A large and diverse group containing crabs, lobsters, and shrimps. The carapace is fused dorsally to all thoracic segments, extending to the base of the legs and enclosing the gills, this forming a gill chamber. The maxilla carries an exopod, the scaphognathite, that pumps water out of the branchial chamber. Eight pairs of thoracic appendages, the first three modified as maxillipeds. Fertilization usually through spermatophores.