Stereom is the calcium carbonate material found in sea urchins, and other echinoderms and in fossilized form in some related marine animals. It is a sponge-like porous structure which in a sea urchin may be 50% by volume living cells, and the rest crystals of calcite. The size of openings in stereom varies in different species and in different places within the same organism. [1] When an echinoderm becomes a fossil, the soft tissue is eventually replaced by calcium carbonate; microscopic examination reveals the structure and is a tool to classify the fossil as an echinoderm or related creature. [2]
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