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. The compound eyes, which in all the crabs have hexagonal facettes, are on short jointed stems placed in deep and nearly circular orbits like cups, so that the stems are scarcely visible. These orbits, whose edges are sometimes smooth and sometimes notched, are so constructed that the crab can bend the eye-stems horizontally to the right and left, and the front of the carapace either conceals the orbit, or forms the eyebrow. In all crabs the antennæ appear in front between the eyes. The first or interior pair are short, jointed, and capable of being bent into cavities, which contain their basal joints; these cavities are near the eye orbits, with which they are connected in certain species. Well-developed ears are placed in their basal joints. Fig. 146 represents a magnified ear seen from behind, and Mr. Gosse mentions that the large eatable crab, whether at rest or feeding, carries these antennæ erect and elevated, always on the watch, and either vibrating them, or incessantly striking the water with them in a peculiar jerking manner. [Illustration: Fig. 146. Ear of Crab.]