Organization of tentacles in competent larvae of Phoronopsis harmeri . Semi-thin cross section (A) and thin cross sections (B-E). (A) Section of the middle part of a tentacle. Around each tentacle, there are several zones of epidermis: frontal (fz), lateral (lz), abfrontal (afz), two laterofrontal (lfz), and two lateroabfrontal (laz). (B) Section of the apical portion of the laterofrontal sensory cell with a cilium (c) and thick microvilli (tm), which are much thicker than regular microvilli (tom). (C) The mediofrontal (at the top) and two laterofrontal (to the sides) neurite bundles (green). A nonsensory perikaryon (pink) is associated with the mediofrontal neurite bundle. (D) Two small lateroabfrontal neurite bundles (green). (E) The portion of the laterofrontal epidermis with the sensory laterofrontal cell (sc), which bears thick microvilli, and an additional sensory cell (blue) that contacts the laterofrontal neurite bundle (green). Abbreviations: afz – abfrontal zone of tentacle; bc – blastocoel; c – cilium; cc – cells of coelomic lining; G – Golgi apparatus; laz – lateroabfrontal zone of tentacle; lf – laterofrontal neurite bundle; lfz – laterofrontal zone of tentacle; lz – lateral zone of tentacle; mc – muscle cells; mf – mediofrontal neurite bundle; n – nucleus; nf – nerve fibers; sc – laterofrontal sensory cell; sr – striated rootlet; tm – thick microvilli, tom – thin microvilli.