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.