Stereom is a calcium carbonate material that makes up the internal skeletons found in all echinoderms, both living and fossilized forms.
It is a sponge-like porous structure which, in a sea urchin may be 50% by volume living cells, and the rest being a matrix of calcite crystals.
The size of openings in stereom varies in different species and in different places within the same organism.
[2] Stereom was the first form of biomineralization to evolve in deuterostomes, predating the evolution of spicules in tunicates and bone in vertebrates.
[5][3] In the largely falsified[6] calcichordate hypothesis, stereom was believed to have been present in the common ancestor of echinoderms and vertebrates.