Films about crabs (infraorder Brachyura), decapod crustaceans which typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax. They are present in all the world's oceans, in freshwater, and on land, often hiding themselves in small crevices or burrowing into sediment. True crabs first appeared in the fossil record during the Jurassic period, around 200 million years ago, achieving great diversity by the Cretaceous period; around 7,000 extant species in 96 families are known. A number of other crustacean groups converged on the body plan of crabs, sometimes closely resembling them; this phenomenon is called carcinisation.