A sea star uses an amazing water vascular system to move it’s tube feet, transport nutrients and gases.
The water first enters through the madreporite and then to the ring canals then to the radials which run down through each arm. The water goes into the tube feet where an ampulla (muscular bulb) and a sucker can be found. The tube feet move through hydraulic pressure.
The water can also leave through the madreporite.
A Study In Biology 