How do echinoderms use calcium carbonate in their skeletons?

Echinoderms utilize calcium carbonate, specifically in the form of calcite, to construct a dermal skeleton that serves as an effective protective armor.12

This skeletal structure is composed of individual units called ossicles, which are formed from granules in the dermal layer. Specialized lime-secreting cells cause these granules to enlarge, branch, and fuse into a complex, three-dimensional lattice known as a stereom. This mineral framework is integrated with living tissue (stroma), allowing the skeleton to grow, undergo resorption, and regenerate throughout the animal's life.1

The application of this material varies significantly across groups:
* Echinoids: Form a rigid, hollow test (shell) and complex feeding structures like Aristotle’s lantern.1
* Asteroids and Ophiuroids: Utilize ossicles of various shapes, such as the vertebrae forming the central axis of ophiuroid arms, held together by muscles and ligaments to provide flexibility.1
* Crinoids: Feature a hollow sheath and series of drum-shaped ossicles.1
* Holothurians: Possess microscopic, highly variable ossicles embedded in the body wall and a calcareous ring surrounding the esophagus.1

Additionally, the stone canal within the water-vascular system is frequently calcified.3