Fig. 4
Pediveliger larvae of Nodipecten nodosus. Scanning electron micrographs (a–b) and transmission electron micrographs (c–g), respectively. a General morphology of the mantle margins exposed by the retracted larval velum; anterior view, ventral to the right. b Detail of the mantle margin showing the periostracum-forming zone (arrows) and the adjacent row of cilia; lateral view, anterior to the top. c Mantle margin of a pediveliger larva, proximal region to the right and distal region to the left; white arrows point to sites of periostracum secretion. d Cross section of cilia present in the periostracum-forming zone showing the classical microtubular arrangement “9 + 2”. e Periostracal layer covering the distal region of the mantle. f Detail of electron-lucent vesicles (black arrows) present in the inner epithelium of the proximal region of the mantle, covered by short microvilli (g) Detail of the rough endoplasmic reticulum around the nucleus of a cell from the proximal region of the mantle margin. Abbreviations: ci, cilia; drm, distal region of the mantle margin; me, mantle epithelium, mi, microvilli; pe, periostracum; pfz, periostracum-forming zone; prm, proximal region of the mantle margin; ve, larval velum