Serotonin-like immunoreactive nervous system in young larvae of Phoronopsis harmeri . In all images, the apical is at the top, except in C where the apical is to the upper right corner. Z-projections (B, C, E-J) of larvae after mono- and double staining for 5-HT (serotonin) (yellow), phalloidin (blue), and alpha-tubulin (cyan). (A) Larva with 18 tentacles (SEM); dorsolateral view. (B) The apical organ viewed from the dorsal side. (C) Dorsal view of larva with 18 tentacles. (D) Live larva with 20 tentacles; ventrolateral view. (E) Perikaryon with cilium and basal process (p) in the apical organ. (F) Dorsal view of the anterior portion of larva with 20 tentacles. (G) Dorsal view of larva with 20 tentacles. (H) Lateral view of the youngest tentacles: the perikarya of dorsolateral group are indicated by arrowheads. (I) Dorsal view of two groups of perikarya (arrowheads), which are located near the youngest tentacles. (J) Lateral view of the anterior portion of larva with 20 tentacles. Abbreviations: am – anterior marginal neurite bundle; ao – apical organ; bm – blood mass; dlp – dorsolateral perikarya; itb – intertentacular branch; la – lateroabfrontal neurites in the tentacle; lp – lower portion of the neuropil of the apical organ; mn – median neurite bundle; ms – metasomal sac; pl – preoral lobe; pm – posterior marginal neurite bundle; t – tentacle; tn – tentacular nerve ring; tr – trunk; tt – telotroch; ttn – telotroch nerve ring; v - varicose (node); up – upper portion of the neuropil of the apical organ; yt – youngest tentacles.