FMRFamide-lir structures (graded scales of dark red to bright yellow) in Lineus albocinctus larvae. Cell nuclei in blue, cilia in green. Boxes indicate regions of the respective detailed micrographs, which partly stem from different individuals. Apical faces upwards. Images are maximum projections of scans through the entire specimen. (A) A circumesophageal neurite proceeds from the apical part of the esophagus to its distal end. An outer marginal neurite bundle is present underneath the ciliary band. Lateral lobes show an additional inner marginal neurite bundle. Scale bar 100 μm. (B) Posterior lobe, detail. Marginal sensory cells are associated with the outer marginal neurite bundle. Scale bar 10 μm. (C) Lobar nerve net, detail. Scale bar 20 μm. (D) Detail of the inner and outer marginal neurite bundle of the lateral lobe. Inner marginal interneurons connect the processes of the lobar nerve net to the inner marginal neurite bundle. Scale bar 20 μm. (E) Larva with incorporated trunk disc stage. The circumesophageal neurite splits and its descendants form a complex lobar nerve net. Scale bar 60 μm. (F) Transition between the posterior and the lateral lobes, detail. The circumesophageal neurite splits into several peripheral lobar neurites, of which one on each side forms the inner marginal neurite bundle. Scale bar 30 μm. (G) Marginal sensory cells, detail. Two neurites connect the marginal sensory cells to the outer marginal neurite bundle. Scale bar 5 μm. Abbreviations: *, mouth; ar, apical region; at, apical ciliary tuft; cb, ciliary band; cen, circumesophageal neurite; cn connective neurite; es, esophagus; imi, inner marginal interneuron; imn, inner marginal neurite bundle; ll, lateral lobe; omn, outer marginal neurite bundle; pln, peripheral lobar neurite; msc, marginal sensory cell; st, stomach; arrows, single cilium; arrowheads, cell soma of multipolar interneuron; double arrowheads, microvilli collar.