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The anatomy of a decapod consists of 20 grouped into two main body parts: the cephalothorax and the pleon (abdomen). Each segment – often called a somite – may possess one pair of , although in various groups these may be reduced or missing.
Maxillipeds are appendages modified to function as mouthparts. Particularly in the less advanced decapods, these can be very similar to the pereiopods. Pereiopods are primarily walking arthropod leg and are also used for gathering food. They are also the ten legs from which decapods take their name. Those pereiopods which are armed with a claw (chela) may be referred to as chelipeds. The moveable fingers of a claw are known as Dactylus. The pereiopods bear the sexual organs, which are the third pereiopod in the female and the fifth pereiopod in the male. Each appendage from the second maxilla to the fifth pereiopod also bears a gill. In the Anomura ( and related animals), the fifth pair of pereiopods is often hidden inside the branchial chamber, where they are used to clean the gills. The cephalothorax is covered by a carapace which protects the internal organs and the gills; the section of the carapace that projects in front of the eyes is called the rostrum.
Pleopods (also called swimmerets) are primarily swimming legs, and are also used for brooding the eggs (except in Dendrobranchiata), catching food (then swept to the mouth), and can sometimes bear their own gills. In some taxa, the first one or two pairs of pleopods are specialised in the males for fertilisation, and are referred to as the gonopods.
In and some other carcinisation decapods, the abdomen is folded under the cephalothorax.
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