The hardness of their shells varies from one species to another. Crustaceans shed and replace their shells several times over the course of their lives to allow their bodies to grow.
Decapod crustaceans have five pairs of limbs.
The thorax of crustaceans is directly attached to the head, forming the cephalothorax from which the limbs protrude. Crustaceans move by walking, usually along the sea floor. Crabs can move very quickly, even from side to side, and certain species venture onto beaches. Female crustaceans are distinguished from males by a sort of flipper, or fin, on the underside of the thorax, which is used to hold the eggs. In males, this appendage is thinner and harder and is not shaped like a fin.