Ossicle (echinoderm)

Ossicles are small calcareous elements embedded in the dermis of the body wall of echinoderms.

Ossicles are created intracellularly by specialised secretory cells known as sclerocytes in the dermis of the body wall of echinoderms.

[1] All the ossicles, even those that protrude from the body wall, are covered by a thin layer of epidermis but functionally they act more like an exoskeleton than an endoskeleton.

[4] They are formed from crystals of calcite and can be solid or hollow, long or short, thick or thin and sharp or blunt.

Their function is to pick off debris so as to keep the surface clean and to prevent larvae of other invertebrates from settling and growing there.

[1][6] Paxillae are small pillar-shaped ossicles with flat tops sometimes found covering the aboral surface of sea stars such as Luidia, Astropecten and Goniaster that live underneath sediment.

Their stalks emerge from the body wall and their tops, each fringed with short spines, and abut each other to form a protective external false skin.

The ambulacral plates are each pierced by a pair of pores through which the active tube feet are connected to the water vascular system.

Perforated plates are sieve-like and often widely distributed and rods provide support for the tube feet and tentacles.

[3] In the order Apodida, members of which lack tube feet, there are anchor-shaped ossicles attached to anchor plates.