A sand dollar is a flattened relative of sea urchins within echinoderms. Its body form and small spines suit life on or within sediment. The familiar pale disk is the skeleton of an animal that was covered by living tissue.[ref]
https://ocean.si.edu/ocean-life/invertebrates/sea-stars-urchins-and-relatives. Smithsonian Ocean, sea urchins and sand dollars
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