In Helix aspersa the ‘cuticle’ on the free surface of the external epithelial cells of the optic tentacles has been shown to consist of a layer of microvilli. Microvilli are also present in the same species on the free cell borders of the body-wall, and in the slug Arion hortensis, on the outer cell surfaces of the external epithelium.
In all three cases the microvilli are arranged in a hexagonal pattern. There are indications that branching may possibly occur. The microvilli have granular cores with cross- and longitudinal-striations and there are fibrillar connexions between their tips. On the tentacular and body surfaces of H. aspersa, the microvilli increase the surface area 15 and 12 times, respectively. On A. hortensis the increase in surface area is only 4 times.
In H. aspersa, beneath the microvilli on the tips of the optic tentacles there is a layer, about 3 to 4 µ deep, composed of vertical, horizontal, and tangential fibres. Some of these fibres are attached to lamellar bodies, which may have a lipid content. Granules are also found among the fibres. Further, a greater depth of cuticle is found to be present on the tips of the inferior tentacles of H. aspersa than on their sides; this seems to indicate that a fibrillar layer, similar to that on the optic tentacles, may lie beneath the cuticle of microvilli on the tips of the inferior tentacles. A thicker cuticle is also found on the tips of the optic tentacles in other stylommatophoran pulmonates. It has not been found possible to ascertain whether the fibrillar layer is intracellular or extracellular, although the evidence points to the latter.
Histochemical tests indicate that mucopolysaccharide is present on the surface of the cuticle. Electron micrographs show a granular precipitate caught on and between the fibrillae connecting the tips of the microvilli. It is suggested that the function of the microvilli is to hold the mucous secretions on the body-surface, which would give protection to the animals.
Caveolins in Epidermal Lamellar Bodies: Skin Is an Interactive Interface, Not an Inflexible Barrier