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A jellyfish-like creature has a neat trick that makes it unique among animals: its anus forms only when it needs to defecate, then disappears without a trace. "That is the really spectacular finding here," says Sidney Tamm of the Marine Biological Laboratory in Woods Hole, Massachusetts, who made the discovery. "There is no documentation of a transient anus in any other animals that I know of." Tamm thinks the discovery might represent an intermediate stage in evolution. In some simple animals, such as jellyfish, the gut has only one opening, which functions as the mouth and anus. It has been known since 1850 that comb jellies - which superficially resemble jellyfish, but belong to a separate group called ctenophores - have a through-gut, with a separate mouth and anus. Some even have more than one anus.