A marine echinoderm that has a spherical or flattened shell covered in mobile spines, with a mouth on the underside and calcareous jaws. Many species are harvested for food.
- Class Echinoidea: several families and genera, and numerous species, including the Atlantic purple sea urchin (Arbacia punctulata, family Arbaciidae) and the green sea urchin (Strongylocentrotus drobachiensis, family Strongylocentrotidae)
- The sea urchin is one of the few marine organisms whose genome has been sequenced.
- The inner shell of a sea urchin is a hollow globe, scored in five curved sections that taper at the ends into a small hole at the top and bottom.
- The red sea urchin's body is domed above and flat below.
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