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The anatomy of adecapod consists of 20body segments grouped into two main body parts: thecephalothorax and the pleon (abdomen).[1][2] Each segment – often called a somite – may possess one pair ofappendages, although in various groups these may be reduced or missing.[citation needed]
The head also bears the (usuallystalked)compound eyes. The distal portion of a mandible or maxilla which has a sensory function is known as a palp.
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 walkinglegs and are also used for gathering food. They are also the ten legs from which decapods take their name.[2] Those pereiopods which are armed with a claw (chela) may be referred to as chelipeds. The moveable fingers of a claw are known asdactyli. 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 agill. In theAnomura (hermit crabs 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 acarapace which protects the internal organs and the gills; the section of the carapace that projects in front of the eyes is called therostrum.
Pleopods (also called swimmerets) are primarily swimming legs, and are also used for brooding the eggs (except inprawns), 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 thegonopods.
At the end of the pleon is the tail fan, comprising a pair ofbiramousuropods and thetelson, which bears the anus. Together, they are used for steering while swimming, and in thecaridoid escape reaction.
Incrabs and some othercarcinised decapods, the abdomen is folded under the cephalothorax.
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