Two morphological sub-systems within the olfactory organs of a terrestrial snail

In the present work, we have re-visited the problem of the olfactory neural system organization in the terrestrial snail. By staining the tentacle's nerves and their intrinsic tracts in different points of the cerebral ganglia–tentacles system we have found that the relatively small part of the primary sensory neurons from the sensory pad (7–8%) send their axons directly to the cerebral ganglia. The axons terminated in the metacerebral neuropil which suggests these receptors being not chemosensory but rather mechanosensory neurons. Majority of the primary sensory neurons are synaptically switching in the areas outside the cerebral ganglia, i.e. digits, glomeruli, tentacular ganglion. No primary sensory neurons of the olfactory pad were projecting directly to the procerebrum — the putative centre of olfactory information processing. The afferent tract innervating the procerebrum neuropil originated from the interneurons located in the tentacle ganglion and its digits. Our results suggest the presence of two different sub-systems within the snail nose – mechanosensory and chemosensory – with two different projection targets.