THE CRUSTACEA.
THE Crustacea are free, locomotive, articulated animals, covered with a
crust or external skeleton, and distinguished by having jointed limbs,
and gills that fit them for aquatic respiration. They are male and
female, and, though extremely diversified, they have a similarity in
their general structure. Many are microscopic.
The Crustacea constitute ten orders, many genera, and innumerable
species. The Decapods, or the ten-footed order, are by far the most
complicated in organization. They have prominent eyes, movable on
jointed stalks, antennæ, gills in a cavity on each side of the throat, a
mouth opening into a digesting apparatus, a heart, liver, circulation of
the blood, and a nervous system, and are therefore animals of a higher
grade than any that have come under consideration.
The Decapods are divided into three tribes:—the Macrura, or long-tailed
Crustacea, of which the Lobster and Astacus fluviatilis, or fresh-water
Crawfish, are types; the Anomura, or tailless tribe, of which the Hermit
crab is the type; and the Brachyura, or short-tailed crustaceans, which
are represented by the common Crab. The greater number of these animals
are marine; some inhabit fresh water; and some are amphibious, living in
holes in the ground; others climb reeds and bushes with their long
claw-feet; the last two kinds come to water to spawn.
_Macrura._
The body of the Macrura, or long-tailed crustaceans, consists of a
number of segments or rings joined end to end, having jointed members on
each side. Every individual joint is covered with a hard crust to afford
support to the muscles. A certain number of the rings, which form the
tail, are always distinct, similar, and movable on one another, whilst
the remainder, which form the carapace or shell, are confluent so as
entirely to obliterate the divisions. But generally the arrangement of
these twenty-one rings is such that seven of them are confluent and form
the head, seven confluent rings form the thorax or throat, and the seven
non-confluent rings form the tail. In the Decapods the three last head
rings greatly expanded are cemented to those of the thorax, so as to
form the carapace or shell, which covers all the body of the animal
except the tail. This structure may be traced on the under-surface of
the crab.
A ring consists of an upper and an under arch, with a space between
them, so as to let the feet and other appendages pass through. In the
long-tailed tribe the tail is bent and unbent by muscles attached to the
under and upper surfaces of each ring, which give the tail a powerful
motive force, for, by bending it suddenly under the body, and then as
suddenly stretching it out, the animal darts backwards through the
water.
The Decapods have five pairs of walking feet; the front pair are claws
employed to seize their prey, and occasionally for walking; the other
four pairs are cylindrical, and end in sharp hooked points.
_Brachyura._
The Brachyura surpass all the other Decapods in compactness and
concentration, and are without exception the highest of the Crustacea.
Though apparently without a tail, they really have one, as their name
implies; but it is short, rudimentary, and folded under the posterior
end of the carapace. The genera and species are exceedingly numerous,
many swim and inhabit the deep oceans, others live on the coasts but
never leave the water; a numerous tribe live as much in the air as in
the water, hiding themselves under stones and sea-weeds on the rocky
coasts, while some dig holes for themselves in the sand, and the land
crabs only come to the sea or to fresh-water lakes to spawn. The
Brachyura have two claws, and are divided into the two chief families of
walking and swimming crabs, according as their posterior pairs of legs
end in a sharp horny nail, or a ciliated lamellar joint.
The great shell or carapace which covers the body varies in form with
the genera; it may be square, oval, or circular, longer than it is
broad, or broader than it is long; it may be straight or beaked between
the eyes; but its lateral edges always extend over the haunches of the
feet. In the Cancri, or walking crabs, of which there are eighteen
genera and many species, the carapace is generally much broader than it
is long, and broader before than behind.
The carapace, or shell, of the common crab is too well known to require
a particular description. The deep lines which indent it correspond with
the limits of the internal organs; the parts between the lines often
bulge very much above the parts occupied by the stomach, heart, gill
chamber, &c., but in the flat crabs these divisions are not so evident.
The compound eyes, which in all the crabs have hexagonal facettes, are
on short jointed stems placed in deep and nearly circular orbits like
cups, so that the stems are scarcely visible. These orbits, whose edges
are sometimes smooth and sometimes notched, are so constructed that the
crab can bend the eye-stems horizontally to the right and left, and the
front of the carapace either conceals the orbit, or forms the eyebrow.
In all crabs the antennæ appear in front between the eyes. The first or
interior pair are short, jointed, and capable of being bent into
cavities, which contain their basal joints; these cavities are near the
eye orbits, with which they are connected in certain species.
