Like the starfish or sea star, sand dollars are "Echinoderms". They are in the same family as sea urchins. They are usually blue or purple, they are covered in spines, and they have tiny "tube feet" that they use to dig in the sand. They crawl on the ocean floor sucking up microscopic organisms with their mouths.
The sand dollars that we see on the beach are just the skeletons. Sand dollars can't live on the beach. They have to stay underwater to survive. When we see them on the beach, they're already dead.