page_title: Ossicle (echinoderm)
section_title: Types of ossicle
hierarchical_section_title: Ossicle (echinoderm) / Types of ossicle
context_page_description: Ossicles are small calcareous elements embedded in the dermis of the body wall of echinoderms. They form part of the endoskeleton and provide rigidity and protection. They are found in different forms and arrangements in sea urchins, starfish, brittle stars, sea cucumbers, and crinoids. The ossicles and spines are the only parts of the animal likely to be fossilized after an echinoderm dies.
context_section_description: Ossicles have a variety of forms including flat plates, spines, rods and crosses, and specialised compound structures including pedicellariae and paxillae. 
  Plates are tabular ossicles that fit neatly together in a tessellated manner. They form the main skeletal covering for sea urchins and sea stars. 
  Spines are ossicles that project from the body wall and articulate with other ossicles through ball and socket joints mounted on tubercles. They are formed from crystals of calcite and can be solid or hollow, long or short, thick or thin and sharp or blunt. The spines serve a protective function and are also used for locomotion.
  Pedicellariae are compound ossicles that articulate with other ossicles and protrude from the aboral (upper) surface of some sea stars (and also the test of sea urchins). They usually have short fleshy stalks and either two or three moveable ossicles forming a set of pincer-like jaws. They may be scattered over the surface or may be grouped around spines. 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.
  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. Beneath this is a water-filled cavity which contains the madreporite and delicate gill structures known as papillae.
caption: Sea urchin tests showing the ball parts of the ball and socket joints that articulate with the spines
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