sponge, common name for members of the aquatic animal phylum Porifera, and for the dried, processed skeletons of certain species used to hold water. Over 4,500 living species are known; they are found throughout the world, especially in shallow temperate waters. All are marine except the members of a single freshwater family. Adult sponges are sessile, attaching themselves to rocks, coral, shells, and other substrates. They show so little movement that until the 18th cent. naturalists considered them plants. Most adults are colonial. Sexual reproduction gives rise to a free-swimming larva, which soon settles on a suitable substrate and develops into the adult form. Asexual reproduction also occurs. The individual sponge is saclike in construction; water is drawn into its central cavity through many tiny holes in the body wall and expelled through a large opening at the top of the body. Hard materials of various kinds, depending on the type of sponge, are imbedded in the body wall, forming a skeleton. A colony consists of a mass of many such individuals. Solitary sponges and colonies range in diameter from about 1/2 in. to 5 ft (1–150 cm) and vary greatly in shape. Some are branched, some more or less globular, and some are thin encrustations on rocks and pilings. Brilliantly colored sponges are common. Bath sponges are the skeletons of certain colonial sponges. These skeletons are composed of a fibrous meshwork of spongin, a material related to horn, and owe their absorbent properties to the fineness of the mesh. Sponges have been used to hold liquid since ancient times. The ancient Greeks used them for bathing and scrubbing, and Roman soldiers used them for drinking. Commercial sponges, species of the genera Spongia and Hippospongia, are harvested principally in the Mediterranean and Caribbean seas and off the Florida coast. They are brought up by divers in deep water, or raked in with long-handled forks in shallow water. They are left in water until the living tissue rots away; the skeletons are then cleaned and dried and sometimes bleached. Sponge fishing has declined in recent decades due to the use of synthetic sponges and to a decline in the population of commercially valuable natural sponges. The block-shaped sponges now commonly sold are the synthetic product. Dried natural sponges are light gray or brown and irregular in shape.