Crabs are decapod crustaceans of the infraorder brachyura, which typically have a very short projecting "tail" (abdomen) (greek: βραχύς, romanized: brachys = short, οὐρά / οura = tail), usually hidden entirely under the thorax. they live in all the world's oceans, in fresh water, and on land, are generally covered with a thick exoskeleton, and have a single pair of pincers. they first appeared during the jurassic period.

Source: Wikipedia