Sand dollars are flat, round marine animals related to sea urchins, sea stars, and other echinoderms. The most common sand dollar, Echinarachnius parma, is widespread in circumpolar ocean waters of the Northern Hemisphere, from the intertidal zone to considerable depths. A sand dollar has a rigid skeleton known as a test. When sand dollars are living, they have a skin of motile spines covering the test. Movement is accomplished by the coordinated action of the spines. Like sea urchins, sand dollars have five paired rows of pores. In sand dollars they are arranged in a petal-like pattern. These pores are perforations in the endoskeleton through which the podia, used in gas exchange, project from the body.
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