Crustacea is a class of arthropods consisting mainly of aquatic species. Crustaceans have two pairs of antennae, a pair of eyestalks with compound eyes, and a pair of mandibles on the head as well as a pair of legs on each body segment. These legs are biramous, or two branches, meaning that the each leg has a joint in it. All crustaceans respire via gills. Crustaceans are found all over Earth, particularly in aquatic environments.