Crabs are decapod crustaceans living in any of the world's oceans, in fresh water, or on land, generally covered with a thick exoskeleton and armed with a single pair of claws. True crabs belong to the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax. Many other animals with similar names – such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs and crab lice – are not true crabs.[Source]