Fig. 1.

The structure of the sea urchin embryonic nervous system. (A and B) A 4-d embryo immunostained for serotonin (green), SynB (red), and nuclei (DAPI; blue). (C) Schematic indicating the three parts of the nervous system from an oral-vegetal view (OVV). These regions are the anterior neuroectoderm, located at the animal pole (AP; dark blue) and containing serotonergic neurons (green) and nonserotonergic neurons (purple in C and red between the dashed arrows in B); the ciliary band (CB; medium blue), which forms between oral (white) and aboral (light blue) ectoderm and contains nonserotonergic neurons (orange ovals); and the pharyngeal neurons close to the lower lip of the mouth (M in A and C) (curved white arrow in A; red ovals in C). (Scale bar in B: 20 μm.)