sea urchin 
noun
A marine echinoderm having a spherical or flattened shell covered in mobile spines, with a mouth on the underside and calcareous jaws.
- Class Echinoidea.
- 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.
- fairskin
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