New Interpretation of a Nudibranch Central Nervous System Based on Ultrastructural Analysis of Neurodevelopment in Melibe leonina. I. Cerebral and Visceral Loop Ganglia.

Development of the `cerebropleural' ganglia in the dendronotid nudibranch Melibe leonina (Gould, 1852) was examined by electron microscopy of semi-serial sections through larval stages. Although comparative neuroanatomical studies suggest that the paired cerebropleurals of nudibranchs are formed by fusion of cerebral and pleural ganglia, plus all other ancestral ganglia of the visceral loop, my study indicates that the pleural ganglia are not part of these compound ganglionic masses. In Melibe larvae, the cerebral, optic, and rhinophoral ganglia, arise from pre-trochal cephalopedal ectoderm. At hatching stage, the visceral loop extends from the two cerebral ganglia, is non-ganglionated, and forms a complete circuit beneath the esophagus. Ganglia that subsequently develop along the visceral loop, which were identified as subintestinal, visceral, supraintestinal, and possibly right parietal ganglia, arise from placodes of visceropallial ectoderm that show torsional displacements. In addition, a cluster of neurons, presumed to be osphradial, lies close to the rim of the right mantle fold. Detorsion of the visceral loop is accomplished by migration of subintestinal neurons along the visceral loop fiber tract, not by visceral loop shortening. Localized elongation of a different segment of this fiber tract during metamorphosis displaces the visceral ganglion to the left, where it fuses with subintestinal and left cerebral ganglia.