WEBA shed carapace of a lady crab, part of the hard exoskeleton. Body structure of a typical crustacean – krill. The body of a crustacean is composed of segments, which are grouped into three regions: the cephalon or head, [5] the pereon or thorax, [6] and the pleon or abdomen. [7] The head and thorax may be fused together to form a ...