<P> The exact structure of the system varies somewhat between the five classes of echinoderm . The system is part of the coelomic cavities of echinoderms, together with the haemal coelom (or haemal system), perivisceral coelom, gonadal coelom and perihaemal coelom . </P> <P> Other terms sometimes used to refer to the water vascular system are "ambulacral system" and "aquiferous system". In the past, "aquiferous system" was also used to refer to many unrelated invertebrate structures, but today, it is restricted to water channels in sponges and the hydrostatic skeleton of some mollusks like Polinices . </P> <P> In sea stars, water enters the system through a sieve - like structure on the upper surface of the animal, called the madreporite . This overlies a small sac, or ampulla, connected to a duct termed the stone canal, which is, as its name implies, commonly lined with calcareous material . The stone canal runs to a circular ring canal, from which radial canals run outwards along the ambulacral grooves . Each arm of a sea star has one such groove on its underside, while, in sea urchins, they run along the outside of the body . </P> <P> Each side of the radial canals gives rise to a row of bulb - like ampullae, which are connected via lateral canals . In sea stars these are always staggered, so that an ampulla on the left follows one on the right, and so on down the length of the radial canal . The ampullae are connected to suckerlike podia . The entire structure is called a tube foot . In most cases, the small lateral canals connecting the ampullae to the radial canal are of equal length, so that the tube feet are arranged in two rows, one along each side of the groove . In some species, however, there are alternately long and short lateral canals, giving the appearance of two rows on each side of the groove, for four in total . </P>

Where does water enter the vascular system of a starfish