Crab, an articulated animal of the class Crustacea, having ten legs, of which the front pair terminate in pincer-like claws. The species of crabs are very numerous, and belong to many different families and several higher groups of their class. Among the crabs proper there are two well marked groups: the true brachyurans, like the common and the blue or edible crab, in which the abdomen, or tail, is small, held closely beneath the body, and in the male without appendages except upon the two segments next the body; and anomurans, of which the hermit or soldier crabs, the purse crabs, etc, are representatives, and in which the abdomen is not so closely appressed to the under side of the body, and has appendages, in both sexes, upon next to the last segment. The king crabs, or horseshoe crabs (limulus), are among the lowest forms of crustaceans, and by many naturalists are regarded as more nearly allied to the spiders and scorpions, or else as representing a distinct class. The majority of the species of crabs are marine, although many, especially in the tropics, inhabit fresh waters, while others are terrestrial, at least for the greater part of their lives.

The marine species are found along the shores, and many of them live at great depths, some species having been brought up from nearly the greatest depths explored by the dredge; while others are truly pelagic animals, living always near the surface of the water and far from land, except when driven ashore by storms. Two such pelagic species are very abundant among floating seaweed in the Gulf stream, and several others are found in the Pacific and Indian oceans. Most of these pelagic species, as well as many others, have the last segments of the hind pair of legs expanded into lamellar paddle-like organs, by means of which they swim with considerable rapidity. These swimming species mostly belong or are allied to one family, the portunidce, but a few are also found in widely different families. - Crabs, as also lobsters, shrimps, etc, belonging to the same class, have the body divided into two regions, the consolidated head and thorax, called the cephalothorax, and the tail or abdomen, composed of seven segments, some of which are, however, partially consolidated together. In the higher groups the cephalothorax is covered with a large shield-like shell, or carapace, and in all the groups it bears 14 pairs of appendages, representing as many segments of the consolidated body.

These appendages are, beginning in front: 1, the pedunculated eyes; 2, 3, two pairs of antennae, which serve as organs of hearing, touch, and perhaps other and special senses; 4, the stout molar-like jaws or mandibles; 5, 6, 7, 8, 9, five pairs of leaf-like jaws which fit closely upon the mandibles and serve to hold and manipulate the food, and of which the outer pair are larger and thicker than the others, and serve when closed to cover the whole mouth opening; 10, the large prehensile claws; 11, 12, 13, 14, the four pairs of walking legs. The joints of the claws and legs are all hinge-like, admitting of motion only in one plane; but, as each joint moves in a different plane, the combination of joints admits of motion in any direction without turning, although they usually run sideways, moving either to the right or left with equal facility. The stomach is situated just above and back of the mouth, and is furnished with strong concentric teeth, which further masticate the food after it is taken into the stomach. The heart is situated just behind the stomach and close to the carapace. The blood is nearly transparent, and is oxygenated by passing into gills attached to the bases of the legs and occupying a large space under each side of the carapace.

The males can be distinguished from the females by having narrower tails without appendages except upon the first two segments, while the females have branching appendages upon the third to the sixth segments. These appendages upon the tail of the female serve for the attachment of the eggs after they are laid, they being fastened to the numerous hairs with which the appendages are clothed. The eggs are carried thus until they hatch. After hatching, the young of nearly all the crabs undergo a remarkable metamorphosis. When first hatched from the egg, the common crab (cancer) of our Atlantic coast is a little free-swimming animal, having a somewhat globular body armed with a long spine projecting down between the enormous eyes, another spine upon the back longer than the whole animal, and another smaller one upon each side, and a slender tail without appendages, but with the last segment expanded into a broad, horizontal caudal fin. In this stage the claws and legs are wholly wanting, and there are no gills; but some of the jaw-like appendages of the mouth are developed into long, two-branched swimming appendages,- by means of which the animal swims rapidly about in the water, where it lives most of the time at the surface.

