Gangliogenesis in the prosobranch gastropod Ilyanassa obsoleta.

We determined that the larval nervous system of Ilyanassa obsoleta contains paired cerebral, pleural, pedal, buccal and intestinal ganglia and unpaired apical, osphradial, and visceral ganglia. We used a modified form of NADPH diaphorase histochemistry to compare the neuroanatomy of precompetent (including specimens 6, 8, and 12 days after hatching), competent, and metamorphosing larvae with postmetamorphic juveniles. This method highlighted ganglionic neuropils and allowed us to identify individual ganglia at various stages of development, thereby laying a foundation for concurrent histochemical studies. The first ganglia to form were the unpaired apical and osphradial ganglia and the paired cerebral and pedal ganglia. In larvae 6 days after hatching, the neuropil had already appeared in the apical and osphradial ganglia. Neuropil began to be apparent in the cerebral and pedal ganglia 2 days later. At that time, the pleural and buccal ganglia were identifiable and adjacent to the posterior edge of the cerebral ganglia. The ganglia of the visceral loop were concurrently recognizable, although the supraintestinal ganglion developed slightly earlier than the subintestinal and visceral ganglia. By 12 days after hatching all of the major adult ganglia were discernible. The apical ganglion was retained by newly metamorphosed juveniles, but not by juveniles 2 days later. After metamorphosis was complete, the central nervous system (CNS) was consolidated into its juvenile form with ipsilateral cerebral and pleural ganglia being partially fused. The metamorphic translocation of ganglia, which included a caudal relocation of the cerebrals and the migration of the buccals from above the esophagus to a position below it, correlated with the movement of the proboscis to the dorsal part of the head.