[Structural characteristics of nematocyst stinging threads from the parasitic cnidarian Polypodium hydriforme].

The structure of discharged nematocyst stinging threads present in free-living individuals of Polypodium hydriforme was studied by scanning and transmission electron microscopy. Not all cnidae of P. hydriforme proved to be atrichous isorhizas (as previously was accepted), but only one of the five nematocyst categories studied. A unique feature of P. hydriforme nematocysts was revealed: their stinging threads possess two strands of spines, rather than 5 or 3, as in Narcomedusae and other Cnidarians, respectively. This fact supplements the evidence in favour of P. hydriforme being a rather isolated branche in the phylogenetic tree of Cnidaria.