All sand dollars have a rigid skeleton known as a "test". This is the typical white disk found washed up on beaches. The living animals have a skin of movable spines on the test. Movement is done by the action of the spines. Like other sea urchins, sand dollars have five paired rows of pores. The pores are arranged in a petal-like pattern. These pores are perforations in the endoskeleton through which the tube feet, used in gas exchange, project from the body.