Whole mounts of subadult specimens of Sagitta sp. labelled with a nuclear marker (bisbenzimide; red channel) and a histochemical reagent to label actin (phalloidin; green chanel). The images A, B', B", C', C", D', D" are black-white inverted micrographs of specimens labelled with fluoresent reagents. A: overview of an entire specimen (nuclear marker) to show the localization of the ventral nerve centre. Arrows identify the cell clusters of fence receptor organs. Scale bar: 500 μm. B: higher magnification of the ventral nerve centre (ventral view) labelled for actin (B') to show the central neuropil core and labelled for nuclei (B") to show the flanking zones of neuronal somata. The overlay is shown in B"'. C, D: higher magnifications (same set of markers) to show details of the neuropil and arrangement of somata (ventral view). The nerve centre cell somata (C', D") appear to be arranged in rows between the emerging fibre bundles (D') that target the neuropil core in a right angle. Scale bars in B,C: 50 μm.