Well-developed ears are placed in their basal joints. Fig. 146
represents a magnified ear seen from behind, and Mr. Gosse mentions that
the large eatable crab, whether at rest or feeding, carries these
antennæ erect and elevated, always on the watch, and either vibrating
them, or incessantly striking the water with them in a peculiar jerking
manner.
[Illustration: Fig. 146. Ear of Crab.]
The exterior or lower pair of antennæ are always longer than the
interior pair; sometimes they are simple and similar to them, as in the
flat crabs; and sometimes they have jointed filaments at their
extremities. In all the species they are attached to the under-side of
the crab, and the organs of smell are openings at the point of junction
between their second and third joints. These openings, which lead into
the mouth, are covered by a membrane, and closed by a calcareous lid.
Each lid is fastened by a little hinge to the side of its cavity, and is
opened and shut by muscles fixed at the extremity of a long tendon. Thus
the lower antennæ are the organs of smell, while the upper pair are the
organs of hearing, and both are probably the organs of touch.
The mouth of the crab is on the under part of the head, its lips are
horny plates, and it has a pair of mandibles to cut the food; their
action is from side to side. On each side of the mouth there are two
pairs of jaws, followed by three pairs of foot-jaws; so called because
they are legs modified to serve as jaws, but in some crustaceans they
are also instruments of locomotion or prehension, and sometimes of both.
The two last pairs have palpi, or feelers, at their base. All the jaws
and foot-jaws, when not in use, are folded over the mouth; the joints of
the two last are so broad that they completely conceal this complicated
apparatus.
Posterior to the mouth and its organs there is a flat broad plate, which
forms the ventral side of the body, with a groove in its surface, into
which the rudimentary tail is folded back, as in the Carcinus mœnas (D,
fig. 148), and the feet are fixed by movable joints on each side of this
sternal plate. The first pair, which are a little in advance of the
others, and bend forwards in a curve towards each other, may be called
hand-feet, as they occasionally serve for both. They have very thick
short arms and swollen hands, having a curved finger and a thumb with a
movable hinge, armed throughout their internal edge with a row of blunt
teeth, and terminated by sharp points. The other four pairs, which are
the real walking feet, spread out on each side of the animal, and often
bend a little backwards; they are rather thin, compressed, and end
either in a horny nail, or flattened blade for swimming.
The gills, which are the breathing organs of the crabs and other
Decapods, are spindle-shaped bundles of long, slender, four-sided
pyramids, fixed by their points on each side of the mid line of the
throat, so that they extend in opposite directions, and their spreading
bases fit and rest upon the vaulted sides of the carapace, or rather
gill chambers, to the right and left. Each of the pyramids is formed of
a multitude of parallel membranous cylinders fixed to the axis of the
pyramid, and an infinity of capillary bloodvessels form a network in
their surfaces.
The crab has nine of these bundles of gills in each gill chamber; a few
of them are shown in fig. 147. Each gill chamber has two openings; the
water is admitted by a slit in the base of the claw feet, and ejected by
another into the mouth. But the act of breathing is regulated by a plate
on the second pair of jaws, so connected with the exterior pair of
foot-jaws that, when the crab applies the latter to its mouth, the plate
shuts the slit, the water in the gill chamber is ejected by the mouth,
and in order to admit a fresh supply, the crab must open the foot-jaws
again, so that they are in constant motion. There are plates called
whips on all the appendages of the crab, from the last pair of foot-jaws
to the fourth pair of walking feet inclusive, which ascend and descend
vertically between the bunches of gills to sweep particles of sand or
other foreign matter out of them.
[Illustration: Fig. 147. Section of a Crab.]
The heart of the crab, as in all the Decapods, is placed under the skin
of the back next to the throat; and the blood, which is white or bluish,
flows from the heart through a complicated system of vessels, and,
having nourished the different organs, it is collected in reservoirs at
the base of the gills, is aërated while passing through them, and
returns to the heart again.
The mouth opens through a short gullet into a large globular stomach,
from the walls of which calcareous toothed organs meet in the centre.
One serves as an anvil, while the others bruise the food on it. Some of
the long-tailed crustaceans can evert this apparatus and push it out of
their mouth. The bruised food is liquefied by solvent juices from the
liver and stomach, and the nutritious part enters the bloodvessels by
imbibition.
