<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> 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>
where does water enter the vascular system of a starfish
where does water enter the vascular system of a starfish
madreporite
madreporite