ABSTRACT
Echinoderms usually have a rigid body wall with an elaborate magnesium-rich calcite endoskeleton. The integumental skeleton of the Holothuroidea contains many microscopic ossicles formed within multinucleated sclerocystes’ syncytia situated in the dermal layer of the body wall. The sequence of ossicle formation of two apodid holothurians, Chiridota rotifera (Pourtalès, 1851) and Synaptula hydriformis (Lesueur, 1824), was observed using light microscopy, scanning and confocal electron microscopy. The first species has clusters of calcareous structures shaped like wheels, gathered inside convex white papillae in the body wall. Those ossicles begin to form as little stars with six or more rays, which develop to form wheels with six rays also, measuring from 40 to 150 m long. Ossicles aggregate in the papillae by addition of new ones at the star stage in its outer layer. The oldest ossicles, in the wheel stage, concentrate in the middle of the papillae. In the second species, the ossicles have two parts, a plate and an anchor, distributed all over the skin. Each anchor measures about 120 m long and is attached to a plate. The anchor begins to form as a small baton. The plate is formed later and measures 95-130 m. In both species ossicle formation occurs during all the life span of the animals.
