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one of the larval stages of the tapeworm. The bladder worm parasitizes an intermediate host, which may be any of a number of invertebrates and vertebrates. The bladder worm resembles a bladder or sac; it contains one or more fully formed everted tapeworm heads. These heads develop into adult tapeworms in the body of the final human or vertebrate animal host. The four types of bladder worms are the cysticercoid, with one head and a caudal appendage equipped with six hooks; the cysticercus, with one head but no appendage; the coenurus, with many heads; and the hydatid, which contains daughter bladders with heads. The bladder worm must be swallowed by the final host in order to develop.