CRAB, the popular name of all the crustaceans of the order decapoda (the highest order of crustaceans, characterized by great concentration of the nervous system and corresponding general concentration, by five pair of thoracic limbs, and by having the gills inclosed in a special cavity on each side of the thorax, covered by the carapace) and sub-order brachyoura (characterized by the small size of the abdomen, which resem bles a short tail curved under the thorax and appressed to it, all the most important viscera being included in the thorax), and extended also to some of the sub-order ano Moura (purse-crabs, hermit-crabs, etc., characterized by a condition of abdomen inter mediate between that of the brachyoura and that of the macroura, or long-tailed.decapod crustaceans, such as the lobster, crayfish, etc.). All the crabs, besides many other crustaceans, were comprehended in the Linntean genus cancer; but the number of spe cies is very great, and the brachyoura alone are now arranged into many genera and even families. The different kinds of crabs differ very much in the form of the carapace (the back), which in sonic is orbicular or nearly so; in some, much broader than it is long; in others; longer than broad; in some, prolonged in front into a kind of beak. etc.; also in its smoothness, or roughness with hairs, tubercles, or spines; in the length of the legs, etc. The eyes are compound, with hexagonal facets, and are elevated on stalks, which are generally short, but sometimes considerably lengthened, and which have the power of motion, so as to turn the eye in different directions. The first pair of limbs are not used for locomotion, but exhibit in great perfection the characteristic claws or pincers (chelce) of the decapod crustaceans. Crabs are inhabitants of almost all seas; most of them, however, having their limbs formed for walking rather than for swimming, are found chiefly near the coast; some inhabiting comparatively deep water, and others abounding in those parts which are left by the receding tide, where they occur equally in the rock pools and among the moist sea-weeds. Some small kinds of crabs (pea crabs) are often found in the inside of mussels and other bivalve mollusks.
Some crabs inhabit fresh water, particularly in the warmer parts of the world; anti others, known as land-crabs (q.v.), live among moist herbage, or burrow in sand or earth. Some have the last pair of limbs expanded at the extremity into a broad blade for swimming, and some have even all the four pair of limbs intended for locomotion thus expanded, and sometimes occur far out at sea. Some of the crabs, with very long legs, are known as spider-crabs. Crabs molt or change their shell, not at fixed inter vals or seasons, but according to the exigencies of their growth; the change being made with great frequency when they are very young, but rarely in advanced age; indeed, from the mollusks, and other animals sometimes found adhering to the carapace, it is inferred that the same covering is sometimes worn for a number of years.—The meta morphosis of crabs is- i noticed in the article CRI:STACEANS.—Crabs are interesting inmates of the aquarium, from their readiness in seizing food, their activity in tearing and eating it, their general habits, and, in particular, their pugnacity. The number of specimens is apt, however, to be soon diminished by the stronger killing and eating the weaker.—Many kinds of crabs are used as articles of food in different parts of the world, as the large edible C. (cancer purus) and small edible C. (carcinus mynas) of the British shores. The latter is extremely common on all parts of the coast, but is not nearly so much esteemed as the former, which is much sought after, and is caught either in the holes of the rocks at low tide, or by means of a kind of trap, a basket which readily permits its entrance but not its escape, and which is baited with meat or animal garbage of some kind. In winter it seems to retire to deeper water. Its black claws and very broad carapace, arched at the sides, readily distinguish it from all other British species. It is sometimes nearly a foot in breadth. The claws of the edible C. were formerly ground to powder and used as a medicine, having, however, no properties but -these of carbonate of lime.