Development of FMRFamide-like (green) and acetylated tubulin-like (red) immunoreactivity in P. dumerilii. Early trochophore (a-c), mid-trochophore (d-e), late trochophore (f-g), early metatrochophore (h-j), mid-metatrochophore (k), late metatrochophore (l-n), early nectochaete (o-s). a: 24 hpf, apical view. The first immunopositive cells are three apical neurons (fa1–3) and a solitary dorsal neuron (fd). b: 24 hpf, apical neurons at high magnification. The neurons fa1,2 are located to the left and fa3 to the right of apical neuropil (an). c: 24 hpf, high magnification of the dorsal neuron fd. Two neurites (arrowheads) extend from it in opposite directions beneath the prototroch. d: 28 hpf, apical view showing the cells fl1 and fl2. dc and vc – dorsal and ventral cerebral commissures. e: 32 hpf, dorsal view showing the cells fa1–3, fd, and fl2. f: 36 hpf, apical view showing neurons in the apical organ (arrows) and ventral cerebral commissure (arrowheads). g: 36 hpf, ventral view of the solitary posterior cell (fp) with two apical cilia. h: 45 hpf, ventral view showing two ventral pretrochal cells (fv1,2) and lateral cells (fl1,2). i: 45 hpf, ventral view on the apical region showing apical neurons (arrows), cerebral ganglion (arrowheads) and ventral pretrochal cells (fv1, fv2). j: 45 hpf, ventral view on the posterior cell (fp). k: 50 hpf, ventral view of the pretrochal region showing FMRFa immunoreactivity in the apical region (arrowheads) and two cells (arrow) close to the cell fl1. Inset: High magnification of the two cells (arrows) close to the cell fl1 (arrowhead).
l: 56 hpf, ventral view. Two groups of neurons associated with the ventral nerve cord (arrows), ventral neurons behind the stomatogastric nerve ring (arrowhead), and two neurons in the first chaetigerous segment (open arrowheads) are seen. m: 56 hpf, the neurons of the first chaetigerous segment (arrows). n: 56 hpf, posterior cell fp (arrow). o: 72 hpf, ventral view showing the brain (asterisk), neurons behind the stomatogastric nerve ring (arrowhead), and paired ventral nerve cord (arrows). p: 72 hpf, brain region with dense neuropil (asterisk). q: 72 hpf, anterior part of the ventral nerve cord showing neurons behind the stomatogastric nerve ring (arrowhead), and two neurons in the first chaetigerous segment (open arrowheads) sending fibers into the ventral cord and segmental nerves (arrows). r: 72 hpf, dorsal cell fd (arrow). s: 72 hpf, posterior cell fp (arrow) with no FMRFa immunoreactivity. Scale bars = 20 μm