CRAB

any short-tailed member of the crustacean order Decapodaespecially the Brachyura, or true crabs, but also other forms such as the Anomura, which may resemble them in appearance and habits. The approximately 4,500 species occur in all oceans, in fresh water, and on land. Unlike those of other decapods (e.g., shrimp, lobster, crayfish), crabs' tails are curled under the thorax, or midsection. The carapace (upper body shield) is usually broad. The first pair of legs is modified into chelae, or pincers.