Schemes of the nervous system organization in Phoronopsis harmeri larvae. (A-B) Competent larvae; the apical organ is at the top; the ventral side is to the right. The number of tentacles has decreased, and the apical organ is simplified. (A) Distribution of serotonin-like immunoreactive perikarya and neurites in a competent larva. (B) The overall organization of the nervous system in competent larvae, including all nerve elements, which were revealed with all used methods. (C-E) The development of the serotonin-like immunoreactive nerve elements of the hood. Larvae are viewed from the ventral side. The organization of larvae is simplified, the number of tentacles decreased. (C) Larva with 18-20 tentacles. (D) Larva with 22 tentacles. (E) Larva with 24 tentacles (competent stage). Abbreviations: am – anterior marginal neurite bundle; ao – apical organ; ar – anal nerve ring; dlp – dorsolateral pericarya; fo – frontal organ; itb – intertentacular branch; la – lateroabfrontal neurites in the tentacle; lf – laterofrontal neurite bundles in the tentacle; mf – mediofrontal neurite bundle of the tentacle; mn – median neurite bundle; mnr – minor nerve ring; ms – metasomal sac; ncs – neurites of the esophagus and cardiac sphincter; nms – neurites and perikarya of the metasomal sac; ofn – neurites and perikarya of the oral field; pm – posterior marginal neurite bundle; pmg – perikarya of the midgut; sf – sensory field; tn – tentacular nerve ring; trn – trunk neurites and perikarya; ttn – telotroch nerve ring; vlb – ventrolateral branch.