Ovid.
The animals of this class, as regards organisation, must be placed higher in the scale than the Arachnidæ, or spiders; but they are beneath the Mollusca, although as regards affinity, the Mollusca in their lower division—the Molluscoïda—more approximate to the Polyp class than to the Crustacea.
The Crustacea is the highest division of articulate animals with feet; they breathe by means of gills, and have no stigmata, or air-passages, as in insects. The name signifies a hard crust or covering, with which the animals are protected. This consists of layers of carbonate of lime with one of pigment, generally, but not always, on the surface. The general outline of these animals is peculiar; unlike insects, they are not divisible into head, thorax, and abdomen; many species truly have no head at all; but a pair of eyes point to the seat of intelligence. Most of these animals have two compound eyes; but a few, like some insects, have both simple and compound eyes. The mouth is situated in the under part of the anterior of the body: in some cases they have jaws, as in crabs; in others suckers only.
Passing over the vast numbers of beings which inhabit the debatable ground—the Annelids, which were for ages confounded with the worms, because of their resemblance in form:—a form which might be declared forbidding, but, as Aristotle has well said, Nature, in her domain, knows nothing low, nothing contemptible; the sea-leeches, whose condition was an impenetrable mystery to Pliny, "Omnia incerta ratione, et in naturæ majestate abdita;" and the singular cirripedes, one species of which, the barnacle (Anatifa lævis), was thought by old Gerard, the herbalist, and in his day by many others, to be the egg from which the barnacle goose was produced—passing over these ocean tribes, we reach the Crustaceans—the Insects of the Sea; of greater size, force, and voracity than any land insect with which we are acquainted. They are armed, also, at all points; for, in place of the coriaceous tunic, they are clothed in calcareous armour, both hard and strong, and bristling with coarse hairs, spiny tubercles, and even serrated spines.
The Crustaceans have nearly all of them claws, formidably hooked and toothed, which they employ as pincers, both in offensive and defensive war. They have been compared to the heavily-armed knights of the middle ages—at once audacious and cruel; barbed in steel from head to foot, with visor and corslet, arm-pieces and thigh-pieces—nothing, in fact, is wanting to complete the resemblance.
These marine marauders live on the sea-coast, among the rocks, and near the shore. Some few of them frequent the deep waters, others hide themselves in the sand or under stones, while the common crab (Carcinus mœnas, Leach) loves the shore almost as much as the salt water, and establishes itself accordingly under some moist cliff overhanging the sea, where it can enjoy both.
One of the necessary consequences of the condition of these animals, enclosed in a hard shell, is their power of throwing it off. The solidity of their calcareous carapace would effectually prevent their growth, but at certain determinate periods Nature despoils the warrior of his cuirass; the creature moults, and the calcareous crust falls off, and leaves it with a thin, pale, and delicate tunic. In this state the Crustacean is no longer worthy of its name—its skin has become as vulnerable as that of the softest mollusc; but it has the instinct of weakness—it retires into lonely places, and hides its shame in some obscure crevice, until another vestment, more suitable for resistance, and adapted to its increased size, has been restored.
The Crustacean has not a vertebral column. The covering of the Crustacean consists of a great number of distinct pieces, connected together by means of portions of the epidermis which have not yet become hardened, in the same way as the bones in the skeleton of the vertebrata are connected by cartilages, the ossification of which only takes place in old age. The covering of the Crustacean consists of a series of rings varying in number, the normal number of the body-segments being twenty-one. Each ring is divisible into two arcs—one upper, or dorsal, the other lower, or ventral; and each arc may present four elementary pieces, two of which are united in the mesial line from the tergum, or back; the lower arc is a counterpart of this, while the others form the two side, or epimeral, pieces. The skin, therefore, performs the functions of a skeleton, so that the Crustaceans, as was said by Geoffroy Saint Hilaire, like the molluscs, live inside and not outside the bony column. The analogue of the Crustacea amongst vertebrata is to be found amongst Sturgeonidæ, whose hard, immovable bony case encloses a softer skeleton; agreeing in its characters with that of the higher divisions of vertebrata, although not possessing the solidity of bone.
The Crustaceans vary greatly in colour; some are of a dark, iron-grey, with a dash of steel-blue, like metal weapons forged for combat; a few of them are red, or reddish-brown; others are of an earthy yellow, or of a livid blue.
"The integument," according to Milne Edwards, "consists of a corium, or true skin, and epidermis, with a pigmentary matter, which colours the latter. The corium is a thick, spongy, and vascular membrane, connected with the serous substance which lines the parietal walls of the cavities, as the serous membrane lines the internal cavities among the vertebrata." This pigment is less a membrane than an amorphous matter diffused through the outer layer of the superficial membrane, which changes to red in the greater number of species in alcohol, ether, acids, and water at 212° Fahr.