The nervous system is condensed to suit the form of the crab. An oval
nervous mass with a hole in its centre surrounds the gullet, from each
side of which a nerve extends to a nerve-centre in the head. The organs
of sense are as usual supplied with nerves from the latter, and, from
the circumference of the massy ring, nerves radiate to every part of the
animal, voluntary or reflex, as may be required.
[Illustration: Fig. 148. Young of Carcinus mœnas in different stages of
development:—A, first stage; B, second stage; C, third stage; D, perfect
form.]
Dr. Carpenter has proved, by microscopic observations, that the shell of
the Decapod, in its most complete form, consists of three strata: the
first is a horny structureless layer covering the exterior; the second,
a cellular stratum; and the third is a laminated tubular substance. In
the large, thick-walled crabs, as the Cancer pagurus, the three strata
are most distinctly marked. The tubuli of the lowest layer rise up
through the pigment stratum in little papillary elevations, which give
the coloured parts of the shell a minutely speckled appearance. There
are various deviations from this general plan. In many of the small
crabs belonging to the genus Portunus, the whole substance of the shell
below the structureless horny investment is made up of hexagonal,
thick-walled cells; and in the prawns there are large stellate coloured
cells.
The eggs of the Brachyura are attached by gluten to the false ciliated
feet of the tail of the female, which being bent up under the body forms
a temporary protection till they are hatched. On leaving the egg the
young have not the smallest resemblance to the parent; it is only after
the fourth moult that they even acquire the crab form. When the young of
our common shore crab, the Carcinus mœnas, leaves the egg, it is
scarcely half a line in length. The body is ovoid, the dorsal shield
large and swelled (fig. 148, A). On the middle of its upper edge there
is a long, hollow spine bending backwards, in which the white blood may
be seen to circulate with a sufficient microscopic power. In front there
is a pair of large sessile eyes, and the circumference of the pupils is
marked by radiating lines: behind, there is a long, six-jointed tail,
the last segment of which is forked and spined. On each side of the
shield there is a pair of swimming feet attached to its waved margin.
Fixed also to the margin, but in advance of these, there are three pairs
of jointed feet ending in slender hairs. Immediately in front, between
the eyes, there is a very long compressed appendage, which is bent
backwards between the claws when the animal moves. Under each eye there
is another appendage, shorter and rather more compressed. There are
three pairs of claws, each composed of three joints, and ending in four
long slender hairs: the claws stand at right angles to the body. The
young, when it escapes from the egg, is quite soft, but it rapidly
hardens by the deposition of calcareous matter on its surface. The
progress of the consolidation is shown by the circulation of the white
blood in the hollow dorsal spine. When the creature is yet soft, the
blood globules may be seen ascending to its apex; but, as the
consolidation advances, the circulation becomes more and more limited
till at length it is confined to the base. This creature, whose shield
is sap green and the rest transparent, swims with great activity,
beating the water with his claws and tail. Such is the first stage in
the life of the common shore crab. At this period the young of the
Decapods bear a strong resemblance to one another, whether they are
afterwards to become long or short tailed crustaceans.
After a time this creature loses its activity, moults, and is no longer
to be recognised as the same, so great is the change (fig. 148, B). The
dorsal spine has vanished, the shield has become flatter, its anterior
part pointed, the eyes raised on stalks, and certain rudimentary organs
that were below the eyes now form long antennæ. The first pair of feet
have got hands, the others are jointed and simple, except the last pair,
which are still natatory: with these and with the tail, which is now
much smaller, these creatures swim and congregate round sea-weeds and
floating objects. After the third moult they have the form of a crab,
though neither that of the genus nor species of the parent (fig. 148,
C). The tail is folded under a square carapace, the four pairs of
walking feet spread widely and laterally, while the great hand-feet
attached to the anterior sides of the carapace stretch straightforwards,
the antennæ are short, and the eye-stalks bent to the right and left. It
requires several moults to bring this creature to its final size and
form.[38]
Crabs sometimes die while moulting, and occasionally are unable to
extricate a limb from its shell, and consequently lose it. But if a limb
be fractured they can cast it off at the second joint, and soon after a
diminutive limb is formed, which attains its full size at the next
moult; but if the crab has not strength enough to cast it off, it bleeds
to death.
_Anomura._
The Anomura is a family of Decapods intermediate between the long and
short-tailed Crustacea. There are nine or ten genera and many species,
chiefly distinguished by the development of the head and thorax, and the
softness of a non-locomotive tail: of these the Pagurus, or Hermit crab,
is assumed as the type or representative.
