Symmetrical Giant Neurones in Asymmetrical Ganglia: Implications for Evolution of the Nervous System in Pulmonate Molluscs

Two of the largest neurones in the terrestrial snail Achatina fulica are RPrl, located in the right parietal ganglion, and V1, located in the visceral ganglion. Several characteristics of these cells were studied in detail, including morphology, passive and active electrical properties, synaptic inputs, sensory inputs, motor outputs and sensitivity to transmitter substances. The results suggest that RPrl and V1 form a bilaterally homologous pair of cells, yet they reside in asymmetrically placed ganglia. An explanation of this paradox is offered in the proposal that RPrl and V1 were formerly locatedin the bilaterally symmetrical intestinal ganglia. Their contemporary asymmetrical locations are accounted for by the hypothesis that, during the evolution of the pulmonate nervous system, the supraintestinal ganglion fused with the right pallial ganglion and the subintestinal ganglion fused with the visceral ganglion.