Figure 4.

Serotonin immunoreactivity in the anterior regionof larval and juvenile Spirorbis cf. spirorbis. Confocal micrographs showing details of Figure 3C-F arranged in the same order. Anterior faces upwards, color bars indicate thickness of specimens in μm. Scale bars: 30 μm (A-B); 40 μm (C-D). (A-B) Free-swimming larva. (A) Dorsal view. Note the growth cones of the branchial crown neurites (bcn) and the collar neurons (cn). The cerebral ganglion (cg) is connected via the circumesophageal connectives (cc) to the ventral nerve cords (vnc) with associated perikarya (encircled) and the subesophageal commissure (sc). The latter is posteriorly followed by one additional commissure (c1). The prototroch nerve ring (pn) is linked to accessory prototroch neurons (apn) on the dorsal side. (B) Ventral view. The prototroch nerve ring (pn) has a dorsal gap (asterisk). (C) Settled larva, dorsal view. Anteriorly, the circumesophageal connectives (cc) split into a dorsal (drcc) and a ventral root (vrcc). The subesophageal ganglion (sg) starts to form. The peripheral nervous system in the thorax region includes an anterior segmental neuron (asn, dotted line), a lateral longitudinal neuron (ltn), and a transversal thorax neuron (ttn). The dashed line marks the very weak signal of the second commissure (c2). (D) Settled juvenile, dorsal view. The two dorsal longitudinal neurons (dln) and the left main collar neuron (mcn) are visible. The ventral commissures (c1 and c2) have been shifted forwards because of the longitudinal compression of the anterior body. The weak signal of the two nerve fibers innervating the operculum is outlined by dashed lines.