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Starfish, brittle stars and their relatives are rarely found as complete fossils, since their skeletons consist of loose (calcareous) elements embedded in a leathery skin. In these so-called ‘asteroids’ there is no clear distinction between the arms and the rest of the body. Ophiuroids have a disc-like body to which five narrow and highly movable arms are attached. These arms allow the animals to 'swim' by rapid sweeping movements. Ophiuroids have no anus: all their food enters the body through their mouth (on the lower surface), with undigested matter leaving through the same orifice.