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Crustacea

The crustacean body is segmented and organised into distinct regions. The segments, or somites, are typically compressed or depressed to a varying degree, but the dorsal tergum, the ventral sternum and the lateral pleuron are usually recognisable. The body wall is basically chitinous and is most usually reinforced by calcium carbonate to form a rigid exoskeleton. Growth involves periodic moulting of the exoskeleton, involving resorption of calcium salts, and their redeposition in the new-formed outer chitinous skeleton.
The body regions comprise a head, thorax (or pereon, mesosome, pereion or peraeon) and abdomen (or pleon or metasome) and an additional tailpiece, the telson. In many taxa a number of the anterior thoracic segments is fused with the head. Although in this case the term head is normally accepted, the name cephalothorax is strictly more correct. In many groups a fold derived from the head extends posteriorly to form a carapace, which covers most or the entire thorax.
Primitively, each body segment bears a pair of biramous appendages.
In its simplest form the crustacean head has 3 pairs of appendages: the antennule or antenna 1, the antenna or antenna 2, and the mandibles. The head is typically fused with the 2 anterior thoracic segments, the appendages of which are modified as accessory feeding structures, the maxillule or maxilla 1 and the maxilla or maxilla 2. In front of and behind the mouth there are the variously developed upper (the labrum) and lower processes (the labium). The labrum is developed from the body wall and the labium is developed from that part of the foregut forming the posterior and ventral margin of the mouth.
Thoracic and abdominal appendages show a wide range of modification throughout the crustaceans as a whole and are descried by the different taxa, but basically the limb consists of 2 segmented rami, the exopodite and the endopodite. They rise from a peduncle of 2 articles, the coxa and the basis. Simple biramous limb structure persists in some crustaceans, but in most groups the exopodite comprises the major functional unit of the limb, while the endopodite is reduced or lost or adapted to serve a separate function.

Sexes are separate, with the exception of the sessile Cirripedia, in which hermaphroditism is generally the rule. The characteristic larval type is the nauplius; it occurs as the first free-swimming larval stage in many crustaceans, while in others it occurs only in the earliest, brooded stages of development. A succession of often very specialised larval instars occurs in the later development of most crustaceans.

Source: Hayward and Ryland.