Echinoderms don't really "breathe" in the same way we do. Breathing is just one method of bringing oxygen into the body and exchanging it for toxic gases such as carbon dioxide. Gas exchange between sea stars and their environments occurs across permeable surfaces such as the tube feet, which have an extensive surface area. Internally, gas exchange is accomplished by movement of fluid called coelomic fluid throughout the body. The movement of this fluid is facilitated by ciliated cells lining the body cavity. This is probably more complicated than you anticipated - sorry, that's nature!