The organization of the serotonin-like immunoreactive nervous system in the mid-gastrula (A-D), late gastrula I (E-F), late gastrula II (G-J), and preactinotrocha (K-O). Photographs of live animals (A, E, K) and Z-projections of embryos after mono-, double, and triple staining for 5-HT (serotonin) (green), phalloidin (grey), and Hoechst (violet). Apical is to the top on all micrographs except for B and C, where it is to the upper right. (A) Mid-gastrula gross anatomy showing the apical plate (ap), the archenteron (ar), the apical tuft (at), the blastocoel (bl), and the mesodermal cells (mc); lateral view, ventral side is to the right. (B) Overview of a mid-gastrula stage with apical organ (ao). (C) The same embryo with one perikaryon (n). Image created from selected optical sections from the mid region of the specimen. (C-insert 1) Detail of a perikaryon with short basal process (bp). (C-insert 2) Detail of the nucleus of the perikaryon, which has an upper thin protrusion (arrowhead). (D) Anterior portion of a mid-gastrula with apical organ, which consists of four perikarya. Some perikarya have a special area (as) devoid of signal under the wide part of the cell containing the nucleus. Ventral view showing mesodermal cells (mc) and mouth (m). (E) Ventral view of live late gastrula I showing apical plate, mouth, and archenteron. (F) Serotonin-like immunoreactive nervous system in late gastrula I: the apical organ is formed by 6–7 perikarya. Ventral view showing mesodermal cells, mouth, and archenteron. Image created from selected optical sections from the mid region of the specimen. (G) Optical sagittal section through a late gastrula II, which has a large apical organ, the precursor of the preoral lobe (ppl) with spacious blastocoels (bl) and apical plate, esophagus (eso), stomach (st), and the precursor of the proctodaeum (ppr). (H-J) Detail of perikarya of a late gastrula II. (H) Two perikarya with two separated apical parts (apt) and one basal process (arrowheads). (I) Two perikarya with special areas (as), which are located under the nuclei and lack staining. (J) Small neuropil (np) and perikaryon with long basal neurite (bp). (K) Lateral view (ventral side on the right) of a live preactinotrocha with complete digestive tract, which consists of esophagus, stomach, midgut (mg), proctodaeum (pr), and small preoral coelom (c1) under the apical plate and blastocoel (bl) in the preoral lobe. (L) Overview of a preactinotrocha, dorso-lateral view showing apical organ, preoral lobe, and tentacular ridge. (M-O) Details of the organization of parikarya of the apical organ. Arrowhead indicates an individual perikaryon, which has a nucleus with apical protrusion. (M) General view of perikarya. (N) Body shape of perikarya with constricted area (double arrow) between the apical and basal parts. (O) Nuclei of perikarya; arrowhead indicates apical protrusion of one nucleus.