The Demospongiae are the largest class in the phylum Porifera. In fact, the demosponges include 81% of all species of sponges with nearly 7,000 species worldwide (World Porifera Database). They are predominantly leuconoid in structure. Their "skeletons" are made of spicules consisting of fibers of the protein spongin, the mineral silica, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges.