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The decapods or Decapoda are a group of crustaceans within the class Malacostraca. They are usually considered to constitute an order, though some authorities may regard them as a subclass or suborder.
The decapods include many familiar groups of crustaceans, including crabs, lobsters and shrimps, but also some families that are less well known.
As their name implies, all decapods have ten legs; these are the last five of the eight pairs of thoracic appendages characteristic of crustaceans. The front three pairs function as mouthparts and are generally referred to as maxillipeds. However, in many decapods, one pair of legs has enlarged pincers; these are called chelae, so those legs are called chelipeds.
Classification within the order Decapoda is currently under debate. Most older authorities divide them into two suborders, Natantia, the swimming decapods (basically, shrimps), and Reptantia, the walking decapods (crabs etc). A more recent scheme, however, makes use of the structure of the gills and legs, and the way in which the larvae develop. This gives rise to two suborders Dendrobranchiata and Pleocyemata. Some of the shrimps (including the common North American shrimps in genus Penaeus, such as the white shrimp P. setiferus) are placed within Dendrobranciata, while Natantia, containing the rest of the shrimps, becomes an infraorder within Pleocyemata (and is sometimes renamed Caridea). On this scheme, Pleocyemata also contains a number of other infraorders such Brachyura, the true crabs.
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