Sensory cells in the head skin of pond snails. Fine structure of sensory endings.

Several types of receptor endings were identified with scanning electron microscopy and silver-impregnation techniques in the skin of the tentacles, lips, dorsal surface of the head and mouth region of the pond snails Lymnaea stagnalis and Vivipara viviparus. Sensory endings at the tips of dendrites of primary receptor neurones, scattered below the epithelium, differ in structure, i.e., the endings exposed to the surface of the skin possess different proportions of cilia and microvilli, which vary in number, length, and packing. Type-I endings have microvilli and a few (1-5) cilia, 5-12 micrometers in length. Type-2 endings have abundant (20-40), interwoven long (9-12 micrometers) cilia and random microvilli. Type-3 endings show typical packing of 10-25 cilia in the form of bundles or brushes. They may be composed either of long (9-18 micrometers) or short (2-7 micrometers) cilia, or of both long and short ones. Microvilli here are absent. Type-4 endings have only microville. Two other types of skin receptors do not extend their sensory endings to the surface and can be identified only in silver-stained preparations. Type-5 endings are branching dendrites of skin receptors cells that terminate among epithelial cells. In type-6, the sensory endings also terminate among epithelial cells but thier cell bodies are located outside of the skin. In both species all skin regions examined possess the receptors of all six types differing only in their relative proportion. Possible functional roles of different receptors are discussed.