ИСТИНА

Nudibranchia is a group of shell-less heterobranch gastropods characterized by high-specialized chemosensory cephalic organs – rhinophores. While the morphology of rhinophore use as taxonomic characters the external structures of the rhinophore have been described for many species. However, their internal morphology studied fragmentarily for several species. Herein, for the first time, the external and internal morphology of the Onchidoris muricata (Doridina) rhinophore is studied in details. The collection of material was carried out in the Kandalaksha Bay of the White Sea near the White Sea biological station MSU (Russia). Rhinophores were studied using light, scanning and transmission electron microscopy, micro-CT and confocal laser scanning microscopy as well as 3D reconstruction based on series of semithin sections. Our studies are confirmed the presence of a thick nerve that located along the lymphatic canal, as well as the muscular system and spicule as was described for several nudibranch species. Nevertheless, we identified the main features that relate mostly to the rhinophore spicular complex. Spicules of the rhinophores significantly differ from the spicules of their body by size and shape however similar in intracellular synthesis in vacuole of sclerocyte. The spicule complex of monoaxones and multiaxones looks like a bonfire from a log cabin. A framework of four spicules located along each rhinophore extends around the central part of the rhinophore. The rows of spicules are circumferentially displaced relative to each other. Based on our data the mechanism of contraction and relaxation is supposed. Contraction of the rhinophore occurs with the participation of the retractor muscles, the relaxation is passive due to the injection of hemolymph.