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cestoda
Cestoda (Cestoidea) is a class of parasitic flatworms, of the phylum Platyhelminthes. Biologists informally refer to them as cestodes.〔Smyth, J. D. McManus, D. P. The Physiology and Biochemistry of Cestodes. Cambridge University Press 2007 ISBN 978-0521038959〕〔Craig P. Cestode Zoonoses. Nato: Life and Behavioural Sciences. Pub: Ios Pr Inc. ISBN 978-1586032203〕 The best-known species are commonly called tapeworms. All cestodes are parasitic and their life histories vary, but typically they live in the digestive tracts of vertebrates as adults, and often in the bodies of other species of animals as juveniles. Over a thousand species have been described, and all vertebrate species may be parasitised by at least one species of tapeworm.
Humans are subject to infection by several species of tapeworms if they eat undercooked meat such as pork (''Taenia solium''), beef (''T. saginata''), and fish (''Diphyllobothrium'' spp.), or if they live in, or eat food prepared in, conditions of poor hygiene (''Hymenolepis'' or ''Echinococcus'' species).
''T. saginata'', the beef tapeworm, can grow up to 20 m (65 ft); the largest species, the whale tapeworm ''Polygonoporus giganticus'', can grow to over 30 m (100 ft). Species using small vertebrates as hosts, though, tend to be small. For example, vole and lemming tapeworms are only in length, and those parasitizing shrews only .
Tapeworm parasites of vertebrates have a long history: recognizable clusters of cestode eggs, one with a developing larva, have been discovered in fossil feces (coprolites) of a shark dating to the mid- to late Permian, some 270 million years ago.〔("Tapeworm Eggs Discovered in 270-Million-Year-Old Fossil Shark Feces", ''ScienceDaily, 30 January 2013 )〕
== Anatomy ==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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