Oksana Tuchina

• The Mollusca are a large and extremely varied animal phylum and especially species of the class Gastropoda are popular subjects for neurobiological research. Eyes of gastropods have already received a great deal of attention, but their central visual pathways are still largely unknown for most of the species. Investigation in this direction should be extremely useful for our understanding of the structural basis of vision and behavioural reactions in gastropods. The aims of this thesis were, therefore, to investigate the central visual pathways and possible neurotransmitters in two species of freshwater pulmonate gastropods: Lymnaea stagnalis and Planorbarius corneus. We discovered that: 1) the central visual pathways in L. stagnalis and P. corneus are extensive, neurons and fibres present in all ganglia except the buccal ones; 2) the distribution of the identified cells in both species is similar; 3) processes of photoreceptors form a dense bundle of many fine axons, i.e., the optical neuropil; 4) the two eyes are connected through the cerebral ganglia; 5) some identified central neurons are probably polymodal; 6) the central pathways of pallial and intestinal nerves are widespread and include projections to the optic nerves as well; 7) structural connections between visual pathways and neurons of the pedal ganglia, as well as the statocysts are present; 8) the optic nerve in L. stagnalis enters the cerebral ganglion under a common sheath with tentacular and small labial nerves and that these three nerves exchange some fibres; 9) serotonin and FMRF-amide are present in the eyes and in the optic nerves of both species and likely perform efferently controlled modulation of the adaptational state of the retinas; 10) the chemical nature of the connection of two eyes in both species is neither serotonin nor FMRF-amide. The obtained results are analyzed and discussed in the light of data available on some known behavioral reactions in these species.