Pleuro-buccal projections in pulmonate molluscs.

Using a nickel lysine axonal filling technique, neurons of the CNS that project to the cerebro-buccal connective in pulmonate snails and slugs were stained. In all species examined, a symmetrical, supposedly homologous pair of small neurons situated in the pleural ganglia have been identified. These neurons possess a unique axonal pathway which traverses the ipsilateral pedal ganglion on its way to the cerebral ganglia. The list of species includes both basomatophoran, fresh-water molluscs (Lymnaea stagnalis, L. auricularia, Planorbarius corneus) and stylommatophoran, terrestrial ones (Fruticicola fruticum, Helix pomatia, H. lucorum, Limax cinerea-niger). It is suggested that the paired pleural cells might be involved in the neuronal mechanism for co-ordination of defensive withdrawal with inhibition of feeding.