawling things that you see
moving about by quick jerks in the same drop of water
are the young recently hatched. They are quite unlike
their parent, having as yet no bivalve shell, no abdomen,
and only three pairs of limbs. The body is a transparent
plate, resembling the bowl of a spoon in form, but ending
in two points which carry pencils of bristles. The large
dark eye is conspicuous in front, and the six jointed and
bristled limbs radiate from the centre, projecting stiffly
on all sides. The second and third pair are seen to be
double, each giving off a branch, which is pencilled with
bristles like the principal stem.

We have not yet done with these tiny Water-fleas.
The sediment at the bottom of this jar of water is quite
alive with a host of nimble atoms, some of which you
may see crawling up the sides of the glass. They are
quite distinct from either of the kinds we nave been ex-
amining, not only in details of structure, which is more
identical indeed than it seems at first sight, but in habit;
for whereas they shoot to and fro through the water with
great force and rapidity, these can scarcely swim at all;
or, if they do, it is with comparative slowness and much


apparent effort; though over the smooth side of their glass
dwelling, or upon the stems of water-plants, they glide
along with much ease and elegance, by the quick vibra-
tions of their pencilled feet.

The form we are now contemplating is distinguished
by the name of Cypris, a genus which contains a good
many British species. It is more completely enclosed in
a shell than even the Lynceus, and its envelope more truly
resembles the shell of a bivalve Mollusk, for the valves
are open for more than three -fourths of their circumfer-

ence; while the portion of the back that is united is
sufficiently elastic to allow of some degree of expansion,
thus answering the purpose of a hinge.

Now look at the elegant little creature. Its most promi-
nent feature is its two pairs of antennae, one projecting
forward and curved upward, the other downward. Both
consist of several transparent joints, and are tipped with
long clear bristles; but the pencils which tip the upper
pair are specially graceful, being as long as the whole
shell, exceedingly slender, beautifully curved, and so
transparent that they seem formed of spun glass.

Another peculiarity is that there seems to be but one


pair of legs, which terminate each in a hooked spine.
You now and then see these awkwardly thrust out from
beneath the hinder part of the shell, but locomotion is
principally effected by the pencilled antennae. There is,
however, a second pair of legs, but these do not usually
make their appearance outside the shell, being curved
backward to sustain the ovaries.

About thirty years ago an Irish naturalist, Dr. J.
Yaughan Thompson, announced a discovery, which, over-
setting conclusions previously received by all, caused no
little dissent and opposition, and gave rise to a lengthened
and widespread controversy. A very minute crustaceous
animal was known, as inhabiting the open sea, to which
the name of Zoea had been given. It had sessile eyes, and
was remarkable for having a long spine projecting from
the face, and a similar one standing" up from the centre
of the back. Another form was known, which constituted
the genus Megalopa : in which the body was broad, the
eyes stalked, and the abdomen projecting behind. This
was also small, but somewhat larger than the preceding.

Nobody suspected that these were other than independ-
ent forms of animal life, distinct from each other, and
equally distinct from every known genus of Crustacea be-
sides. It was supposed that no animal of this class under-
went metamorphosis or that change of form in different
periods of life which distinguishes Insects; but that these
creatures retained through life the general shape, slightly
modified by development of parts and organs, which they
each displayed when hatched from the egg.

But these conclusions were quite set aside by the bril-
liant discovery of Thompson, that Zoea and Megalopa were
the same animal in different stages of existence; and that,


moreover, both were but the early states of well-known
and familiar forms of larger Crustacea, which therefore un-
dergo a metamorphosis as complete as that by which the
caterpillar changes to a chrysalis, and the chrysalis to a
butterfly, and in every essential point parallel to it.

In the Cove of Cork this naturalist met with a consid-
erable number of Zoeas, which he kept in captivity. Some
of these passed into the Megalopa form, which in turn
changed to the most abundant of our larger Crustacea, the
common Shore-crab (Carcinus mcenas). "Thus, in its prog-
ress from the egg to its final development, the Crab was
proved to pass through two temporary conditions, which
had previously been regarded as types, not of genera only,
but of different families; and both strikingly dissimilar
from the group to which, in its perfect state, it belongs."

I have not myself examined the transformations of this
species; but, as they have been well worked out, and as
the animal is so abundant everywhere on the coast that
you may easily verify what has been observed, I will cite
you the elaborate account of Mr. E-. Q. Couch of Pen-
zance, who has investigated the subject with great skill,
zeal, and success.

Having procured some specimens of the Shore-crab
laden with eggs, just ready for shedding, he goes on to
say "these were transferred to captivity, placed in sepa-
rate basins, and supplied with sea- water; and in about
sixteen hours I had the gratification of finding large num-
bers of the creatures alluded to above swimming about
with all the activity of young life. There could be but
little doubt that these creatures were the young of the cap-
tive Crabs. In order, however, to secure accuracy of re-
sult, one of the Crabs was removed to another vessel, and



supplied with filtered water, that all insects might be re-
moved; but in about an hour the same creatures were ob-
served swimming about as before. To render the matter,
if possible, still more certain, some of the ova were opened,
and the embryos extracted ; but shortly afterward I had the
pleasure of witnessing, beneath the microscope, the natural
bursting and escape of one precisely similar in form to
those found so abundantly in the water. Thus, then,
there is no doubt that these grotesque-looking creatures
are the young of the Garcinus
mcenas; but how different they
are from the adult need hard-
ly be pointed out any further
than by referring to the figure.
When they first escape they
rarely exceed half a line in
length. The body is ovoid,
the dorsal shield large and in-
flated; on its upper edge and
about the middle is a long
spine, curved posteriorly, and
rather longer than the diam-
eter of the body, though it
varies in length in different specimens; it is hollow, and
the blood may be seen circulating through it. The upper
portion of the body is sap-green, and the lower semi-
transparent. The eyes are large, sessile, and situated in
front, and the circumference of the pupil is marked with
radiating lines. The lower margin of the shield is waved,
and at its posterior and lateral margin is a pair of natatory
feet. The tail is extended, longer than the diameter of the
shield; and is composed of five equal annulations, besides



the terminal one; its extremity is forked, and the external
angles are long, slender, pointed, and attached to the last
annulation by joints. Between the external angles, and on
each side of the median line, are three lesser spines, also
attached to the last ring by joints. Between the eyes, and
from near the edge of the shield, hangs a long, stout, and
somewhat compressed appendage, which, as the animal
moves, is reflexed posteriorly between the claws. Under
each eye is another appendage, shorter, and slightly more
compressed. The claws are in three pairs; each is com-
posed of three joints, and terminates in four long, slender,
hair-like appendages. These claws are generally bent on
the body, but stand in relief from it. If the animal be
viewed in front, the lower margin of the dorsal shield will
be found to be waved into three semicircular festoons, the
two external of which are occupied by the eyes, and be-
tween which the middle one intervenes ; the general direc-
tion of the claws will be seen to be at right angles to the

Online LibraryPhilip Henry GosseEvenings at the microscope (Volume 1) → online text (page 14 of 32)