A member of the phylum Porifera, marine and freshwater invertebrates that live permanently attached to rocks or other surfaces. The body of a sponge is hollow and consists basically of an aggregation of cells between which there is little nervous coordination, although they do have specialized sets of cells that perform different functions. One set of cells causes water to flow in through openings in the body wall and out through openings at the top; food particles are filtered from the water by these cells. Other cells construct a stiffening skeletal framework of spicules of chalk, silica, or fibrous protein to support the body.