Largest class of sponge

The largest taxonomic class of sponge is Demospongiae, which contains almost 7,000 species worldwide and therefore accounts for 90% of all sponge species as well as for all of the largest species (i.e., those over 1 m across). Their bodies' so-called skeletons consist of spicules that are composed of fibres of the protein spongin, or of the mineral silica, or of both. Most species are marine, but the members of one taxonomic order, Spongillida, occur in freshwater.

The other three taxonomic classes of sponge that still exist today are Calcarea, Hexactinellida and Homoscleromorpha. Those belonging to the class Calcarea are the calcareous sponges, their spicules being composed of calcium carbonate. The hexactinellids are the glass sponges, whose spicules are composed of silicon, and which are either four-pointed or six-pointed, and include such famously fragile forms as the Venus's flower basket. And homoscleromorphs are massive or encrusting in form but with only very tiny spicules; they were formerly included within the demospongid class but are now deemed separate.