Sclerosponges secrete a basal crystalline and aspicular skeleton of calcium carbonate, either aragonite or calcite, above which lies a thin layer of living tissue which also secretes siliceous spicules and collagenous fibers. The tripartite skeleton of sclerosponges distinguishes them from all other sponges and also from all other multicellular animals, no one of which has an abundant quantity of two disparate minerals helping to make up its skeleton. The cell types and their organization as well as what little is known about their development indicate that the sclerosponges are related to the demosponges. Sclerosponges are inhabitants of shaded crevices, caves and tunnels on coral reefs in both the Caribbean and Indo-Pacific regions. The range of only one species, Merlia normani Kirkpatrick, extends from the tropics into the warm temperate waters of the Mediterranean Sea.