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The Myxozoa (etymology: Greek: μύξα ''myxa'' "slime" or "mucus" + thematic vowel o + ζῷον ''zoon'' "animals") are a group of parasitic animals of aquatic environments. Over 1300 species have been described〔()〕 and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. The average size of a myxosporean spore usually ranges from 10 μm to 20 μm〔Fiala, Ivan. 2008. Myxozoa. Version 10 July 2008 (under construction). http://tolweb.org/Myxozoa/2460/2008.07.10 in The Tree of Life Web Project, http://tolweb.org/〕 whereas that of a malacosporean spore can be up to 2 mm. Myxozoans can live in both fresh and marine habitats. Early thought to be unicellular, they are in fact multicellular organisms, probably extremely reduced cnidarians.〔http://www.haaretz.com/israel-news/science/1.687159〕〔Fiala, Ivan. 2008.〕 Infection occurs through valved spores. These contain one or two sporoblast cells and one or more polar capsules that contain filaments which anchor the spore to its host. The sporoblasts are then released as a motile form, called an amoebula, which penetrates the host tissues and develops into one or more multinucleate plasmodia. Certain nuclei later pair up, one engulfing another, to form new spores. Despite their inclusion in Cnidaria, the World Register of Marine Species still classifies myxozoa as a separate phylum under Animalia. ==Phylogenetics== The Myxozoa were originally considered protozoan,〔 〕 and were included among other non-motile forms in the group Sporozoa.〔(Edwin Lanfranco, 2007, ''A phylogenetic classification of organisms other than animals'' ).〕 As their distinct nature became clear through 18S ribosomal DNA (rDNA) sequencing, they were relocated in the metazoa. Further classification was hindered by conflicting evidence: although 18S rDNA suggested an affinity with Cnidaria,〔 〕 other rDNA sampled,〔 〕〔 〕 and the HOX genes of two species,〔 〕 were more similar to those of the Bilateria. The discovery that ''Buddenbrockia plumatellae'', a worm-like parasite up to 2 mm in length, is a myxozoan〔 initially appeared to strengthen the case for a bilaterian origin, as the body plan is superficially similar. Nevertheless, closer examination reveals that ''Buddenbrockias longitudinal symmetry is not twofold, but fourfold, casting doubt on this hypothesis. Further testing has sourced the first three HOX genes found in previous research (''Myx1-3'') to the bryozoan ''Cristatella mucedo'', and the fourth (''Myx4'') to Northern pike. This explained the confusion: the original experiments had used samples contaminated by tissue from host organisms, leading to false positives for a position among the Bilateria. More careful cloning of 50 coding genes from ''Buddenbrockia'' established the clade as severely modified members of the phylum Cnidaria, with medusozoans as their closest relatives. Similarities between myxozoan polar capsules and cnidarian nematocysts had been drawn for a long time, but were generally assumed to be the result of convergent evolution.〔 〕 Taxonomists now recognize the outdated subgroup Actinosporea as a life-cycle phase of Myxosporea.〔Kent, M. L., Margolis, L. & Corliss, J.O. (1994). "The demise of a class of protists: taxonomic and nomenclatural revisions proposed for the protist phylum Myxozoa Grasse, 1970." ''Canadian Journal of Zoology'' 72(5):932-937.〕 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「myxozoa」の詳細全文を読む スポンサード リンク
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