[2] 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.

[3] Due to the variable rates of 18S rDNA sequences, these results were however suggested to be an artifact of long branch attraction,[2] and myxozoans have in the meantime, also been classified within cnidarians.

[4] Evans et al. (2008)[2] 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.

[2] Polypodium hydriforme is an endocellular parasite with an unusual life cycle, a peculiar morphology, and high rates of DNA evolution.

[6] Just prior to host spawning, Polypodium everts to the normal position of cell layers, revealing tentacles scattered along the stolon.

[2] During eversion, the yolk of the host oocyte fills the gastral cavities of the parasite, supplying the future free-living stage with nutrients.

Finally, upon emerging from the host egg in fresh water, the free-living stolon fragments into individual medusoid-like organisms that go on to multiply by means of longitudinal fission.