Fig. 1

Veliger larvae of Nodipecten nodosus. a Specimen observed by light microscopy applying differential interference contrast; lateral view, anterior to the top, ventral to the right. b Same individual, anterior view, ventral to the right. c Cross section of a veliger with the larval velum retracted (anterior to the top). Toluidine blue and basic fuchsin. d Cross section of a veliger with the larval velum exposed. Arrows point to the membrane connecting the larval velum and the unfolded mantle margin. Hematoxylin and eosin. e General morphology of a decalcified veliger, scanning electron micrograph (SEM); ventral view, anterior to the top. f Detail of the periostracum-forming zone (arrow), as indicated in the inset of e, defining the distal and proximal regions of the mantle margin. Lateral view. g Detail of the newly formed periostracum with smooth and wrinkled zones, as indicated in the inset of e. Lateral view. Abbreviations: ci, cilia from the larval velum; drm, distal region of the mantle margin; dt, digestive tract; hi, hinge line; me, mantle epithelium; mm, mantle margin; pe, periostracum; pfz, periostracum-forming zone; prm, proximal region of the mantle margin; se, shell edge; st, stomach; ve, larval velum