Ultrastructure of sensory cells on the mantle tentacles of the gastropod Notoacmea scutum.

Previous studies have indicated that the mantle margin of the gastropod mollusc Notoacmea scutum is sensitive to chemical, photic, and mechanical stimulation. Here, the ultrastructure of sensory cells on the mantle tentacles of N. scutum is examined by transmission electron microscopy to determine if morphological types of sensory cells can be correlated with known sensory capabilities. The sensory cells of the mantle tentacles are found to be ciliated, primary receptors with subepithelial nuclei. The ciliated sensory endings are concentrated at the tip of the tentacles, but also occur in smaller numbers along the shaft. Ultrastructural differences between cilia form the basis of distinguishing two types of sensory ending. Type 1 sensory endings, which are over 90% of the endings, bar unusual cilia that typically are filled with an electron-dense material. Type 2 sensory endings bear cilia that have a 9 + 2 arrangement of longitudinal elements and thus more closely resemble previously reported sensory cilia of molluscs.