Details of the organization of the median neurite bundle (A, B) and frontal organ (C, D) in competent larvae of Phoronopsis harmeri . (A) Thin cross section of the exumbrella epidermis of the hood. The median neurite bundle is represented by three bundles of neurites, which intimately contact the basal lamina (bl) and muscle cells (mc). (B) Longitudinal section of the median bundle of the median neurite bundle containing a huge neuropil (np) and sensory cell (shown in blue). (C) Semi-thin sagittal section of the hood edge. The huge neuropil (np) of the frontal organ is visible. (D) The frontal organ neuropil accompanied by sensory (blue) and nonsensory (pink) perikarya. The hood edge is to the right; the dorsal side of larva is to the left. Abbreviations: am – annular muscle of the hood; bc – blastocoel; bl – basal lamina; mc – muscle cells; mi – microvilli; np – neuropil; pr – preoral ciliated band.