Embryonic development and organogenesis in the snail Marisa cornuarietis (Mesogastropoda: Ampullariidae). V. Development of the nervous system.

The nervous system is ectodermal in origin. All nerve ganglia arise separately by proliferation and later delamination from the ectoderm, not by invagination. They become secondarily connected to one another by commissures and connectives developing as extensions from the peripheral layer of ganglionic nerve cells. Rudiments of the cerebral, pedal, pleural and intestinal (parietal) ganglia arise almost simultaneously at a relatively early stage (Stage V). The cerebral ganglia develop from the ectoderm of the head plates. Rudiments of the pedal and pleural ganglia are separate at their inception. They later fuse (Stage VI) to form a pleuro-pedal ganglionic mass on each side. The 2 intestinal ganglia are symmetrical at the beginning, but they soon lose their symmetry as a result of torsion. The right ganglion crosses to the left over the gut and persists as the supraintestinal ganglion. The left or subintestinal ganglion shifts to the right and forward, and fuses with the right pleural ganglion (Stage VIII), thus obscuring the chiastoneury. The paired buccal and single visceral (abdominal) ganglia start differentiating in Stage VII. The former develop from the ectodermal wall of the stomodaeum, while the visceral ganglion delaminates from the right wall of the visceral sac, then shifts to the left during torsion. The statocysts develop early (Stage V) from 2 ectodermal invaginations on either side of the rudimentary foot. They later separate from the overlying ectoderm and statoconi appear in their lumina. Contrary to earlier reports on related ampullariids, the osphradium proved to be ontogenetically older than the mantle and mantle cavity. It starts differentiating as a thickened ectodermal plate in the right wall of the visceral sac (Stage V). During torsion, it becomes engulfed in the mantle cavity and shifts to the left side, then is carried forward as the mantlegrow. The eyes develop late (Stage IX) as ectodermal invaginations which rapidly separate from the ectoderm to form closed vesicles. Their cells start differentiating before hatching to form the retina, in which pigment is deposited, and the inner cornea. The lens is secreted in the lumen of the eye and grows by addition of concentric layers of secretion.