SEM photomicrographs of the ossicles from Ypsilothuria bitentaculata bitentaculata (MZUSP 1306). (6–7) tables from body wall in dorsal and lateral views, respectively; (8) detail of the spire; (9) rod from tentacles; (10–11) knobbed plates from introvert (12–14); arched rods from anal and oral siphon. (15–16) plates from oral and anal siphons, respectively. Scale bars: 6 = 500 μm, 7–8 = 200 μm, 9–11 = 50 μm, 12–16 = 20 μm.