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By their stolonal parasitic budding stage and their cnidome, the Polypodiozoa seem to present some affinities with the Narcomedusae, to which they were previously associated. P. hydriforme was, until recently, the only known metazoan adapted to an intracellular parasitic life.
("Polypodium hydriforme." Grzimek's Animal Life Encyclopedia. The Gale Group, Inc, 2005. Answers.com 15 Mar. 2010. http://www.answers.com/topic/polypodium-hydriforme)
Polypodium hydriforme is a parasite with an unusual life cycle and peculiar morphology, both of which have made its systematic position uncertain. Polypodium has traditionally been considered a cnidarian because it possesses nematocysts, the stinging structures characteristic of this phylum. However, recent molecular phylogenetic studies using 18S rDNA sequence data have challenged this interpretation, and have shown that Polypodium is a close relative to myxozoans and together they share a closer affinity to bilaterians than cnidarians. Due to the variable rates of 18S rDNA sequences, these results have been suggested to be an artifact of long-branch attraction (LBA). A recent study, using multiple protein coding markers, shows that the myxozoan Buddenbrockia, is nested within cnidarians. Polypodium was not included in this study. To further investigate the phylogenetic placement of Polypodium, we 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.
(Evans, et al. "BMC Evolutionary Biology: Phylogenetic placement of the enigmatic parasite, Polypodium hydriforme, within the Phylum Cnidaria." Volume:8. Issue:139. <http://www.biomedcentral.com/1471-2148/8/139> 03.15.2010.)