Serotonin-lir neural structures in an early Lineus albocinctus larva. Serotonin-lir is in graded scales of dark red to bright yellow, cell nuclei are in blue, tubulin in green. Apical faces upwards. All images are maximum projections of scans through the entire specimen. Boxed areas in A indicate details of B, C and D. Image D is from a different specimen. (A) Larva with early anlagen of cephalic and trunk discs. The apical plate gives rise to the apical ciliary tuft and shows two serotonin-lir neurons. An oral nerve ring encircles the stomach and gives rise to two serotonin-lir suboral neurites, which merge with the marginal neurite bundle at the junction of the posterior and the lateral lobes. Scale bar 100 μm. (B) Detailed view of the marginal neurite bundle of the posterior lobe. Marginal serotonin-lir perikarya bear a single cilium each. Scale bar 30 μm. (C) Detail of the apical plate region and the oral nerve ring. Two serotonin-lir neurons of the apical plate send their processes to the four lobes and form an apical neurite plexus. Scale bar 30 μm. (D) Detail of the oral nerve ring and the suboral neurites. The suboral neurite are always associated with conical serotonin-lir cells. Scale bar 30 μm. Abbreviations: *, mouth; anp, apical neurite plexus; ap, apical plate; at, apical ciliary tuft; cd, cephalic disc; mnb, marginal neurite bundle; msp, marginal serotonin-lir perikaryon; onr, oral nerve ring; son, suboral neurite; st, stomach; td, trunk disc; arrows, single cilium; arrowheads, serotonin-lir neurons; double arrowheads, lateral processes.