Retinal differences between light-tolerant and light-avoiding slugs (Mollusca: Pulmonata)

The fine structure of photoreceptors in the eye of a light-tolerant slug, Ariolimax californicus, was compared with that of a nocturnal slug, Limax maximus. There is a marked difference in the distribution of the 800 Å microvesicles (photic vesicles) in the Type I sensory cells: massed distally beneath the microvilli, the light-sensitive organelles, in A. californicus; and aggregated basally near the nuclei in L. maximus. This finding supports our hypothesis that the vesicles are involved in photoreception. Additionally, L. maximus has a unique modification of endoplasmic reticulum in the receptoral process of the Type I sensory cell that is not seen in A. californicus. And the distribution and nature of the pigment granules in the supportive cells differ in the two species of slugs. Type II sensory cells and ganglion cells in A. californicus are described briefly.