CANCER, or CRAB, in zoology, a genus of insects belonging to the order of insects aptera. The generic characters are these: They have eight legs, (feldom ten or six), besides the two large claws which answer the purpose of hands. They have two eyes at a considerable distance from each other, and for the most part supported by a kind of pedunculi or foot-stalks; the eyes are likewise elongated and moveable. They have two clawed palpi; and the tail is jointed. There are no less than 87 species of cancer, distinguished principally by the length of their tails and the margins of their breasts. This genus includes the lobster, shrimp, &c.