heart urchin

Any sea urchin of the order Spatangoida, characterized by having heart-shaped or oval bodies, modified for burrowing in sand. Their spines are reduced in number and size. Clavules, flattened modified spines, form clearly defined tracts across the animal and move to produce a water current through the burrow for cleaning purposes.

The tube feet are restricted to the oral and aboral (opposite to the mouth) surfaces and have specialized functions. On the aboral surface the tube feet are capable of great extension and play a role in respiration. The oral tube feet are modified to pick up particles of food from the surrounding sand.