1. Four British species of Littorina, viz. L. littoralis (L.), L. saxatilis (L.), L. littorea (L.) and L. neritoides (L.), responded when crawling on horizontal surfaces in air and in sea water to plane polarized light incident from above. Photonegative winkles crawled parallel to the plane of vibration (e vector) and photopositive molluscs at right angles to the plane of vibration.
2. The investigations with L. neritoides confirmed the observations of Fraenkel (1927) on the reversal of phototaxis from photonegative to photopositive when this winkle was crawling in the inverted position immersed in sea water. This change of response was considerably influenced by the sea-water temperature, the optimum temperature being within the range from 10° to 12° C.
3. Examination of the eye failed to reveal birefringent structures. It is suggested that the mechanism of analysing plane polarized light is a simple reflexion/refraction phenomena based on Fresnel's laws of refraction of polarized light, the minimum amount of light being refracted into the eye when the animal is crawling parallel to the plane of vibration.