OSCAR BURRIEL / SCIENCE PHOTO LIBRARY OSCAR BURRIEL / SCIENCE PHOTO LIBRARY
Sand dollar. Shell, or test, of a sand dollar, a bottom-dwelling marine scavenger related to the sea urchin. Like other echinoderms, it has five- fold radial symmetry, as seen by the arrangement of the pores on its test. The pores are used to take in and expel water. It does this to power its water vascular system, with which it moves its body parts. The sand dollar inhabits muddy or sandy seabeds, and feeds on organic matter. Sand dollars are found in most of the world's seas.
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