Ultrastructure of some nerve elements in competent larvae of Phoronopsis harmeri . Thin sagittal sections. (A) The most ventral portion of the main tentacular neurite bundle, which consists of several bundles of nerve fibers (nf). (B) Synaptic contact (arrows) between neurites of the main tentacular neurite bundle. (C) A part of the minor tentacular neurite bundle, which gives rise to the mediofrontal neurite bundle that extends into a tentacle. (D) Neurosecretory cell and a bundle of neurites (nf) in the midgut epithelium. (E) Perikaryon (p) and nerve fibers (nf) in the epidermis of the oral field. (F) Neurite bundle (indicated by a circle) of the telotroch nerve ring, which passes between the epidermis of the body wall (bw) and the epidermis of the telotroch (tt). Abbreviations: bc – blastocoel; bl – basal lamina; bw – epidermis of body wall; mc – muscle cell; mf – miofilaments; mi – microvilli; n – nucleus; nf – nervous fibers; p – perikaryon; tt – epidermis of telotroch.