Ultrastructure of the hood edge in competent larvae of Phoronopsis harmeri . Sagittal semithin (A) and thin (B-D) sections. (A) The hood edge with distal part of the median neurite bundle (fo). (B) Fine structure of the marginal neurite bundles. (C) Biciliated cells in the epidermis of the hood edge. Two basal bodies (bb) are evident. (D) Sensory cell (blue) and perikaryon, which does not contact the surface of the epidermis (pink). Abbreviations: ac – accessory centriole; am – annular muscle of the hood; bc – blastocoel; epr – epidermis of the preoral ciliated band; es – epidermis of the subumbrella; ex – epidermis of the exumbrella; G – Golgi apparatus; mc – muscle cell; mi – microvilli; n – nucleus; nf – nerve fibers; pr – preoral ciliated band; sc – sensory cell; sr – striated rootlet; sv – synaptic vesicles.