The carapace is long and convex, scarcely extending over the basal
joints of the feet. The claw feet are short, with a very broad hand and
sharp pincers; but the Hermit crab and some of its congeners are
irregularly formed; for the last pair of walking feet, instead of being
attached to the thorax, like the others, are fixed to the first part of
the tail, are generally folded over the back, and are employed to sweep
foreign matter out of the gills. The mouth and its masticating organs
are similar to those in the crab, except the exterior pair of foot-jaws,
which are longer and move like feet. But that which distinguishes the
Pagurus and its fellows from every other Decapod is the softness of its
unsymmetrical tail, all the appendages of which are abortive, and the
extremity, instead of ending in a swimming fin, terminates in a pair of
grasping organs. In order to protect this soft-skinned tail, the Hermit
crab folds it up and thrusts it into some old empty shell, clasps the
column of the shell with its grasping organs, draws in the rest of its
body, and covers it with the broad hands folded in such a manner as to
close the mouth of the shell, and to defend itself if attacked. It holds
so fast that it cannot be drawn out; but, when in search of food, it
stretches out its mailed head and legs, and walks off with its house on
its back. However, it sometimes comes out of its shell to feed, and,
like some other crustaceans, it holds its prey with one claw, and tears
it to pieces with the other. They are very pugnacious, and come out of
their shells to die. The larvæ of the Paguridæ undergo transformation,
and they moult when full grown.
_Stomapoda._
The Stomapods are all swimmers; they have long bodies with a carapace;
but it is so varied in form and size, that no general description of it
can be given. They have external, instead of internal, organs of
respiration; gills in the form of tufts are in some cases attached to a
few of the foot-jaws, but they are much more frequently fixed to the
basal joints of their swimming feet, so that the blood in their
capillary veins is aërated through their thin skin as they float in the
water. In the Squilla mantis, or S. Desmarestii, members of a genus of
this family, the gills, which are fixed to the basal joint of their last
pair of feet, consist of a long conical tube, on each side of which
there are numerous parallel tubes, like the pipes of an organ, and each
of these has a row of many long cylindrical filaments that drag in the
water. The mouth and its appendages are similar to those of the common
Decapods, with the exception of the anterior jaw-feet, which are of a
singular and formidable structure. They are bent outwards, and their
basal joint is exceedingly large, broad, and compressed; the next joint
is less, with a groove in its side; the third joint is a blade like a
scythe, whose cutting edge is furnished with long pointed teeth. The
Squillæ are carnivorous, and, if any unfortunate animal comes within
their grasp, they bend back the toothed edge of the first joint into the
groove of the second joint like a clasp-knife, and cut it in two. These
prehensile foot-jaws, or ‘pattes ravisseurs,’ are like the fore-feet of
the praying Mantis, and like them weapons of defence.
The genus Mysis, or Opossum Shrimps, have a long straight carapace,
which covers most of the thorax, and folds down on each side so as to
conceal the base of the feet: in front it is narrow, and ends in a
flattened beak; at the posterior end it is deeply scooped out. The two
last rings of the thorax are more or less exposed; the tail is long,
almost cylindrical, tapering to the end, and terminating in a swimming
fin composed of five plates spread like a fan. Both pairs of antennæ
have jointed stems ending, the outer in one, the inner in two very long
many-jointed filaments. On the top of the basal joint of the outer pair
there is a very long lamellar appendage, ciliated on the side next the
joint. Between the second and third joints of the exterior antennæ, Mr.
Spence Bate found the organ of taste: the aperture is simply covered by
a membrane, as in the lobster. The ears are in the last appendage of the
tail.
The Mysis has two pairs of jaw-feet differing little from feet; five
pairs of thoracic feet, all thin and divided into two branches, which
increase in length as they are nearer the tail, and are all provided
with a ciliated appendage to adapt them for swimming. In the female,
broad horny plates, attached to the two last pairs of legs, are bent
under the body so as to form a kind of pouch, destined to lodge the eggs
and the young during the first period of their lives, whence their name,
‘Opossum Shrimps’: the young are crowded in this pouch, and acquire
their adult form before they come into the water. The circulation of the
white blood of the Mysis was discovered by Mr. Thompson: the pulsations
of the heart are so rapid that they resemble vibrations. There are many
species of these small shrimps.
