Members of the class Crustacea include lobsters, crabs, crayfish, shrimp, copepods, barnacles, and several other organisms. All of them have two pairs of antennae, a pair of mandibles, a pair of compound eyes and two pair of maxillae on their heads. Along with this, all crustaceans contain a pair of appendages on each body segment. Crustacean bodies usually are made up of head, thorax, and abdomen. Most crustaceans are free-living, but some are sessile and a few are even parasitic. Most use their maxillae and mandibles to take in food. The walking legs, including specialized chelipeds, may be used to help capture prey.