Crab, any
short-tailed member of the crustacean order Decapoda (phylum
Arthropoda)—especially the brachyurans (infraorder Brachyura), or true
crabs, but also other forms such as the anomurans (suborder Anomura),
which include the hermit crabs. Decapods occur in all oceans, in fresh water, and on land; about 10,000 species have been described.
Unlike those of other decapods (e.g.,
shrimp, lobster, crayfish), crabs’ tails are curled under the thorax,
or midsection. The carapace (upper body shield) is usually broad. The
first pair of legs is modified into chelae, or pincers.
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