Line drawings illustrating development of the serotonin-lir nervous system in Lineus albocinctus . Larval serotonin-lir components in red, juvenile serotonin-lir structures in blue except for F, which depicts the overall adult neuroanatomy. Apical faces upwards in A, B, C. Anterior is to the left in D, E, F. (A) Larva with incorporated trunk disc stage. Two apical sensory neurons send their processes to the four lobes. The oral nerve ring encircles the stomach and gives rise to two suboral neurites. Size is approximately 400 μm. (B) Larva with almost fused cephalic and trunk discs. Compared to the specimen in A, a higher number of serotonin-lir processes, which originate from the two apical neurons and project into all four lobes, is present. Size is approximately 470 μm. (C) Juvenile worm inside the larval episphere. The oral nerve ring and the suboral neurites (dark blue) have been incorporated into the juvenile. Two lateral neurite bundles appear on the ventro-lateral sides. Size is approximately 830 μm. (D) Isolated juvenile, lateral left view. The two suboral neurites merge with the lateral neurite bundles. Size is approximately 380 μm. (E) Juvenile, dorsal view. Two lateral neurite bundles run along the ventro-lateral sides. Size is approximately 430 μm. (F) Anterior region of an adult “Anopla” after . The brain encircles the rhynchocoel but not the foregut. Abbreviations: anp, anterior neurite plexus; asn, apical sensory neuron; br, brain; cb, ciliary band; cd, cephalic disc; dc, dorsal commissure; es, esophagus; ey, eye; fg, foregut; juv, juvenile worm; lnb, lateral neurite bundle; lnn, lobar nerve net; lp, lateral plexus; mnb, marginal neurite bundle; mo, mouth opening; msp, marginal serotonin-lir perikaryon; onr, oral nerve ring; pb, proboscis; pp, posterior plexus; rc, rhynchocoel; rd, rhynchodaeum; sn, stomach neurite; son, suboral neurite; st, stomach; td, trunk disc; vc, ventral commissure.