Animals with segmented, chitin-encased bodies and articulated appendages. The mouthparts called mandidbles are specialised for for piercing, cutting, chewing etc. Other appendages are maxillae (accessory feeding organs), first and second pairs of maxillulae and two pairs of antennae. The body segments, or somites compressed or depressed. Somites organized into head, thorax, and abdomen. The somites are fused in variously diagnostic combinations.
Crustacean have a very characteristic larva, the nauplius. It hatches with only three pairs of appendages, viz. antennules, antennae, and mandibles. In later stages of its development the characteristic crustacean head or cephalon is formed by fusion of two somites. Frequently, thoracic segments fuse with the cephalon to form the cephalothorax, which is covered by the carapace.
The paired appendages of crustaceans consist of two branches, the endopodite and the exopodite, which arise from a peduncle, the protopodite. The protopodite consists of the coxa and the basipodite; the coxa articulated with the body.
Crustaceans take up oxygen through gills, and/or through parts of the body surface. In most crustacean groups sexes are separate.