Sponges are the most primitive multicellular animals in that their cells function independently, with no coordination via a central nervous system, and they do not form true tissues nor organs. Their metabolism is largely dependent on the flow of water through its dermal pores and inner canals, thus they are exclusively aquatic and chiefly marine. They are sessile animals, and are often found attached to rocks, submerged timber, corals and the shells of other animals.