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''Polypodium hydriforme'' is a species of parasite attacking the eggs of sturgeon and similar fishes (Acipenseridae and Polyodontidae). It is one of few metazoans living inside the cells of other animals. It is also the only known intracellular cnidarian parasite.〔 ''Polypodium hydriforme'' is the only species in the genus ''Polypodium'' (monotypic genus). It is also the only species and genus within the whole family Polypodiidae. == Taxonomy == Unusual characteristics have led to much controversy regarding its phylogenetic position within metazoans.〔 ''Polypodium'' has traditionally been considered a cnidarian because it possesses nematocysts, the stinging structures characteristic of this phylum.〔 However, molecular phylogenetic studies using 18S rDNA sequence data have challenged this interpretation, and have indicated that ''Polypodium'' is a close relative to myxozoans and that together they share a closer affinity to bilaterians than cnidarians (Zrzavý & Hypša 2003).〔Zrzavý J. & Hypša V. (2003). "Myxozoa, ''Polypodium'', and the origin of the Bilateria: The phylogenetic position of "Endocnidozoa" in light of the rediscovery of ''Buddenbrockia''". ''Cladistics'' 19(2): 164–169. 〕 Due to the variable rates of 18S rDNA sequences, these results have been suggested to be an artifact of long branch attraction.〔 Evans et al. (2008)〔 have performed phylogenetic analyses of metazoans with 18S and partial 28S rDNA sequences in a large dataset that includes ''Polypodium'' and a comprehensive sampling of cnidarian taxa. This support the placement of ''Polypodium'' within Cnidaria.〔 This accords with the fact that ''Polypodium'' possesses nematocysts and a cnidarian-like body plan.〔 The ML combined analysis places ''Polypodium'' as its own class Polypodiozoa.〔 By contrast, the combined parsimony analysis and the ML analyses of 28S alone place ''Polypodium'' within the hydrozoan clade Leptothecata.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Polypodium hydriforme」の詳細全文を読む スポンサード リンク
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