Crabs are decapod crustacean animal member of infraordo brachyura, which is known to have a "tail" which is very short (Greek: brachy = short, ura = tail), or the stomach (abdomen) was hidden under the chest (thorax).
Crab's body is protected by a very hard outer skeleton, composed of chitin, and armed with a pair of tweezers. Into brachyura including crab, crayfish, and small crab. Crabs found in all oceans of the world. There are also freshwater and terrestrial crabs, particularly in tropical regions.
True crabs have five pairs of legs; the first pair of legs is modified into a pair of tweezers and not used to moving. In almost all types of crabs, except a few (eg, Raninoida), abdomen folded under the cephalothorax.
Crab's mouth parts are covered by the maxilliped is flat, and the front of the carapace does not form a long rostrum. Crab gill plates are formed from a flat (phyllobranchiate), similar to the gills of shrimp, but with a different structure.
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