(And remember...if they look like this, PUT THEM BACK, they are ALIVE!) Sanibel Sea school. sand dollars! These spiny-skinned creatures belong to the class Echinoidea and are flattened versions of their cousins the sea urchins. Did you know that it takes 6-12 minutes for a sand dollar to rebury itself after being excavated from the sand? And if they get flipped over, it may take them up to an hour to flip right side up again. Exhausting but fascinating!