Fig. 10
From: Development of the nervous system in Platynereis dumerilii (Nereididae, Annelida)
Summary diagram of the ontogeny of serotonergic structures in P. dumerilii. The earliest 5HT-lir posterior sensory cell sp1 appears at the pretrochophore stage before hatching. Soon after hatching, a solitary sensory cell sa1 appears at the apical extreme and basal fibers of these two cells outline the future prototroch nerve, paired ventral cord and brain commissure. In metatrochophores, more sensory cells add in the apical region and along the developing ventral cord. Starting from the nectochaete stage, 5HT-lir cells and neurites rapidly develop in the adult CNS. Apical end is always up. Relative dimensions are not maintained