Development of the Larval Serotonergic Nervous System in the Sea Star Patiriella regularis as Revealed by Confocal Imaging. Development of the nervous system in the larvae of the sea star Patiriella regularis was reconstructed in three dimensions. The optical sectioning and image processing capabilities of the confocal microscope made it possible to identify the precise location and timing of development of serotonergic cells in relation to subsequent development of larval features. Similarities between this system and the serotonergic systems in larvae of other echinoderms were explored. Neuronal-like immunoreactive cells and processes first appeared in late gastrulae as a collection of cells scattered across the animal pole. These cells subsequently gave rise to basal axons positioned along the basal lamina. Immunopositive cells located in the stomodaeal region marked the beginnings of formation of the adoral ciliated band. Cells were also present in the mid-dorsal epithelium. Advanced bipinnaria had pyramidal immunoreactive cells within the adoral band and ovoid immunoreactive cells within the preoral and postoral ciliated bands. Processes originating from neurons in the transverse region of the preoral ciliated band extended into the buccal cavity, suggesting that these cells have a sensory role in feeding. An anterior ganglion formed in the late bipinnaria, innervating the preoral and postoral ciliated bands. This connection has not previously been described. It thus appears that the ciliated bands in the bipinnaria larvae of P. regularis communicate via serotonergic nerve tracts.