The effects of light and dark on photoreceptors of the brackish-water polychaete annelidNereis Hmnicolawere studied by electron microscopy. Animals dark-adapted for one or two days exhibited well-formed straight microvilli (rhabdomeres) on the sensory cell processes. Continuous illumination of worms for one or two days caused extensive breakdown of the microvilli into vesicles and debris. Thirty minutes to three h of exposure of dark-adapted animals to light produced increasing severity of degradation of photoreceptoral microvilli. Light-adapted worms placed in darkness for one-half to three h showed progressive restoration of the microvilli to the dark-adapted condition. The products of degradation were internalized by both sensory and pigmented supportive cells by phagocytosis and pinocytosis.
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