The Echinoid Directory

The sea urchin skeleton is formed in the mesoderm and lies embedded within soft tissue throughout life. It is composed of high magnesium calcite, a calcite (CaCO₃) that incorporates up to 15% MgCO₃. Each plate behaves optically as though it were composed of a single calcite crystal. However, under high magnification (pictured above) the skeleton can be seen to be composed of a three-dimensional meshwork termed stereom. The size of the pores and the thickness of the rods forming the stereom varies according to the type of soft tissue that attaches to that part of the skeleton. In the picture above you can see the boundary between two stereoms, one associated with muscle attachment (top right) and the other with normal epithelium (skin - bottom left). In life the pore space is filled with connective tissue which is termed stroma. The design of the echinoid skeleton is thought to be particularly effective at preventing cracks from propagating through the plate.

See Smith, A. B. 1981. The stereom microstructure of the echinoid test. Special Papers in Palaeontology, 25, 1-85.