Polypodium hydriforme
Polypodium is a genus of cnidarians that parasitizes in the eggs of sturgeon and similar fishes (Acipenseridae and Polyodontidae). It is one of the few metazoans (animals) that live inside the cells of other animals. Polypodium hydriforme is the only species of this monotypic genus. The parent family (Polypodiidae), order (Polypodiidea) and class (Polypodiozoa) are also monotypic. Taxonomy Unusual characteristics have led to much controversy regarding the phylogenetic position of Polypodium within metazoans. Polypodium has traditionally been considered a cnidarian because it possesses nematocysts, the stinging structures characteristic of this phylum. Molecular phylogenetic studies using 18S rDNA sequence data temporarily challenged this interpretation, by finding that Polypodium is a close relative to myxozoans and suggesting that together they share a closer affinity to bilaterians than cnidarians. Due to the variable rates of 18S rDNA sequences, these results were however suggested to be an artifact of long branch attraction, and myxozoans have in the meantime, also been classified within cnidarians. Evans et al. (2008) 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 supports the placement of Polypodium within Cnidaria. This accords with