A small disc-shaped marine animal closely related to the sea urchin. They are from the class of marine animals known as Echinoids, spiny skinned creatures. Their relations include sea urchins, star fish, sea lilies and sea cucumbers.
When exposed to a steady flow of water, they gather in groups, forming aligned rows in the sand, while digging their front edge in and raising their back edge into the flow of water, lined up so it passes from right to left across their bodies. Because the shape of a sand dollar is a hydrofoil, this draws particles of food closer in to their mouths during feeding, a benefit enhanced by the alignment of many individuals together into a communal feeding group.