Abstract. The eyes of aquatic pulmonates differ from those of terrestrial pulmonates; the latter, in species such as Cepaea nemoralis and Trichia hispida, possess conventional, cup‐shaped retinas, but the aquatic species Lymnaea stagnalis, Radix peregra, Physa fontinalis, and Planorbarius corneus have retinas that are partitioned into dorsal and ventral depressions (“pits”). The pits are separated by an internal ridge, called the “crest”, and on account of their pigmentation can be seen in vivo. The dominant cellular components of the retinae of terrestrial as well as aquatic snails are pigmented cells and microvillar photoreceptors, the latter occurring in two morphologically distinct types (I and II). Aquatic snails with preferences for shallow water possess eyes with both type I and type II photoreceptive cells, but Pl. corneus, an inhabitant of deeper water, only has type‐I receptors, supporting an earlier finding that type I cells represent dim‐ and type II cells bright‐light receptors. On the basis of histological and optical comparisons, we conclude that the eyes of L. stagnalis and R. peregra, species that are known to escape and seek temporary refuge above the water surface, are well adapted to function in water as well as air, but that the eyes of P. fontinalis and Pl. corneus are less modified from those of their terrestrial ancestors.

Abstract. Our results suggest that freshwater pulmonates like Lymnaea stagnalis, Radix peregra, Physa fontinalis, and Planorbarius corneus have inherited from their terrestrial ancestors eyes with a spherical, immobile lens with fixed focal‐length optics. Unable to change the dioptric apparatus to form an image under water, modifications to the retina had to occur if sharp vision was required. Computer‐assisted calculations and 3‐D eye reconstructions demonstrate that the photoreceptors in the deeper, ventral pit are in a position to perceive focused images under water. Vision in air, however, would favour photoreceptive cells located in the shallower, dorsal pit. On the basis of histological, ethological, and optical comparisons, we conclude that the eyes in L. stagnalis and R. peregra, species that are known to escape and seek temporary refuge above the water surface, are well adapted to function in water as well as air, but that the eyes in P. fontinalis and Pl. corneus are less modified from those of their terrestrial ancestors. We also conclude that good resolving power may be of greater importance in the aquatic pulmonates than the terrestrial species, since the former have to locate thin, vertical stems of reeds and sedges to ascend in order to reach the surface to breathe.

A comparative analysis of the camera eyes of gastropod pulmonate molluscs Cepaea nemoralis and Cepaea hortensis was carried out on the basis of original and published data. It was demonstrated that the eyeballs and their main components in C. nemoralis and C. hortensis have more similarities in morphological and optical properties than in anatomical parameters.

Gastropod mollusks are convenient model organisms for studying the functioning of the visual system. The purpose of this work is to estimate the value of the optical sensitivity to white light for the camera-like eyes of gastropod mollusks and humans and analyze its effect on the spatial resolving power in two regions of the retina: in the center—for single photoreceptors of the first/second type in a mollusk and single cones in humans—and in the periphery—for single photoreceptors of the first/second type in a mollusk, as well as for single rods/cones and their groups, subject to spatial summation in humans. The methods of histology, light and transmission electron microscopy, morphometry, calculations and methods of statistical analysis are used in the work. In a mollusk, with a fixed pupil area, the value of the optical sensitivity of the eye to white light in the center of the retina for single photoreceptors of the first/second type is 0.5/0.006 μm2·sr and in the periphery of the retina, 0.9/0.009 μm2·sr. In humans, at the minimum and maximum pupil area, respectively, the value of the optical sensitivity of the eye to white light in the center of the retina (foveola) for single cones varies from 0.00053 to 0.028 μm2·sr, and in the periphery of the retina (far periphery) for single rods from 0.011 to 0.575 μm2·sr, for single cones from 0.025 to 1.319 μm2·sr, for the groups of rods from 3859 to 204,094 μm2·sr and for the groups of cones from 2.5 to 131 μm2·sr. The value of the optical sensitivity of the eyes to white light for single photoreceptors of the first/second type in both regions of the retina in a mollusk, as well as for single cones in the center and groups of rods/cones in the periphery of the retina in humans, corresponds to the ambient light conditions during periods of activity and does not affect the spatial resolving power.

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