All true crabs have 10 legs that are arranged in pairs. The front-most pair is modified into a pair of claws or pincers, and the other four pairs are used for locomotion. For some swimming crabs, the hindmost pair of legs is flattened to form paddles.
Crabs are crustaceans of the infraorder Brachyura, and they make up about half of all decapods, or 10-footed creatures. There are 6,793 species of crab. They are a diverse group, ranging in size from the tiny pea crab, which is less than 1 inch across, to the Japanese spider crab, which has a leg span of up to 13 feet.