It’s interesting that these animals use sea water to help them with movement.  Sea water is drawn into the sea star and by using a form of hydraulics this animal is able to flex and relax its many hundreds of tube feet allowing it to move across the ocean floor.  Sea stars also have an open circulatory system with internal channels where sea water flows into and out of these invertebrates, unlike humans with a closed circulatory system.  Their open circulatory system allows for the movement of gasses and nutrients within the animal, as well as the release of metabolic waste products back into the ocean, and all this is done without a heart.