Our results suggest that freshwater pulmonates like Lymnaea stagnalis, Radix peregra, Physa,fhntinalis, and Planorbarius corneus have inherited froin 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 PI. 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.