An echinoderm is a marine animal in the group containing sea stars, sea urchins and sand dollars. Adult organization commonly has five-part radial structure, whereas larvae can have bilateral symmetry. The skeleton consists of internal mineralized elements.[ref]
https://ocean.si.edu/ocean-life/invertebrates/sea-stars-urchins-and-relatives. Smithsonian Ocean, symmetry and skeleton
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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