The young in this condition were long ago described under the name of zoea, and it is still called the zoea stage. After casting the skins several times and increasing much in size, swimming legs, like those of shrimps, are developed on the under side of the tail, and soon the claws and legs appear, the mouth appendages become much like those of the adult, and the spines upon the body have either disappeared or become much smaller. This is called the megalops stage, the name having been applied to them before it was known that they were the young of crabs. Finally, at one casting of the skin, the swimming legs upon the abdomen and the spines upon the body disappear, and the little crab, now about an eighth of an inch long, comes forth in something like the adult form, gives up its free-swimming existence, and afterward lives upon the shore or bottom. Most species pass through similar changes, but there are apparently exceptions in some of the land and fresh-water crabs, which are said to be hatched in a form much like the adult. The subsequent increase in size takes place only at the times of casting the shell, which occurs frequently at first, but finally, in most species, only once a year, and in old individuals of some species still less frequently.

At this period the animal seeks some sheltered situation, the whole integument becomes detached from the new one forming beneath it, the carapace separates from the body just above the bases of the legs nearly all round, and the legs and other appendages are slowly drawn out of their old shells. The shells of the big claws and other appendages are not cracked or broken, but the new integument is so soft and yielding and the muscles in such a flaccid condition that the limbs are drawn through the small openings at the joints, much as a sack nearly filled with some fluid may be drawn through an opening much smaller than the sack itself. For some time after leaving the old shell the crabs are in a very soft and defenceless condition, as the new shell hardens quite slowly. In this state the common edible or blue crabs (callinectes hastatus) of our Atlantic coast are sold in the markets as "soft-shelled crabs," and are much more highly esteemed for food than when in the normal or hard-shelled condition. Many species are extensively used for food in different countries, and nearly all of them would probably be found edible. They are eagerly devoured by fishes, and constitute a large part of the food of many valuable kinds.

The one of most importance as an article of food on the Atlantic and gulf coast of the United States is the blue crab just referred to. This is one of the swimming species, and is found especially in estuaries and brackish waters, from Cape Cod to the gulf of Mexico. Other species of the same genus, found in the West Indies and on both coasts of Central and South America, are extensively used for food. The common or rock crabs (cancer irroratus and borealis), large, mottled, reddish brown species, with nine small teeth on each side of the carapace, are also sometimes sold in the markets of New England; and similar species of the same genus are common in San Francisco, while another species (cancer pagurus) is much used on the coast of Europe. The species most extensively used in many parts of Europe is, however, the green crab (carcinus maenas), which is smaller and has only five teeth on each side of the carapace. This is common upon the southern coast of New England, but is here seldom if ever used for food. Upon the coast of the southern states a very large and massive species (menippe mercenaria), with the body often five or six inches across, is caught in great numbers for the market.

In tropical countries very numerous species, and among them many land and freshwater ones, are much used as food. - The sand crab, or swift-footed crab (ocypoda arenarid) and the fiddler crabs (species of gelasimus) are among the most highly developed species. The sand crab is almost exactly the color of common beach sand, has a nearly square body, often two inches or more across, long legs, and short claws, nearly alike on the two sides. It is found from New Jersey to Brazil, and closely allied species of the same genus are found upon the west coast of Central and South America and in the old world. Our species digs deep holes in sandy beaches just above high-water mark, and when overtaken away from its hole runs so swiftly over the sand that it is difficult to catch. It is carnivorous, catching beach fleas, etc, or feeding upon fish or other dead animals thrown upon the shore. Although the adults are almost entirely terrestrial, the young as soon as hatched swim freely in the water until, in the megalops stage, they are fully a fourth of an inch across the body, but still very unlike the adult. In this stage they are often thrown upon the beaches as far north as Block island.

