PERIPHERAL AND CENTRAL ELEMENTS OF CHEMOS ENSORY SYSTEM IN THE POND SNAIL LYMNAEA-STAGNALIS (GASTROPODA, LIMNAEIDAE)

Location of primary sensory neurons in tentacles, lips, pneumostom are a and in the chemoreceptor organ (osphradium) of the pond snail Lymnae a stagnalis has been described after nickel-lysine and biocytin dye-fi lling into the corresponding nerves. Central projections of the osphra dial sensory system to the CNS were determined. Most of the neurons in the osphradial ganglion are primarily sensory cells which send proces ses directly to the CNS. The fibres originating from these neurons are located in the neuropil of all central ganglia except buccal and peda l. Back-filling the CNS through the osphradial nerve showed that only two pairs of neurons in me cerebral ganglia, two neurons in the right parietal ganglion, and cell in the left pleural ganglion sent processe s into the osphradium. Back-filling with biocytin revealed that only f ive to seven neurons in the right parietal and visceral ganglia were d ye-coupled with osphradial neurons. Such a small number of central neu rons projecting to the osphradial sensory system differs it from the t entacular and lip systems where hundreds of central neurons innervate the periphery. Two different systems with either central or peripheral processing of sensory information are proposed to be included into ch emoreception of Lymnaea. The obtained results are the basis of further physiological studies.