Cnidocyst nomenclature is based on the structure of the tubule and its armature as viewed in the light microscopy (LM). Investigations utilising optically improved LM and scanning electron microscopy have revealed some errors in the interpretation of the fine structure of tubules and armature of nematocysts. Categories of nematocyst have been modified, therefore, to incorporate observations made with new visualising techniques. Isorhizas are defined as nematocysts whose tubule is of uniform or nearly uniform thickness proximal to the midpoint, while b-mastigophores are nematocysts with a rod-shaped encapsulated shaft and a prominent armature on the everted shaft. The category of amastigophores is retained but redefined as p-amastigophores for the V-shaped notch at the base of encapsulated shaft. The trirhopaloids are found to be similar to the birhopaloids. Mesobasic is redefined as an intermediate length of discharged shafts between micro- and macrobasic. Astomocnidae is redefined as entangling nematocysts and stomocnidae as penetrants.