Class and subphylum of the phylum Arthropoda. Crabs, hermit crabs, shrimps, and lobsters together with smaller and less familiar arthropods form some 42000 species. Although the species differ considerably in size and shape, they are all variations on a rather strict design of the body, which consists of somites (segments), that can bear one pair of appendages each. In all crustaceans the head is formed by the first 6 somites, of which the first has no appendages. The trunk is less uniform than the head, with, depending on the species, varying numbers of somites and appendages that are modified for different functions. The first part of the trunk is the thorax, built of 6-11 somites, with appendages that are the actual limbs. The thoracic limbs can be very large with claws or modified for swimming, digging, crawling, carrying eggs, etc. The number of remaining abdominal somites is variable.

Alternative forms for crustacea : crustacean, crustaceans.