Subphylum Crustacea Brünnich, 1772
The crustaceans are among the arthropods the most diverse assemblage, although not reaching the degree of differentiation of the insects. Their importance in freshwater and marine habitats can be compared with the insects on land. Most are marine, but there are many freshwater species and there have been a number of invasions of the terrestrial environment. About 75.000 species of crustaceans are known. Crustaceans are animals with segmented, chitin-encased bodies and articulated appendages. They typically possess five pairs of head appendages: antennules, antennae, gnathobasic mandibles, maxillules and maxillae. The mouthparts called mandibles are specialized for piercing, cutting, chewing etc. The other appendages are maxillae (accessory feeding organs), first and second pairs of maxillulae and two pairs of antennae. The body segments, or somites are compressed or depressed. The somites are organized into head, thorax, and abdomen. The anus is located on the posterior anal somite or telson, which often bears caudal rami. The somites are fused in variously diagnostic combinations. Crustaceans have a tendency to fuse the segments of the head to form a cephalic shield and to develop from the posterior aspect of the cephalon a posteriorly directed shield over the body (a carapace). Crustaceans usually have a median naupliar eye and a pair of lateral compound eyes. The excretory system normally consists of antennal and/or maxillary glands.
Crustaceans 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.
The following classes belong to the crustaceans: Remipedia (nectiopods), Malacostraca (crabs, lobsters, shrimps, woodlice, etc.), Phyllopoda (fairy shrimp, water fleas, etc.) and Maxillopoda (ostracods, copepods, barnacles).
based on Fransen, 1995