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.
- ‘The red sea urchin's body is domed above and flat below.’
- ‘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.’
- ‘This sea urchin defends itself with two sets of mobile spines.’
- ‘The sea urchin occupies an important evolutionary position with respect to vertebrates and humans.’
We take a look at several popular, though confusing, punctuation marks.