The sponges, a phylum of the animal kingdom that includes more than 8000 described species. The body plan of sponges is unique among animals. Currents of water are drawn through small pores, or ostia, in the sponge body and leave by way of larger openings called oscula. The beating of flagella on collar cells or choanocytes, localized in chambers on the interior of the sponge, maintains the water current. Support for the sponge tissues is provided by calcareous or siliceous spicules, by organic fibers, or by a combination of organic fibers and siliceous spicules. Some species have a compound skeleton of organic fibers, siliceous spicules, and a basal mass of aragonite or calcite. The skeletons of species with supporting networks of organic fibers have long been used for bathing and cleaning purposes. Because of their simple and possibly primitive organization, sponges are of interest to zoologists as an aid in understanding the origin of multicellular animals.