Serotonergic nervous system development in P. lamarckii. Scale bars = 50 μm. Apical is to the top of the figure in each panel unless indicated. A. Gastrula, left lateral view. One posterior cell (psc) is highly reactive for anti-serotonin. Another cell in the vicinity of the mouth is also reactive (*). Very faint, scattered fluorescent signal marks the beginning of formation of nerve fibres. B. Early trochophore, left lateral view. The posterior serotonergic cell sends processes apically. These fibres are the rudiments of the ventral nerve cords (vnc) and associate with processes that innervate the prototroch nerve ring (pnr). Fibres encircle the mouth (m). The second serotonergic cell (*) now lies posterior to the mouth and is associated with the suboesophageal ganglion.C. Early trochophore, left lateral view. The prototroch nerve ring is complete and runs behind the stomach (st). The neuropil has begun to appear in the apical organ (ao). Some gut autofluorescence is visible. D. Complete trochophore, left lateral view. A metatroch nerve ring appears (mnr). Some gut autofluorescence is visible. E. Elongating trochophore, left lateral view. The cerebral ganglion (cg) is developing and is connected to the ventral nerve cords by the circumoesophageal connectives (cc). The prototroch nerve ring now consists of multiple fibres. Serotonergic cells near the anus are connected to the ventral nerve cords by small fibres (arrow). F. Metatrochophore, ventral view. The paired ventral nerve cords gain commissures and converge posteriorly (arrowhead). Circular nerves can be seen in the periphery of each segment (arrows). Chaetae are autofluorescent. G. Juvenile, ventral view, anterior is to the top. The branchial nerves (bn) and opercular nerve net (onn) are serotonergic. A serotonergic fibre can be seen running along the intestine (arrowhead). Chaetae and uncini are autofluorescent.