Abstract
Given equal choices of light (40 foot candles) and dark (0.13 foot candles) visual fields (elevation 30°, width 45°), the Moroccan land snail, Otala lactea, Mu¨ll., moves towards and enters dark fields. The wider the dark sector for a constant height, the more likely is the entry. Probability of entry changes monotonically proportional to width. Presented with a 45° dark sector in a surrounding ring of light, intact O. lactea differ highly significantly in locomotor behaviour from blinded specimens. They do not differ from specimens unilaterally blinded nor with crushed anterior tentacles. Anterior tentacles appear to have no photosensory function in this experimental setting. Camera lucida drawings of typical paths suggest O. lactea can orient with respect to a target as small as 221/2° × 30° at 25 cm distance. Pertinent literature is reviewed and discussed with emphasis on the issue of uncertainty in photic responses of several species of land snail.
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