<P> Contraction of the ampullae causes the podia to stretch as water is brought into them . This whole process allows for movement, and is quite powerful but extremely slow . </P> <P> The central ring canal, in addition to connecting the radial canals to each other and to the stone canal, also has a number of other specialised structures on the inner surface . In between each radial canal, in many sea star species, there lies a muscular sac called a polian vesicle . The ring canal also has four or five pairs of complex pouches, called Tiedemann's bodies . These apparently produce coelomocytes, amoeboid cells somewhat similar to the blood cells of vertebrates . </P> <P> Although the contents of the water vascular system are essentially sea water, apart from coelomocytes, the fluid also contains some protein and high levels of potassium salts . </P> <P> Ophiuroids, the group including brittle stars and basket stars, have a somewhat different water vascular system from sea stars, despite their superficially similar appearance . The madreporite is located on the underside of the animal, usually in one of the jaw plates . The stone canal runs upwards to the ring canal, typically located in a circular depression on the upper (i.e. internal) surface of the jaws . The ring canal has four polian vesicles . </P>

Where does water enter and leave a starfish