[Illustration: Fig. 149. Lucifer, a stomapod crustacean.]
The genus Lucifer is one of the most singular of the crustaceans from
its almost linear form (fig. 149), the excessive length of the anterior
part of the head, the extreme shortness of the thorax, the smallness of
the carapace or shell, and the great development of the tail, which is
more than three times as long as the thorax. The thin eye-stalks, which
are of exaggerated length, extend at right angles from the top of the
long cylindrical part of the head, and terminate in large, staring,
dark-coloured eyeballs covered with a multitude of facettes. The two
pairs of antennæ are placed between and below the eye-stalks. The
undermost pair, which are the shortest, have a little lamellar appendage
at their base: in some Lucifers, when viewed in front, it looks like a
cross. The salient mouth is placed at the base of the long organ that
carries the eye-stems. It has strong toothed mandibles, two pairs of
jaws with plates attached to each jaw, and three pairs of foot-jaws. The
tail is very narrow, consisting as usual of seven rings movable on one
another; but they are quite abnormal, for each of the rings is at least
as long as the thorax; the last has five plates spreading like a fan.
All the bristly feet, which seem to hang loosely down from the animal,
are fitted for swimming; those of the tail have long ciliated plates in
their basal joints. These creatures are small, and inhabitants of warm
seas.
_Amphipoda._
The Amphipods are very numerous, and abound in the British seas. They
have long, slender, and many-jointed bodies which have no carapace: the
tail in some genera is more fitted for swimming, in others for leaping.
The Talitrus, or Sandhopper, common on every sandy shore in Europe, is a
well-known example of the leaping genus. It is very small and
exceedingly active. The upper antennæ are very short, the inferior pair
are large, and longer than the whole body. The anterior feet are thin
and not prehensile. The first pair end in an immovable claw; the second
pair have a kind of hand, and are folded beneath the body; the following
feet end in a crooked nail. The appendages of the last three rings of
the tail are thick and spiny, and the tail serves as a leaping organ.
The sandhoppers hide themselves between tidemarks in large communities
under masses of wet sea-weeds, on which they feed. When disturbed they
leap away with great agility, and bury themselves in the sand by digging
with their fore-feet, and kicking the sand away with their tail-feet.
They have a strong sense of smell, for if a dead fish be buried in the
sand, it is devoured by these little voracious animals in a few days.
In the fin-tailed genera the gills are suspended between the bases of
the thoracic legs: they swim lying on their side, and their feet are
very varied in form, but always more or less furnished with spines and
hairs.
There are several genera of Amphipods that are nest-building animals;
all have hooks at the end of their tails, The Amphithoæ enclose
themselves in a cylindrical tube open at both ends. The animal is very
active, running along the branches of the sea-weeds by means of its
antennæ instead of its feet, which remain within the tube. In general
only the first pair of antennæ are put out to catch prey. If the animal
be prevented from advancing, it immediately turns its body within the
tube, and protrudes its head from the other extremity.
_Isopoda._
The order of Isopoda are so called because of the sharp and equal claws
of their walking feet, which are often prehensile. Their body is short
and flattened, and their small head is almost always distinct from the
throat. They are very numerous, and are divided into walking, swimming,
and sedentary animals; the females have horny plates on some of their
feet, which fold under the throat and form a pouch, in which the eggs
are hatched.
The Oniscus, common Wood-louse, or Slater, is a terrestrial Isopod. It
is an oval jointed creature, which rolls itself into a ball when
touched. The second of its six pairs of posterior limbs perform the part
of lungs: they contain hollow organs in their interior, into which the
atmospheric air penetrates directly through openings in their exterior
covering: so the Oniscus and its congeners, which live on land, are
drowned when put into water.
In the swimming Isopods, the five first pairs of tail-limbs are false
feet, and are suspended under the tail. The gills, consisting of two
great oval leaves, are fixed to them by a stalk; and are dragged through
the water. This group is very numerous; many live among the sea-weeds on
the coasts, others perforate submerged wood in all directions, and live
in the winding galleries they have formed. The Limnoria lignorum is
particularly destructive in the harbours on the British coasts, and in
the locks of the canals. The tortuous holes it bores are from the
fifteenth to the twentieth of an inch in diameter, and about two inches
deep. The female Isopod is not more than a line or two in length, the
male is a third less, and of a grey or greenish brown. These minute
creatures bore their holes with their mandibles, which are so sharp and
strong that they can penetrate the hardest wood, and appear to feed on
it, from the quantity found in their stomachs. Their bodies are covered
with pinnated hairs, their antennæ are short, and their posterior end or
tail is rounded.
