Zhukov, V.V., Borissenko, S.L., Zieger, M.V., Vakoliuk, I.A. and MeyerRochow, V.B. 2006. The eye of the freshwater prosobranch gastropod Viviparus viviparus : ultrastructure, electrophysiology and behaviour. -Acta Zoologica (Stockholm) 87 : 13 -24We used light and electron microscopy to study the retinal organization of the eye of Viviparus viviparus . Electroretinogram (ERG) recordings were used to investigate the electrophysiological responsiveness to flashes of light of varying intensity and colour, behavioural observations were made of phototactic reactions, and optical measurements and calculations related to the path of light rays in the eye were made. The retina contains principally two types of cells: first, photoreceptor cells with both microvilli and cilia, and second, cells, often strongly pigmented, that are supportive in nature. The ERGs obtained were essentially similar in form, amplitude and duration to those known from other gastropods that have exclusively rhabdomeric photoreceptors. Spectral sensitivity curves closely fitted the absorption spectrum of a rhodopsin-like pigment. The spectral sensitivity peak was at 475 nm. Measurements of the refractive indices of the lens gave values of 1.55 for the outer layer and 1.57 for the lens core. None of the snails tested exhibited a 'defensive reflex' and although no preference between light and dark regions was expressed, we nevertheless argue that, on the basis of optical measurements and calculations, the eye of V. viviparus is well-adapted for seeing under water. Our main conclusion is that in the eye of V. viviparus with its 'mixed photoreceptor' cell type, there is an equal probability for microvilli and cilia to function as principal photoreceptive elements.

 Abstract Bobkova, M.V., Tartakovskaya, O.S., Borissenko, S.L., Zhukov, V.V. & Meyer-Rochow, V.B. 2004. Restoration of morphological and functional integrity in the regenerating eye of the giant African land snail Achatina fulica . -Acta Zoologica (Stockholm) 85 : 1-14To determine whether vision returns to its original state following eye removal in Achatina fulica , light and electron microscope examinations, electrophysiological recordings and behavioural tests were carried out on the regenerating snails. Reparative morphogenesis can result in the restoration of the peripheral sense organ even in the absence of complete regrowth of the tentacle, but it can also lead to the formation of aberrant regenerates. We found that anatomically and ultrastructurally the eyes of the 'most normal' regenerates were basically the same as the original eyes. Under normal conditions each eye is composed of a principal and an accessory eye, both sharing a common cornea. The only difference between regenerated and native eyes is the smaller size of the former, as a result of a reduced number of retinal cells. Electroretinographic responses revealed that the molecular mechanism of phototransduction is restored, in principle, but that flicker fusion frequency in the regenerated eye is significantly lower than in the normal eye. The directional movement to a visual stimulus (a black stripe of 45 ° width) had not completely recovered even 6 months after amputation. This suggests that the central projections of the optic nerve had not become fully re-established at the time of testing.