Demospongia is the largest and most diverse of the classes of sponges, comprising approximately 90 percent of all sponges. The spicules of a demosponge can be made of either spongin, an organic substance, or silica, a mineralized substance, and in some cases, both. Demosponges can grow to be very large, and are found in virtually any tropical or subtropical water, as well as in other marine environments.