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1 The Biology Department, University of Victoria, Victoria, B. C., Canada, and the Station Zoologique, Université de Paris VI, Villefranche-sur-Mer, 06230 France
1. The locomotory cilia of Mangelia and Pneumoderma larvae undergo arrests spontaneously and in response to tactile stimulation. These events are often associated with muscular contractions in an overall response thought to be protective in nature.
2. Isolation of the ciliated bands from the central nervous system abolishes the ability for coordinated ciliary arrests and the cilia show continuous metachronal beating.
3. Recordings with suction electrodes attached to the surface show patterns of electrical signals during periods of ciliary arrest. Intracellular recordings with glass microelectrodes from single ciliated cells in Pneumoderma show rapidly rising, slowly decaying, all or none 50 mV spikes when the cilia undergo arrest. There are no fluctuations in membrane potential during metachronal beating.
4. The existence of a rich motor innervation supplying the ciliated epithelium in Mangelia has been established using optical and electron microscopy. The nerve endings appear to derive from neurons whose cell bodies are located in or near the central nervous system. The evidence for a local system of neurons forming a nerve net, as described by some authors, is not supported by the present work.
5. Under the electron microscope, neurociliary synapses have been identified. Each ciliated cell in the preoral band of Mangelia probably receives at least one synapse. These junctions presumably mediate the arrest response. Synapses are characterized by small, clear presynaptic vesicles in the range 335-560 Å. Similar junctions are made with muscle cells and with supporting (presumed secretory) cells which lie adjacent to the ciliated cells.
6. Neurites containing dense-cored vesicles (560-835 Å) are found near the ciliated cells, but such vesicles are rarely found at synapses and never predominate in them. Taken in conjunction with findings from other gastropods, this observation appears to complement existing pharmacological evidence for an excitatory role for serotonin in molluscan veligers. Comparisons with the dual system of ciliary control found in lamellibranch gills are suggested.
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