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Tagmata

The head bears two pairs of antennae, the first of which is often biramous (branching into two parts) and the second pair bear exopods (outer branches) which are often flattened into antennal scales known as scaphocerites. The mouthparts consist of pairs each of mandibles, maxillules (second pair of mouthparts) and maxillae. Except for fairy shrimps, malacostracans are the only extant arthropods with compound eyes placed on moveable stalks, although in some taxa the eyes are unstalked, reduced or lost.

Up to three thoracic segments may be fused with the head to form a cephalothorax; the associated appendages turn forward and are modified as maxillipeds (accessory mouthparts). A carapace may be absent, present or secondarily lost, and may cover the head, part or all of the thorax and some of the abdomen. It is variable in form and may be fused dorsally with some of the thoracic segments or occasionally be in two parts, hinged dorsally. Typically, each of the thoracic appendages is biramous and the endopods are the better developed of the branches, being used for crawling or grasping. Each endopod consist of seven articulating segments; the coxa, basis, ischium, merus, carpus, propodus and dactylus. In decapods, the claw is formed by the articulation of the dactylus against an outgrowth of the propodus. In some taxa, the exopods are lost and the appendages are uniramous.

There is a clear demarcation between the thorax and the six or seven-segmented abdomen. In most taxa, each abdominal segment except the last carries a pair of biramous pleopods used for swimming, burrowing, gas exchange, creating a current or brooding eggs. The first and second abdominal pleopods may be modified in the male to form gonopods (accessory copulatory appendages). The appendages of the last segment are typically flattened into uropods, which together with the terminal telson, make up the "tail fan". It is the sudden flexion of this tail fan that provides the thrust for the rapid escape response of these crustaceans and the tail fan is also used in steering. In Leptostraca, the appendages on the telson instead form caudal rami (spine-like protrusions).