[Ultrastructure of the osphradium of Viviparus sp. (Mollusca, Prosobranchia)].

By means of scanning and transmissive electron microscopy, osphradial structure has been studied in the freshwater herbivorous mollusc Viviparus sp. Ciliary supporting and microvillous cells are described; they do not form any specific for the osphradium zones on its surface. The number of the villi, belonging to one supporting cell, is within 20-40. Peripheral processes of the receptory subepithelial cells have microvilli and 1-2 kinocilia on their apical surface. Central processes of the primary sensitive receptor cells go into the neuropil of the osphradial ganglion, that is situated in the center of the osphradial torus. Among uni- and multipolar ganglionic cells typical neurosecretory cells with electron opaque granules 200 nm in diameter are revealed. Release of neurosecretion takes place in the neuropil and in the connective tissue of the osphradial ganglion. A suggestion is made on modulating role of neuropeptides in the osphradial ganglion, that enables to change sensitivity of this receptor organ depending on physiological state of the animal and on the environment.