Fig. 9
From: Nervous system development in the Pacific oyster, Crassostrea gigas (Mollusca: Bivalvia)
Schematic presentation of the ground structures of the nervous system in Bivalvia and Mollusca based on morphological data of neurogenesis in studied species. a suggested ground pattern of AO of the last common ancestor of Bivalvia consist of three flask-shaped cells. b the most conserved nervous structures in Mollusca are an AO consisting of three flask-shaped cells and a paired ventral nerve cord (black lines). Note, the neurites of pioneer peripheral sensory cells are located in the episphere (1st scenario for Mytillus trossulus, Bivalvia) or hyposphere (located dorsally (2nd scenario for oyster) or caudally (3rd scenario for chitons and gastropods); all trace the future VNC. Thus, the position of first sensory cells in larvae does not matter, but the route that the neurites make is necessary for positioning of the VNC later