NEW INTERPRETATION OF A NUDIBRANCH CENTRAL-NERVOUS-SYSTEM BASED ON ULTRASTRUCTURAL ANALYSIS OF NEURODEVELOPMENT IN MELIBE-LEONINA .2. PEDAL, PLEURAL, AND LABIAL GANGLIA

Electron microscopical analysis of semi-serial sections through larval stages of the dendronotid nudibranch Melibe leonina (Gould, 1852) revealed paired placodes of neurogenic ectoderm at the base of the foot. The location of these laterocephalic placodes corresponds to descriptions of the ectodermal site generating pleural neurons in prosobranchs. In Melibe, there are two sites of neuronal ingression within each laterocephalic placode. Neurons ingressing from one of these sites join the cerebral ganglia, and their initial axons extend into the cerebrobuccal connectives or run distally along the esophagus. I identify these neurons as homologues of labial ganglia neurons in archeogastropods. However, neurons derived from the second ingression site within each laterocephalic placode join the pedal ganglia. Pedal ganglia are present in hatching veligers and are linked to the cerebral ganglia by cerebropedal connectives associated with the statocyst nerves. A second connective between each cerebral and pedal ganglia appears at the onset of neuronal ingression from the laterocephalic placodes. Peripheral axons branching from this second pair of connectives are associated with laterocephalic neurons that ingress to the pedal ganglia. I argue that these are pleural neurons, meaning that the pleural ganglia in Melibe are uncoupled from the visceral loop.