In certain types of marine animals, such as starfish and sea urchins, water allows the creature to maintain control and movement over the many different tube feet that live in its ambulacral grooves. To do this, the water vascular system is composed primarily of complex canals and reservoirs that are filled with fluid. These canals and reservoirs contribute to the organisms' ability to move, exchange gases and perform other functions.
One of the most important parts of the water vascular system is the opening, known as the madreporite, which can act like a pressure valve and filter water in and out of the organism. This is connected to a short stone canal which connects to yet another canal, the gut canal. The gut canal in these organisms circles around the gut and is usually found below the mouth.
There are also polian vesicles that branch out from the stone canal and reach all the way into the body cavity. Eventually, these polian vesicles connect to oral tentacles. The water pressure in these vesicles, which is controlled by the madreporite, and the contractions of the vesicles themselves dictate the movement of these oral tentacles. Other types of muscles, called podia and ampulla, are also attached to these canals. These are controlled by hydraulic water pressures and through muscle contractions. They can be used to grip rocks and maintain positions.