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._