The sand dollars are the most specialised sea urchins. All their features are adaptations for burrowing. They live just beneath the sand, either flat or at an angle. At an angle they keep the edge facing the oncoming currents, presumably to catch food. This excellent habitat has been taken up by other groups of sea urchins. The heart urchins, another order, are even more effective in burrowing. Sand dollars can live for up to 10 years.