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Each somite, or body segment can bear a pair of appendages: on the segments of the head, these include two pairs of antennae, the mandibles and maxillae ; the thoracic segments bear legs, which may be specialised as pereiopods ( walking legs ) and maxillipeds ( feeding legs ).
The abdomen bears pleopods, and ends in a telson, which bears the anus, and is often flanked by uropods to form a tail fan.
The number and variety of appendages in different crustaceans may be partly responsible for the group's success.
Crustacean appendages are typically biramous, meaning they are divided into two parts ; this includes the second pair of antennae, but not the first, which is uniramous.
It is unclear whether the biramous condition is a derived state which evolved in crustaceans, or whether the second branch of the limb has been lost in all other groups.
Trilobites, for instance, also possessed biramous appendages.

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