[Cerebral division of the visual sensory system in pulmonate mollusks]

The distribution of receptor cell axons and location of neurons sending their axons to the optic nerves were revealed in cerebral ganglia of pulmonate molluscs Helix pomatía and Lymnaea stagnalis by the method of cobalt chloride injection. Afferent fibers of the optic nerves give terminal branches dorsally in the central area of the cerebral ganglion. Some of them pass through the comissure to the symmetrical area of the opposite cerebral ganglion. Neurons innervating eyes are situated in several areas of both cerebral ganglia. In Lymnaea stagnalis they are located near the optic nerve entrance, in the comissural area, in the mesocerebrum and the posterodorsal part of the ganglion. In Helix pomatía these neurons were found in the same areas except the posterior part of the ganglion. In electrophysiological experiments neuronal responses to the adequate stimulation of the eyes were recorded in the mentioned areas of the cerebral ganglia. The variable character of the responses and the presence of cells indifferent to photic stimulation demonstrate the functional diversity of these zones. On the basis of these data it is suggested that there are no morphologically distinct visual centres in the cerebral ganglia of pulmonate molluscs.