Sponges can be calcareous, siliceous, or composed of spongin. This sponge has neither spicules nor spongin fibres in its tissues. Many sponges have internal skeletons of spongin and/or spicules of calcium carbonate or silicon dioxide. The poriferan 'skeleton' consists of microscopic calcareous or siliceous spicules or a spongin network. Species of the genus "Grantia" contain spicules and spongin fibers. More commonly the mesohyl is stiffened by mineral spicules, by spongin fibers or both. True spongin is found only in members of the class Demospongiae. More commonly, the mesohyl is stiffened by mineral spicules, by spongin fibers, or both. They are bleached and marketed; the spongin gives the sponge its softness. Demosponges use spongin; many species have silica spicules, whereas some species have calcium carbonate exoskeletons.