Structure and Abundance of Receptors of the Unspecialized External Epithelium ofNassarius Reticulatus[Gastropoda, Prosobranchia]

Abstract
The epithelial receptors ofNassariusare primary sense cells. The sensory endings are abundant on the tentacle tips, anterior border of the foot and ventral external surface of the siphon tip. Densities are a small multiple of 1032 endings/mm2.Type 1 ciliated endings are of ubiquitous occurrence. The ultrastructure of type 1 endings is uniform, though the static and motile bundles of sensory cilia may be distinguished by the random or uniform orientation of the ciliary subfibrils and basal feet. Ciliated receptors probably include both chemo- and mechanoreceptors.A distinct ultrastructural type of sensory ending (type 2), confined to the siphon tip, consists of a small pit in the peripheral dendrite close to the epithelialsurface. The pit contains whorled microvilli and a few cilia. It may be photoreceptive.A small amount of convergence of sensory information may occur in peripheral nerve.