Wirenia argentea larva at day 7, TEM micrographs of cross sections through the apical cap. A. Section anterior to prototroch depicting cerebral depressions (arrows), pretrochal cells (prc), and gland cells (gl); yg: yolk granules; scale bar 20 μm. B. Detail of A; surface of cerebral depression with microvilli (arrow); ne: neurons; scale bar 2 μm. C. Lumen of cerebral depression (cd) surrounded by neurons (ne); yg: yolk granule of pretrochal cell; scale bar 2 μm. D. Section through prototrochal plane; note compound prototrochal cilia (pc), large nuclei of prototroch cells (n), stomodaeum (arrowhead), and mesodermal cell with nucleus in mitosis (arrow); scale bar 20 μm. E. Detail of D, nuclei of prototrochal cells (n) larger and more electron-translucent than nuclei of adjacent mesoderm cells (asterisks); scale bar 10 μm. F. Detail of stomodaeum in cross section, stomodaeal lumen (st) surrounded by epithelial cells (ste); note that some epithelial cells contain numerous yolk granules (yg); scale bar 3 μm. G. Section posterior to prototroch, slightly oblique section plane; first tier of posttrochal cells (pt1) constituting outermost layer of apical cap; asterisk marks stomodaeal opening; scale bar 20 μm. H. Detail of protonephridial duct (pdu) opening into cleft between trunk (tr) and apical cap (ac); scale bar 2 μm. I. Terminal cell (tc) with ultrafiltration weir (arrows marking two pores) and protonephridial duct (pdu); note cilia and microvilli in the duct lumen; scale bar 1 μm.