The Class Asteroidea mainly includes seastars. A typical seastar is composed of a central disc and five arms, or rays. The mouth is on the underside and have tiny tube feet that pump water in order for the seastar to move. The suckers on the end of the arms hold firmly onto solid surfaces as their muscles contract and force water back that causes the tube feet to shorten. This pulls the animal over into the desired direction. This locomotion is used mainly for getting across rocks whereas on the sand they usually move in a walking motion.