Siliceous or carbonate spicules provide support and defense to marine sponges. The inorganic envelope usually embodies a protein core. Our SAXS study of the siliceous spicules from the demosponge Thetya aurantium proves the very ordered structure assumed by the protein core inside the spicules. Indeed, not only the very sharp diffraction spots already found in previous studies on spicules from different sponges are confirmed, but also the 11 sharp spots in the diffraction pattern recorded after thermal treatment at 250 degrees C can only be interpreted in terms of a natural nanocomposite mesostructure with an hexagonal lattice formed by a three-dimensional periodic arrangement of silica cages in which the protein units act as structure directing agent.