Cytomorphological characters of Polypodium hydriforme—a unique cnidarian, adapted to intracellular parasitism inside oocytes of Acipenseriform fish—are reviewed and analysed. Polypodium's unique characters, those shared with the Myxozoa, and the characters shared with bilateral triploblastic animals are listed. The set of unique features has allowed us to allocate Polypodium to a separate cnidarian class Polypodiozoa Raikova, 1988. The combination of characters displayed by P. hydriforme suggests that this animal is not an aberrant cnidarian, as it was formerly believed, but a relic of a major cnidarian group (class Polypodiozoa) that has undergone reduction due to adaptation to parasitism.
