Figure 9.

Organization of the FMRFamide-like immunoreactive nervous system in the late gastrula (A, B), preactinotrocha (C), and young larva (D-K). Z-projections of larvae after double and triple staining with antibodies against FMRFamide (cyan), as well as staining with phalloidin (blue), and Hoechst (violet). Apical is to the top in all cases except C, where it is to the upper left. (A) Overview showing apical organ, which is represented by a horseshoe-shaped neuropil (np), archenteron (ar), and precursor of the proctodaeum (ppr). Dorso-lateral view. (B) Median region of the anterior body part showing apical plate (ap) with neuropil (np), mesodermal cells (mc), and mouth (m); ventral view. (C) Overview of a preactinotrocha with large apical neuropil (np) and marginal neurite bundle (mn) running along edge of the preoral lobe (pl). Lateral view; ventral side is to the right. (D) Overview of nuclei and FMRFamide-like immunoreactive nervous system showing marginal neurite bundle (mn), neuropil (np), first perikarya (n) of the apical organ, oral nerve ring (or), first neurites of the ventral neurite bundle (vnb), and tentacular ridge (tr). Ventral view. (E) Overview of the musculature and FMRFamide-like immunoreactive nervous system showing marginal neurite bundle (mn), perikarya (n) of the apical organ, the tentacular neurite bundle (tn), oral ring (or), lower ventro-lateral perikarya (lvl) of the oral field, perikarya associated with the ventral neurite bundle (vnb), and perikarya associated with the midgut (mgp). Ventral view. (F) Lateral view of a young larva with apical organ (ao), marginal neurite bundle (mn), thin tentacular neurite bundle (tn), upper ventro-lateral perikaryon (uvl), lower ventro-lateral perikaryon (lvl), perikarya of the midgut (mgp), and perikarya of the ventral neurite bundles (vnb). Lateral view; ventral side is to the right. (G) Monopolar perikaryon of the apical organ with bright apical cell part (apt), nucleus (nu), and basal process (bp) projecting to the neuropil (np). (H) Ventral body wall of a larva with perikarya (pe) and neurites (ne) of the ventral neurite bundles. (I) Bipolar perikaryon of the apical organ with two processes: anterior process (apr), which runs into the neuropil (np), and posterior process (pth), which is the precursor of one stem of the tentacular neurite bundle. Lateral view; anterior is to the right. (J) Ventro-lateral body wall with large upper ventro-lateral perikaryon (uvl) and the ventro-lateral neurite (pvln). Lateral view; ventral is to the right. (K) Posterior body part with midgut (mg), proctodaeum (pr), and perikarya of the midgut, which contain the non-stained nuclei (nu) and form basal processes (bp). Lateral view; ventral is to the right.