A class of sponges that lay down a compound skeleton comprising an external, basal mass of calcium carbonate, either aragonite or calcite, and internal siliceous spicules and protein fibers. The living tissue forms a thin layer over the basal calcareous skeleton and extends into its surface depressions; the organization of the tissue is similar to that of encrusting demosponges. Sclerosponges are common inhabitants of cryptic habitats on coral reefs in both the Caribbean and Indo-Pacific biogeographic regions. Their discovery has reopened questions of the affinities of certain fossil reef–inhabiting organisms, the stromatoporoids and the chaetetid and favositid tabulate “corals,” all of which have been generally regarded as members of the phylum Cnidaria.