Primary sensory neurons and their central projections in the pond snail Lymnaea stagnalis.

We describe the location of primary sensory neurons in the tentacles, lips, pneumostome area and osphradium of the pond snail Lymnaea stagnalis after Nickel-lysine and biocytin dye-filling into the corresponding nerves. In addition, central projections of the osphradial sensory system were determined. Most of the neurons in the osphradial ganglion are primary sensory cells that send processes directly to the CNS. Fibres originating from these neurons are located in the neuropil of all central ganglia except buccal and pedal. Back-filling the CNS from the osphradial nerve showed that only two pairs of neurons in the cerebral ganglia, two neurons in the right parietal ganglion, and one cell in the left pleural ganglion send processes into the osphradium. Back-filling with biocytin showed that only five to seven neurons in the right parietal and visceral ganglia are dye-coupled with osphradial neurons. Such a small number of central neurons projecting to the osphradial sensory system represents a large difference from the tentacular and lip systems, where hundreds of central neurons innervate the pheriphery. We propose that two different systems with either central or peripheral processing of sensory information may be involved in chemoreception in Lymnaea. Our results provide a basis for physiological studies in this field.