GENERAL HISTOLOGY

Detailed explanation-1: -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.

Detailed explanation-2: -Echinoid tests have a variety of shapes; they can be globular or flattened, rounded or heart-shaped. The spines, held in place by soft tissue covering the test during life, usually became detached and fossilized separately.

Detailed explanation-3: -Sea urchins and sand dollars have ossicles that are fused, forming a rigid skeletal shell, known as a ‘test’.

Detailed explanation-4: -Echinoderms have an internal skeleton made of bony plates (ossicles) of calcium carbonate. They deposit this material after extracting dissolved calcium and carbonate (bicarbonate) ions from sea water. In some species, such as the sea urchin, plates of the skeleton are locked together to form a rigid structure.

Detailed explanation-5: -The endoskeleton of echinoderms (Deuterostomia: Echinodermata) is of mesodermal origin and consists of cells, organic components, as well as an inorganic mineral matrix.