Typical sea urchins have spines about 1 to 3 cm ( 0.39 to 1.18 in ) in length , 1 to 2 mm ( 0.039 to 0.079 in ) thick , and not terribly sharp .
Typical sea urchins have spines that are 1 to 3 centimetres ( 0.39 to 1.18 in ) in length , 1 to 2 millimetres ( 0.039 to 0.079 in ) thick , and not terribly sharp .