Wirenia argentea larvae, day 1-3 after hatching, anti-α tubulin labeling, overlay of confocal image stacks. A, B. Day 1 larva. A. Overlay of CLSM maximum projection (anti-α-tubulin labeling) and light micrograph. Note strong signal from prototrochal (pro) ciliary band and cilia (arrow) of second posttrochal cell row (pt2). Pretrochal cells (prt) and cells of first posttrochal row (pt1) bear scattered cilia; B. Phalloidin labeling showing areas of high actin content (cell borders, cilia); note irregular size of pretrochal (prt) and central cells (asterisk) and regular arrangement of prototrochal (p) and posttrochal (pt1, pt2) cells; a: apical organ, pro: prototrochal ciliary band, tr: trunk rudiment; scale bar 35 μm; C, D. Day 3 larva in lateroventral view, scale bars 50 μm; C. Overlay of Hoechst nuclear staining and light micrograph, outermost cell layer of apical cap omitted, thus showing small cell nuclei of mesodermal cells and ectodermal cells of the trunk (tr) and apical cap inner surface (arrows); note ring-shaped zone devoid of nuclei (arrowheads) and cerebral ganglion anlagen (asterisks), D. Hoechst nuclear staining, showing nuclei of outermost cell layers; note size difference of posttrochal cell tier nuclei (arrows) and cerebral depression nuclei (asterisk); nuclei of prototroch cells (p) only faintly labeled. E. Detail of outermost cell layers of apical cap of a Hoechst-labeled three day old larva showing a large prototroch cell nucleus (pcn), several smaller mesoderm nuclei (n), including one in mitosis (mn); note dense yolk granules (arrows); scale bar 12 μm.