Line drawings showing the larval FMRFamide-lir nervous system of Lineus albocinctus . FMRFamide-lir structures are in red. Apical faces upwards. (A) Larva with circumesophageal neurite that descends on each side of the esophagus. At the level of the lobar junctions it splits into several peripheral lobar neurites. In the lateral lobes some of these peripheral lobar neurites form a complex nerve net with numerous interconnected multipolar interneurons. The neurites of the lateral lobar net are connected to the inner marginal interneurons, which are associated with the inner marginal neurite bundle. The inner marginal neurite bundle is only present in the lateral lobes and originates from one peripheral lobar neurite on each side of the lobe. An outer marginal neurite bundle underlies all four lobes and is associated with the marginal sensory cells. Scale bar 100 μm. (B) Detail of the distal part of the lateral lobe. The lateral nerve net is connected to the inner marginal neurite bundle via the inner marginal interneurons. The lobar nerve net shows multipolar interneurons which interconnect the neurites. The inner marginal neurite bundle is only present in the lateral lobes. Scale bar 20 μm. (C) Detail of a marginal sensory cell of the lateral lobe. Its single sensory cilium projects into the ciliary band and is surrounded by a microvilli collar. The marginal sensory cell is connected to the outer marginal neurite bundle via two neurites. Scale bar 5 μm. Abbreviations: ap, apical plate; cb, ciliary band; cen, circumesophageal neurite; ep, epidermis; es, esophagus; imi, inner marginal interneuron; imn, inner marginal neurite bundle; lnn, lateral nerve net; mpi, multipolar interneuron; msc, marginal sensory cell; mvc, microvilli collar; omn, outer marginal neurite bundle; pln, peripheral lobar neurite; sc, sensory cilium; st, stomach.