Serotonin-like immunoreactive nervous system in Phoronopsis harmeri larvae with 22 tentacles. In all images, the apical is at the top, except in A where the apical is to the upper left corner. Z-projections (B-G) of larvae after mono- and double staining for 5-HT (serotonin) (yellow), phalloidin (blue), and alpha-tubulin (cyan). (A) Ventral view of live larva. (B) Whole larva, viewed from the right side. (C) Ventrolateral view of the preoral lobe and the esophagus of the larva. (D) Dorsolateral view of the whole larva. (E) Two groups of neurite bundles, viewed from the right side. (F, G) Top view of the preoral lobe; the anterior edge is to the right. Abbreviations: am – anterior marginal neurite bundle; ao – apical organ; bm – blood mass; dp – distal portion of the median neurite bundle; es – esophagus; fg – neurite bundles of the first group; la – lateroabfrontal neurites in the tentacle; mn – median neurite bundle; ncs – neurites of the cardial sphincter; pm – posterior marginal neurite bundle; pr – proctodaeum; s – septum of the stomach; sg – neurite bundles of the second group; st – stomach; t – tentacle; tn – tentacular nerve ring; tp – neurites and perikarya of the trunk; tr – trunk; tt – telotroch; ttn – telotroch nerve ring; vlb – ventrolateral branch.