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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.