Figure 6

Details of organization of the apical organ in competent larvae of Phoronopsis harmeri . (A) Semi-thin parasagittal section of the apical organ. The locations of different types of perikarya are shown. The apical side is at the top; the hood edge is to the right; the dorsal side of the larva is to the left. A portion of the median neurite bundle is visible on the right. (B) Ultrastructural organization of a portion of the apical organ. Two types of perikarya and the neuropil (np) are visible. (C) Ultrastructural details of type 1 perikarya (p1), which have long microvilli (open arrowheads) around the cilium, a root apparatus, large and abundant mitochondria (m), and clear (electron light) synaptic vesicles (cv). (D) Ultrastructural details of a type 2 perikaryon (p2), which contains a dense-core (dc), clear (cv) vesicles, and rudiments of a cilium (highlighted by a circle). (E) Type 3 perikarya (p3) contain a nucleus (n) with a large nucleolus (nu), a large Golgi apparatus (G), and dense-core vesicles (dc). (F) The central portion of the neuropil contacts the basal lamina (bl). Neurites contain numerous synaptic vesicles (closed arrowheads) spread along the thickened membrane. The basal membranes of muscle cells (mc) are also thickened. Abbreviations: bc – blastocoel; bl – basal lamina; c1 – preoral coelom; cc – cells of coelomic lining; cv – clear synaptic vesicles; dc – dense-core vesicles; G – Golgi apparatus; hr – horizontal rootlet; m – mitochondria; mc – muscle cells; mn – median neurite bundle; n – nucleus; np – neuropil; nu – nucleolus; p1 – perikarya of 1 type; p2 – perikarya of 2 type; p3 – perikarya of 3 type; vr – vertical rootlet.