An aquatic, multicellular animal (phylum Porifera) that lacks distinct organs and tissues, but consists of flagellate cells, pores, and canals. Sponges have an outer covering and an internal skeleton made from jelly-like collagen, often with siliceous or calcareous spicules, interconnected protein fibres, or a combination of these. A sponge depends on a constant flow of water through its body to transport food (bacteria) and oxygen to its cells, and to remove wastes. A small piece broken from a sponge will grow into a whole animal. Sponges reproduce asexually by budding and from fragments detached by wave action, and sexually, producing motile larvae. Most sponges are marine. The phylum is divided into classes according to the type of skeleton, of which three are extant, with up to 10,000 species.