Decapod anatomy

Thedecapod(crustaceanssuch as a crab, lobster, shrimp or prawn) is made up of 20body segmentsgrouped into two main body parts: thecephalothoraxand the pleon (abdomen).Each segment may possess one pair ofappendages,although in various groups these may be reduced or missing. They are, from head to tail:

Antennules

Pereiopods

Carapace

Pleopods

Telson

Cephalothorax

Head

1. antennules
2. antennae
3. mandibles
4. first maxillae
5. second maxillae

The head also bears the (usually stalked)compound eyes.The distal portion of a mandible or maxilla which has a sensory function is known as apalp.

Thorax / pereon

1. first maxillipeds
2. second maxillipeds
3. third maxillipeds
4. first pereiopods
5. second pereiopods
6. third pereiopods
7. fourth pereiopods
8. fifth pereiopods

Maxillipedsare appendages modified to function as mouthparts. Particularly in the less advanced decapods, these can be very similar to the pereiopods.Pereiopodsare primarily walkinglegsand 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 aschelipeds.The moveable fingers of a claw are known asdactyls.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 crabsand 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 acarapacewhich protects the internal organs and the gills; the section of the carapace that projects in front of the eyes is called therostrum.

Abdomen / pleon

Folded abdomen of a cooked crab severed from its body, showing its pleopods hiding beneath.

1. first pleopods
2. second pleopods
3. third pleopods
4. fourth pleopods
5. fifth pleopods
6. uropods

Pleopods(also calledswimmerets) 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.

Telson

At the end of the pleon is thetail fan,comprising a pair ofbiramousuropodsand thetelson,which bears the anus. Together, they are used for steering while swimming, and in thecaridoid escape reaction.

Incrabsand some othercarciniseddecapods, the abdomen is folded under the cephalothorax.

The anatomy of krill, usingEuphausia superbaas a model.

The pleopods ofEuphausia superba.

Crab moult with carapace lifted, exposinggills.

See also

• Arthropod leg