SILICEOUS SPONGE

any sponge in which the main skeletal component is silica as opposed to calcium carbonate or fibrous organic materials only. More than 95 percent of all known sponge species have a siliceous skeleton and belong to the class Demospongiae (phylum Porifera). The siliceous skeleton is usually composed of discrete elements known as spicules that vary greatly in size and shape from species to species and are hence often used to aid species identification. Siliceous sponges are mainly marine, but there are a few freshwater species. The siliceous glass sponges of the class Hexactinellida are found in deep water and include such spectacular forms as the glass rope sponge (Hyalonema) and the Venus's flower basket (Euplectella), whose beautiful lattice-like skeleton was highly prized as a collector's item in Victorian times. Compare calcareous sponge; horny sponge.