  crabs are decapod crustaceans of the infraorder brachyura, which typically have a very short projecting "tail" (abdomen) ( = short,  = tail), usually entirely hidden 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 claws. many other animals with similar names â such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs and crab lice â are not true crabs. ==evolution==  crabs are generally covered with a thick exoskeleton, composed primarily of calcium carbonate, and armed with a single pair of chelae (claws). crabs are found in all of the world's ocean
Commercial_crustaceans Crabs Edible_crustaceans