Decapod anatomy
Encyclopedia : D : DE : DEC : Decapod anatomy
The body of a decapod crustacean is made up of nineteen body segments grouped into two main body parts, the cephalothorax and the abdomen. Each segment may possess one pair of appendages, although in various groups these may be reduced or missing. They are, from head to tail:
Cephalothorax
Head
- antennules
- antennae
- mandibles
- first maxillae
- second maxillae
The head also bears the (usually stalked)
compound eyes.
Thorax / Pereon
- first maxillipeds
- second maxillipeds
- third maxillipeds
- first pereiopods
- second pereiopods
- third pereiopods
- fourth pereiopods
- fifth pereiopods
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
legs and are also used for gathering food. Those pereiopods which are armed with a claw (
chela) are sometimes referred to as
chelipeds. 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 (
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 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.
Abdomen / Pleon
- first pleopods
- second pleopods
- third pleopods
- fourth pleopods
- fifth pleopods
- uropods
Pleopods (also called
swimmerets) are primarily swimming legs, and are also used for brooding the eggs (except in
prawns). 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. At the end of the pleon is the
tail fan, comprising a pair of biramous
uropods and the
telson, which bears the anus. Together, they are used for steering while swimming, and in the
caridoid escape reaction.
In crabs and some other carcinised decapods, the abdomen is folded under the cephalothorax.
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