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Structure of nematocysts from Polypodium hydriforme was studied by scanning and transmission electron microscopy. Two nematocyst types were observed: holotrichous isorhizas with minute spines, and true atrichous isorhizas. Among holotrichous isorhizas four morphological categories were distinguished based on differences in morphology of their discharged stinging tubules. The cnidom of P. hydriforme possesses no specialized penetrant types. Instead, Polypodium uses holotrichous isorhizas as penetrants during prey capture. Stinging tubules of holotrichous isorhizas display two helical rows of tiny spines in the discharged state and possess two helical folds in the undischarged state, thus corresponding to double-helical symmetry. Double-helical symmetry has never been described for nematocysts of any cnidarians, thus being a peculiar feature of P. hydriforme.

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