After coming upon the shore they bury themselves in the sand, and probably soon after change to the adult form. The fiddler crabs are more abundant in sheltered situations, especially on salt marshes; they are found upon the coasts of nearly all the warmer countries, and three species are common from Cape Cod to Florida. Our species are all much alike, and are at once distinguished by the males having one of the claws, sometimes the right, sometimes the left, enormously developed and much larger than the whole body, while the claw upon the other side is very small, as both are in the female. They dig holes somewhat in the manner of the sand crabs, but usually between tides, and the males run over the shores and marshes, holding up the big claw in front of them in a threatening manner. This way of carrying the big claw has probably given rise to the common name. The little oyster crab (pinnotheres ostreum) is found wherever oysters occur on our coast. The female lives, at least when mature, within the shell of the oyster, in the gill cavity. The males are much smaller than the females, being no larger than a small pea, and are rarely if ever seen within the oyster, but are sometimes found swimming at the surface of the water.

Other similar species live in many kinds of bivalve shells, and an allied species, found upon the west coast of South America, lives inside a sea urchin or echinus. The spider crab (li-oinia canaliculata) of our Atlantic coast has a somewhat pear-shaped body and exceedingly long legs, often spreading more than a foot across, and is always covered with mud, barnacles, and other foreign substances, which tend to conceal it from its enemies. It is a representative of a very large class of crabs, some of which are among the largest known. One of them, a species of macrocheira, found in Japan, sometimes measures 10 ft across the expanded legs. The land crabs (species of ge-carcinus and other allied genera) have the gills so constructed that they are able to live for a long time away from the water, and are even found high up upon mountain sides. They are said, however, to journey to the sea once a year, probably for the purpose of depositing the eggs. - Among the anomurans there is even a greater diversity in form and habits than among those previously mentioned, but the species are not so numerous or important. The hermit or soldier crabs are the most familiar and widely distributed of the group, numerous species being found from the Arctic ocean throughout the tropics.

Their peculiar habit of living in the spiral shells of mollusks has long attracted attention. The shells which they inhabit are often covered with hydroids or other foreign substances, and some species always have an actinia upon the shell. The hind part of the body and the abdomen are soft, protected only by a thin integument, and the abdomen is onesided and curved spirally, so as to fit the shells which they always inhabit and drag about with them, while the legs and claws are all directed forward and occupy the opening of the shell. Most of the species pass through a metamorphosis, the first stage of which is much like that of the common crabs; but after they have attained considerable size and have acquired the full complement of legs and claws, the abdomen is perfectly symmetrical and furnished with appendages, as in shrimps and lobsters. In this and the earlier stages they swim about freely at the surface of the water, but soon change their form and begin to inhabit small shells, which are from time to time exchanged for larger ones as the crabs increase in size. When changing the shells, combats between individuals sometimes take place for the possession of the more desirable shells, and this has given rise to their common name.

Some of the species in the tropics are terrestrial, like the land crabs, and travel long distances from the sea and even up among mountains, carrying the marine shells with them. The robber crab (birgus), inhabiting the islands of the Indian and Pacific oceans, is an enormous terrestrial crab allied to the hermit crabs, but does not inhabit shells, the abdomen being shorter and covered with a hard integument. It feeds upon cocoanuts, climbing the trees and breaking open the fruit with its claws, which are adapted to the purpose. Some of the anomu-rans approach closely in form to the brachy-urans, resembling the common crabs except that the abdomen in both sexes has appendages upon the segment next to the last. The species of hypoconcha, which inhabit both coasts of tropical America, are of this form, but they always carry one of the valves of a bivalve shell upon the back, which is soft and unprotected. The shell is held in place by the hind legs, which are specially adapted to the purpose. Species of an allied genus carry compact sponges, or similar substances, in the same way. Others have a hard shell wholly unprotected.

Such are the gigantic species of li-thodes and its allies in the arctic and antarctic seas, and, in the warmer seas, the little, brightly colored porcellanians, with flattened bodies and large compressed claws.

American Edible Crab (Callinectes hastatus).

Common Crab of Europe (Cancer pagurus).

Fiddler Crab (Gelasimus vocans).

Hermit Crab and Actinia.