Most of the genus Cymothea are parasitical; they can bend the sharp nail
of the three first pairs of feet upon the preceding joint, so as to form
hooks with which they fix themselves to the fishes on whose juices they
feed.
The Isopods bear a strong resemblance, an almost identity of structure,
with the Trilobites, a jointed race of Crustaceans long extinct. Some of
the Isopods roll themselves into a ball, as these most ancient
inhabitants of the ocean were wont to do; whose large compound eyes are
exactly like those of the Isopods; whence it was inferred by Dr.
Buckland, that neither the constitution of the sea nor the light of the
sun had changed for innumerable ages. The discovery of the Eozoön has
proved that Nature has not varied during a period immeasurably prior
even to that.
_Entomostraca._
The Entomostraca form an immense group of the lower Crustacea,
consisting of five orders. A vast number are just visible to the naked
eye, and many are microscopic; they teem in every climate along the
coasts, and in the deep blue oceans. The horny coat, enclosing the
minute bodies of these animals, is often so transparent that their
internal structure, and occasionally the process of the assimilation of
the food, is distinctly seen by the aid of a microscope. Small as they
are, their beauty is often very great; when transparent they sometimes
radiate all the prismatic colours; when opaque, they are frequently of
the most brilliant and varied hues, others shine with vivid
phosphorescent light. The segments of their bodies are often very
numerous, and similar to one another; but their appendages are very
different. They form two distinct natural groups of the bristly-footed
and gill-footed Crustacea.
_Copepoda._
The first order, Copepoda, or oar-footed tribe, have a distinctly
articulated body formed of movable rings, bristly swimming limbs; and
the females carry their eggs in huge pouches suspended on each side of
the posterior part of their bodies.
The Sapphirina fulgens is a beautiful example of the two-eyed tribe; its
body is nearly oval, divided into nine distinct joints, and so flat that
it is almost foliacious. The head has two brilliantly coloured eyes,
with large cornea so connected with the shell that they look like
spectacles. The two pairs of antennæ are silky, and the last pair of
foot-jaws that cover the mouth are garnished with silky plumes. It has
five pairs of swimming feet, and the tail ends in two little plates.
The Sapphirina is about a line and a half long, of a rich sapphire blue,
and floats on the surface of the Mediterranean and tropical oceans. It
shines with the most brilliant phosphorescent colours, passing from deep
blue to a golden green, or splendid purple. The brilliant colouring is
seated in the layer of cells that secrete the firm substance of the
body. With a microscope the cells are seen to pass alternately from one
colour to another. There is a little three-lobed body between the eyes
connected with the central nervous system by a small nerve; it contains
several corpuscules, which Professor Gegenbaur regards as the remains of
the single eye of the larva which undergoes many transformations before
it arrives at its adult form.
According to Professor Gegenbaur, the Sapphirina fulgens is a true
Copepod and the Mediterranean Phyllosoma is a Decapod, although it has a
lacunar blood system.
Some genera of the order Copepoda inhabit salt water, others fresh, as
the Cyclops quadricornis (fig. 150), which abounds in the water with
which London is supplied.
[Illustration: Fig. 150. Female Cyclops:—_a_, body; _b_, tail; _c_,
antenna; _d_, antennule; _e_, feet; _f_, plumose setæ of tail; B, tail,
with external egg-sacs; C, D, E, F, G, successive stages of development
of young.]
The genus Cyclops is a type of the bristly-footed group, distinguished
by a single compound eye placed in the middle of the forehead. The head
and thorax are almost entirely covered with an oval jointed buckler,
which has an opening below to let the bristly limbs pass through (fig.
150); and the tail, which is five-jointed, ends in two plates furnished
with bristly plumes. It is traversed by the intestine, which ends near
its extremity. The brilliant little eye in front consists of a number of
simple eyes placed under one glassy cornea. It rests upon the base of a
cone of muscular fibres, which give it a movement of rotation upon its
centre. Its upper pair of antennæ, situated below the eye, spread to the
right and left. In the female they have numerous joints with a bristle
at each joint; the lower pair of antennæ are short-jointed and bristled.
