Crabs are decapod crustaceans of the infraorder Brachyura, which usually have a really short projecting "tail" usually hidden entirely under the thorax. They sleep in all the world's oceans, in water, and ashore are generally covered with a thick exoskeleton and have one pair of pincers. Many other animals with similar names – like hermit crabs, king crabs, porcelain crabs, horseshoe crabs, and crab lice – aren't true crabs. They were previously thought to be a monophyletic group, but are now believed to represent a minimum of two distinct lineages, one within the Old World and one within the New World. Crabs often show marked sexual dimorphism. Females can store the sperm for an extended time before using it to fertilizer their eggs