CANCER, tropic of, in astronomy, a lesser circle of the sphere, parallel to the equator, and passing through the begin ning of the sign Cancer.
Cesczn, the crab, in natural history, a genus of insects of the order Aptera. The generic charactet : eight legs in ge neral, sometimes six or ten, besides two chelated arms ; two eyes, distant ; in ge neral foot-stalked, elongated, and move able ; tail unarmed, and jointed. Ani mals of this genus at particular periods cast their shells, previously to which the limbs shrink, to facilitate their extrica tion. The loss of a limb, with other ani mals irreparable, is of little consequence to these, as a few weeks suffice to repro duce one: and in cases of bruise or mu tilation, a consciousness of this eventual, and indeed speedy, reproduction induces them violently to rid themselves of the injured member, and to await in seclu sion the formation of a complete substi tute for it. Some species, which are un provided by nature with any shelly cover ing, uniformly have recourse to such shells as they find best accommodated to their purpose, and in which their bo dies are immersed, while their claws are protruded and unprotected. The corres pondence pf parts in both sides of al most all other animals is far from being universally observable in these. The claspers on one side are often of extraor dinary size, and on the other slender and small ; and in some instances the large arm is obliged to be supported by the back of the animal, both while walking and at rest, from its unwieldy and extra vagant size. The genus comprehends an
immense variety of species ; but the chief division is into the Brachyouri and the Macrouri, or the short-tailed and the long-tailed : under the form of which the crab commonly used in this country, for food, is the principal. It is found chiefly on the rocky coasts. Among the Macrouri, the common lobster is the principal, and a well-known specimen. It inhabits in the clearest water, and at the base of rocks which project over the sea. It is extremely prolific, depositing about 12,000 eggs each time of laying. The warmth of summer is required for maturing them. The C. Norwegicus, or Norwegian crab, is naturally of a pale red colour, and variegated with yellow It is longer, and more slender, than the ordinary lobster. For a representation of it, see Entomology, Plate IL fig. 1. C. grapsus, or the streaked crab, is an inhabitant of the American and Indian seas. Its general pale yellow is finely interspersed with red streaks and spots. For a specimen, see Entomology, Plate II. fig. 2.