The mouth of the Cyclops has a pair of jaws, and two pairs of foot-jaws
covered with bristles. The five pairs of branching legs, which are
fitted for swimming, are thickly beset with plumose tufts. In the female
the egg-sacs are hung on each side of the tail (B, fig. 150) by a
slender tube, through which the eggs pass from the ovary within the
mother into the sacs where they are deposited in rows, and there they
remain till hatched. When the larvæ come into the water the sacs drop
off, and the young undergo various changes before coming to maturity, as
shown in fig. 150. The Cyclops swims with great activity, striking the
water with its antennæ, feet, and tail; and the rapid movement of its
foot-jaws makes a whirlpool in the water which brings minute animalcules
to its mouth, and even its own larvæ, to be devoured.
Some species of the Calanus, a marine genus of the one-eyed group, are
eminently social. Professor Dana found that the colour of those vast
areas of what the sailors call bloody water, met with off the coast of
Chili, was owing to shoals of the Calanus pontilla; and another immense
area of bloody water he met with in the North Pacific was owing to a
vast multitude of the Calanus sanguineus. Although this genus abounds
more in individuals in the temperate seas, the species are more varied
in the tropical. Those figured and described in Captain Maury’s works
were mostly microscopic and very beautiful; one fished up was grey with
a bunch of yellow feathers at the end of its tail. The egg-bags were
purple, another was green marked with scarlet tufted antennæ longer than
itself spread out at right angles from its head. This creature shone
with a bright phosphorescent light, visible even when a candle was
burning. These and many more were taken in tropical seas. They were
remarkable for the length of their antennæ; and it was observed that no
eyes were perceptible in such Crustacea as had these exaggerated
antennæ; these organs of intelligence and warning were probably
sufficient for their wants. When animals live without eyes on the
surface of a tropical sea, it is quite conceivable that similar
instruments of touch may suffice for those who live in the dark abyss
below.
The Ostrapods, which form the second order of the bristly-footed
Crustacea, are defended by a bivalve carapace; they have swimming limbs
and a confluent eye; that is, a number of simple eyes placed under a
glassy cornea.
[Illustration: Fig. 151. Cypris.]
The genus Cypris belongs to this group. Several species may be seen
swimming in our streams and fresh-water pools. The body of the common
Cypris (fig. 151) is enclosed between two flat oval shells, united by a
hinge on the back. The little animal can open and shut the valves by
means of two slender muscles, extending from its back to the shells,
which are much curved above and rather flat below. There are two pairs
of antennæ beneath the eye, they are perfectly transparent,
many-jointed, and end in tufts of filaments. One pair projects forward
and then bends gracefully backwards; the other pair are bent downwards.
The mouth has no foot-jaws, and there are only two pairs of feet. Only
one pair is seen in the female, for the other pair is bent upwards to
support the egg sacs. The Cypris attaches her eggs to the leaves of
aquatic plants by a greenish fibre. Not more than twenty or thirty eggs
are deposited by one individual, while the heaps contain several
hundreds; so many females contribute to form one heap. The young are
hatched in the form of their parent in about four days and a half. As
the pools dry up, the Cyprides bury themselves in the sand or mud at the
bottom; if that remain moist they survive, if it becomes dry they
perish; but the eggs remain dormant till the return of rain, when they
are hatched, and the surface of the water is soon crowded with a swarm
of young Cyprides.
[Illustration: Fig. 152. Section of Daphnia pulex.]
The Cladocera is the first order of the gill-footed Crustacea: their
body is defended by a bivalve carapace; they have from four to six
gill-footed limbs, one compound eye, and two pairs of antennæ, one pair
of which is large and adapted for swimming. The Daphnia pulex, or
Arborescent Water-flea, of which fig. 152 is a section, is a common form
of this tribe. It is very abundant in pools and ditches, coming in
groups to the surface in the mornings and evenings in cloudy weather.
The bivalve shell is transparent, flexible, and open below; it ends
behind in sharp toothed peaks. The eye placed in front is moved by four
muscles, and on each side of it are the great antennæ, which are
jointed, branched, and garnished with feathery filaments, and are the
chief organs of locomotion. This animal has no foot-jaws, but it has a
nervous system and a heart, whose pulsations are repeated two or three
hundred times in a minute, and the blood is aërated by gills at the
extremities of six pairs of bristly feet situated behind the mouth, and
only used for respiration and prehension.
