Polypodium hydriforme is the only coelenterate adapted to intracellular parasitism in oocytes of acipenserid and polyodontid fishes. It occurs in both the Old and the New worlds, being parasitic in 12 species of Acipenseridae and in 1 species of Polyodontidae. Its earliest parasitic stages are binucleate cells that occur in previtellogenic oocytes. All embryonic and postembryonic development (which seems to be parthenogenetic) up to the budding stolon stage takes place inside fish oocytes and lasts several years. The planula and stolon have inverted germ layers. All parasitic stages are encircled with a highly polyploid unicellular trophamnion that is homologous to the second polar body. Before spawning, eversion of the stolon takes place inside the oocyte. At spawning, the everted stolons get into water and the free-living phase of the life cycle begins. The stolon fragments into individual specimens that can move and feed. They multiply by longitudinal fission (paratomy). In mid-summer they form 2 kinds of endodermal gonads. The so-called "female" gonads (2 ovaria, each with a gonoduct encircled with a common envelope) produce diploid cells that display no meiotic phenomena. The so-called "male" gonads have no gonoducts, but their sex cells undergo 2 meiotic divisions, giving rise to binucleate cells with unequal nuclei. The entire gonad becomes a gametophore with an ectodermal lid carrying nematocysts and containing many binucleate cells. Gametophores can be deposited onto the skin of prelarvae of fishes. How the parasite gets into young fish oocytes is not known.