Serotonin-like immunoreactive nervous system in competent larvae of Phoronopsis harmeri . In all images, the apical is at the top. Z-projections (B-F) of larvae after mono- and double staining for 5-HT (serotonin) (yellow), phalloidin (blue), and alpha-tubulin (cyan). (A) Competent larva, viewed by SEM from the ventral side. (B) Lateral view of competent larva. (C) Optical section through the metasomal sac. The perikarya are indicated by arrowheads. (D) Anterior part of the larva stained for alpha-tubulin and viewed from the right side. (E) Anterior portion of the same larva stained with phalloidin and with 5-HT. (F) The base of tentacles. (G) Three-dimensional reconstruction of the apical organ viewed from the top. Anterior edge of the preoral lobe is to the right. (H) Three-dimensional reconstruction of the apical organ viewed from the bottom. Anterior edge of the preoral lobe is to the right. Color code: yellow – ciliated perikarya, blue – non-ciliated (underlying perikarya), pink – tentacular nerve ring, magenta – median neurite bundle. Abbreviations: am – anterior marginal neurite bundle; ao – apical organ; cp – ciliated perikarya; dp – distal portion of the median neurite bundle; fo – frontal organ; itb – intertentacular branch; la – lateroabfrontal neurites in the tentacle; lfo – “loop” of the frontal organ; lmn – lateral branches of the median neurite bundle; m – mouth; mn – median neurite bundle; ms – metasomal sac; ncp – non-ciliated perikarya; np – neuropil; of – oral field; pam – place of the anterior marginal neurite bundle; pl – preoral lobe; pm – posterior marginal neurite bundle; ppm – place of the posterior marginal neurite bundle; sf – sensory field; t – tentacle; tn – tentacular nerve ring; tr – trunk; tt – telotroch; ttn – telotroch nerve ring; vlb – ventrolateral branch.