Figure 3

Serotonin-lir neurogenesis in the pilidium larva and the juvenile of Lineus albocinctus . Serotonin-lir is shown in graded scales of dark red to bright yellow, cell nuclei are in blue. Apical of the larvae faces upwards in A, C and D. B is an abapical view. Larva with incorporated trunk disc stage in A and B. Anterior of the dissected juvenile is to the left in E and D. All images are maximum projections of scans through the entire specimen. White dotted lines encircle the imaginal discs of the developing juvenile. (A) Same specimen as in 2A. The apical plate shows two serotonin-lir neural cell bodies. A serotonin-lir oral nerve ring encircles the stomach and sends two suboral neurites towards the transition between the posterior and the lateral lobes. (B) The marginal neurite bundle underlies all four lobes. It is associated with marginal serotonin-lir perikarya. (C) The processes of the apical neurons form a complex lobar nerve net with numerous interconnected multipolar interneurons. (D) All imaginal discs and anlagen have fused and have formed a juvenile worm. The oral nerve ring and the two suboral neurites have been incorporated into the juvenile body. (E) Dissected juvenile. Two lateral neurite bundles emerge from the fused cephalic discs and run along the ventro-lateral side of the juvenile. (F) Dissected juvenile. The lateral neurite bundles show a high density of associated serotonin-lir cells within the cerebral organ discs. All scale bars equal 100 μm. Abbreviations: *, mouth; ap, apical plate; cd, cephalic disc; ced, cerebral organ disc; ey, eye; juv, juvenile worm; lnb, lateral neurite bundle; lnn, lobar nerve net; mnb, marginal neurite bundle; msp, marginal serotonin-lir perikarya; onr, oral nerve ring; pc, posterior cirrus; pr, proboscis anlage; sn, stomach neurite; son, suboral neurite; st, stomach; td, trunk disc; arrowheads, apical neuron.