Sand dollars are actually the skeletons of marine echinoderms and take their off-white, chalky form after the animal dies. They are closely related to flat sea urchins, starfish and sea cucumbers. When alive, they are covered in short, fine spines and range in color from yellow to blue and purple.
Sand dollars get their name from their size, shape and the coloring of their skeletons. They are 2 to 4 inches across and coin-shaped. The round shape and bleached white color of their skeleton on the beach at low tide is reminiscent of silver dollars.
Sand dollars in their living form exist in marine environments on the sandy ocean floor, usually in shallow waters. They use their spines to burrow into the sand as well as to move over it. In tranquil waters, they stand on end, partially buried in the sand. As ocean currents increase, they flatten out or burrow completely into the sand. They can also grow heavier skeletons by ingesting sand in order to weigh themselves down.
There are male and female sand dollars. The animals reproduce sexually by releasing eggs and sperm into the water. The eggs are fertilized and form larvae, which eventually settle to the ocean floor and slowly change into the flattened, spine-covered creature. They can live anywhere from 6 to 10 years.