Demosponges are the largest class of Phylum Porifera. Their 'skeletons' are made of spicules consisting of spongin fiber (a protein), silica, or both. They contain 90% of all species of sponges, and are predominantly Leuconid structural grade. There are many diverse orders in this class, and all of the large sponges are within this class.
However, the vast majority of living demosponges do not possess skeletons that would easily fossilize, thus their fossil diversity, which peaks in the Cretaceous, is probably an enormous underestimate of their true diversity.
Demosponge skeletons are composed of spongin fibers and/or siliceous spicules, though one genus (Oscarella) has neither.
Demosponge spicules, if present, are siliceous, have one to four rays not at right angles, and have axial canals that are triangular in cross section.
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