Decapod

Decapoda is an order of crustaceans in the class Malacostraca. Many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp are in this order. Many decapods are scavengers - they eat dead plants and animals. Crabs are mixed feeders, taking algae and  shellfish such as molluscs. Lobsters eat mostly live prey.

Anatomy
As their name suggests, all decapods have ten appendages. 

The last five pairs of appendages on the front half of the body, the cephalothorax, are used for walking.So called because the head and thorax are fused together. 

In many decapods, the front pair of appendages carry large pinching claws. The claws are called chelae, so these appendages may be called chelipeds.

The front three pairs of appendages on the thorax are used as jaws, and called maxillipeds.

On the abdomen are swimming appendages called pleopods, and the body ends in a tail fan.

Classification
Classification of the order Decapoda depends on the structure of the gills and legs, and the way in which the larvae develop. There are two suborders: Dendrobranchiata and Pleocyemata. Prawns (including many so-called "shrimp", such as the Atlantic white shrimp) make up the Dendrobranchiata. The other groups, including true shrimp, are the Pleocyemata.

The following classification  follows Martin and Davis, with some changes based on more recent structural and molecular studies.x

Order Decapoda Latreille, 1802  (Selection, not everything is listed)
 Suborder Dendrobranchiata Bate, 1888  prawns

 Suborder Pleocyemata Burkenroad, 1963

 Infraorder Caridea Dana, 1852  true shrimp

 Infraorder Achelata Scholtz & Richter, 1995  spiny lobsters, slipper lobsters, and furry lobsters
 Infraorder Astacidea Latreille, 1802  lobsters and crayfish

 Infraorder Anomura MacLeay, 1838

 Superfamily Galatheoidea Samouelle, 1819  squat lobsters

 Superfamily Paguroidea Latreille, 1802  hermit crabs
 Infraorder Brachyura Latreille, 1802  crabs

References

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