[Distribution of the afferent elements in the central nervous system of the pond snail (Lymnaea stagnalis L.)].

To study distribution of afferent fibres of all nerves of the cerebral ganglia in the central nervous system (CNS) of the pond snail Lymnaea stagnalis, CoCl2 and horseradish peroxidase solutions were administered. In the CNS certain neurons were revealed which could be considered as primary sensitive according to some signs. It has been stated that the sensitive fibres are situated in neuropil as well formed, definitely localized fasciculi. Most of them terminate immediately in the cerebral ganglion, some - go further through commissures and connectives into the neighbouring ganglia of its own and on the contralateral side of the CNS, sometimes reaching the visceral ganglion. Thus, not only the cerebral, but also all the pleuro-visceral are ganglia can directly get certain sensory information from the primary sensitive receptory cells of the sensory organs and the head skin of the mollusk. The sensitive fibres of every nerve form several clusters - sensitive nuclei, some of them overlapping each other. The CNS neuropil of the L. stagnalis possesses not the dorsoventral differentiation, specific for the truncal brain of all arthropodous forms of the invertebrates, but a nuclear type of the structure peculiar mainly for suprapharyngeal ganglia of the annelides and arthropods.