Crab is a beast of decapod crustaceans from Infraorder Brachyura, which is known to have a "tail" is very short (Greek: brachy = short, ura = tail), or the belly (abdomen) was hidden under the chest (thorax). Crab's body is protected by the outer frame of a very hard, composed of chitin, and armed with a pair of tweezers. Crabs and small crab is also included in the Infraorder Brachyura.
Crabs found in all oceans of the world. There is also a freshwater crabs and land, particularly in areas tropis.Kepiting varied in size, from crab beans, which only a few millimeters wide, until the crab Japanese spider, with leg spans up to 4 m.
True crabs have five pairs of legs; the first pair of legs modified into a pair of claws and are not used to moving. In almost all kinds of crabs, except a few only (eg, Raninoida), abdomen folded under the cephalothorax. Crab's mouth parts are covered by a flat maxilliped, and the front of the carapace does not form a long rostrum. Formed from crab gill plates are flat ( "phyllobranchiate"), similar to the gills of shrimp, but with a different structure.