The eggs, when laid, are deposited in a receptacle between the back and
the shell of the female Daphnia, and after the young come into the water
they undergo no transformations. Between each brood the Daphnia moults,
and the egg receptacle is thrown off with the exuvia. After several
changes of skin the young Daphniæ come to maturity and lay eggs, which
produce successive generations of females throughout the spring and
summer; but in the autumn males appear, and then the eggs are retained
in the receptacle of the female and are not hatched till spring. If the
female should moult after this, the case with the eggs in it is cast off
with her outer skin, which then becomes a protection to the eggs during
the winter, and they are hatched in spring, producing females.
_Phyllopoda._
The second order of gill-footed Crustacea are called Phyllopoda, because
they have gills like the leaves of a book attached to their lamelliform
swimming feet. Their bodies are divided into many segments, and they
form two groups, one of which has a carapace, the other has not. The
Apus cancriformis is an example of the first. It is about two inches and
a half long, and is a large animal compared with the others of its
class. Its head and thorax are covered by an oval carapace, and its
cylindrical body is composed of thirty articulations. It has a compound
movable eye in the middle of its forehead, and a sessile eye on each
side of it. All the members that follow the apparatus of the mouth have
a foliaceous form, and are in constant motion even when the animal is at
rest. The Apus has sixty pairs of jointed legs; the number of joints in
these and in the other appendages is estimated to be not less than two
millions. However, the instruments chiefly used for locomotion are the
first pair of feet, which are very long and serve for oars; with these
the animal can swim freely in any position, but when they are at rest it
floats on the surface of the stagnant water in which it lives, and the
fin feet maintain a constant whirlpool in the water, which brings the
small animals on which it feeds to its mouth.
The Branchipes stagnalis, which may be taken as a type of the second
order, has a perfectly transparent segmented body nearly an inch long,
eleven pairs of pale red gill-feet, antennæ of bluish green, and a long
tail ending in red bristles. The head has two large eyes on movable
stems, and a sessile black oculus between them. Filiform antennæ spring
from the upper part of the head; the other pair, like two large horns,
are turned downwards. The last ring of the swimming tail has two plates
with ciliated appendages.
The Artemia salina differs very little from the Branchipes. It abounds
so much in the brine pans at Lymington and other salt works, as to give
a red tinge to the nearly concentrated brine, the temperature of which
is so high that no other animal could live for a moment in it.
_Pycnogonoïdea or Spider Crabs._
Some of the Spider crabs hook themselves to fishes, while others live
under stones, or sprawl with their long hairy legs over sea-weeds, and
feed on the gelatinous matter these weeds afford. The throat with its
members, and the head soldered to its first ring, forms nearly the whole
animal. It has a pair of antennæ and four rudimentary eyes, set on a
tubercule. A proboscis-like projection extends from the front; the mouth
is furnished with cilia and one pair of foot-jaws. Four pairs of long
hairy legs proceed from the throat, spread widely on each side, and end
in a hooked claw. The stomach, which occupies the centre of the animal,
sends off five pairs of long closed tubes like rays; one pair enters the
foot-jaws, the others penetrate the legs. This digesting system is in a
state of perpetual vermicular motion, which, as well as the movements of
the animal itself, aërate its transparent blood through the skin, by
keeping it in circulation. So this insignificant-looking creature has a
very curious and complicated mechanism.[39]
_Fossil Crustacea._
Analogues to the Anomura are found in the Chalk formation, but the
Macrura are the prevailing forms. Extinct species of lobster, crawfish,
and shrimps are met with in the secondary strata, from the Chalk to the
Coal measures. In the Coal formation all these higher forms disappear,
but then the gigantic King Crab, or Limulus, is found accompanied by the
minute Entomostracan forms in infinite variety of species.
_Epizoa, or Suctorial Crustacea._
The Epizoa infest the skin, eyes, and gills of fishes. Many of them in
their adult state bear a strong resemblance to the lowest of the
Crustacea; but, in general, the resemblance between these two classes of
animals can only be traced during the extraordinary changes which the
Epizoa undergo in their early life, and they differ so much in their
perfect state that it is wonderful any connection should ever have been
discovered between them. The Epizoa are extremely varied in their
perfect forms, and the class generally is supposed to be more numerous
than the whole race of fishes. In the lower orders of the Epizoa the
mouth is suctorial; the higher orders adhere to their victim by jointed
mandibles ending in hooks. The Epizoa are male and female: the male is
small and free, the female is fixed, and generally has a pair of long
egg-sacs hanging from her body.