[Electron microscopic study of presumptive photoreceptor cells in the aboral organ of the ctenophore, Beroë cucumis].

Presumptive photoreceptor cells are located in the receptor epithelium of the aboral organ and exhibit some common morphological characters with flagellar photoreceptors of some invertebrates, as well as with photoreceptors of vertebrates. These cells are specifically distinguished by the presence of a special membrane structure in their epinuclear zone, this structure being the derivative of a cilium. Presumptive photoreceptor cells which have a short central projection and form in their basal part the synaptic contacts ("triads") with the adjacent nervous endings, may be classified as pseudosecondary receptor cells. Acetylcholinesterase was found in the synaptic regions. Cytoplasm of the presumptive photoreceptor cells is rich in granular vesicles which contain biogenic amines.