Schematic of the neck circular muscle and closing system in Echinoderes . Apical views, with ventral side to the bottom. (A) Neck circular muscle is contracted, and placids are rotated down toward the center of the neck; the neck is closed. (B) Neck circular muscle is stretched along interplacid attachment sites (dashed lines), and the placids are ‘flipped open’. The neck is open. Note the attachments of longitudinal fibers from the trunk to interplacid soft cuticle. Abbreviations: mvp, midventral placid; s1, segment 1.Numbers refer to the placid numeration starting with the midventral placid clockwise.