Test (biology)

The test of sea urchins is made of calcium carbonate, strengthened by a framework of calcite monocrystals, in a characteristic "stereomic" structure.

These two ingredients provide sea urchins with a great solidity and a moderate weight, as well as the capacity to regenerate the mesh from the cuticle.

According to a 2012 study,[2] the skeletal structures of sea urchins consist of 92% of "bricks" of calcite monocrystals (conferring solidity and hardness) and 8% of a "mortar" of amorphous lime (allowing flexibility and lightness).

This lime is constituted itself of 99.9% of calcium carbonate, with 0.1% structural proteins, which make sea urchins animals with an extremely mineralized skeleton (which also explains their excellent conservation as fossils).

[4] On a strictly scientific point of view, the term "test" should be restricted to the hard shell protecting sea urchins and foraminiferans.

Test of a shingle urchin ( Colobocentrotus atratus )
Test of a sea urchin.
Sea urchin test detail. External plates made of carbonate, silica, and chitin create a dense protective layer that is often covered in serrations, ridges and spines [ 1 ] .
Foraminiferan tests.