Chiton integument: ultrastructure of the sensory hairs of Mopalia muscosa (Mollusca: Polyplacophora).

The dorsal integument of the girdle of the chiton Mopalia muscosa is covered by a chitinous cuticle about 0.1 mm in thickness. Within the cuticle are fusiform spicules composed of a central mass of pigment granules surrounded by a layer of calcium carbonate crystals. Tapered, curved chitinous hairs with a groove on the mesial surface pass through the cuticle and protrude above the surface. The spicules are produced by specialized groups of epidermal cells called spiniferous papillae and the hairs are produced by trichogenous papillae. Processes of pigment cells containing green granules are scattered among the cells of each type of papilla and among the common epidermal cells. The wall or cortex of each hair is composed of two layers. The cortex surrounds a central medulla that contains matrix material of low density and from 1 to 20 axial bundles of dendrites. The number of bundles within the medulla varies with the size of the hair. Each bundle contains from 1 to 25 dendrites ensheathed by processes of supporting cells. The dendrites and supporting sheath arise from epidermal cells of the central part of the papilla. At the base of each trichogenous papilla are several nerves that pass into the dermis. Two questions remain unresolved. The function of the hairs is unknown, and we have not determined whether the sensory cells are primary sensory neurons or secondary sensory cells.