The familiar sand dollar cast up on the beach is white, with an obvious five-pointed shape on the back. But a live sand dollar looks completely different. Densely packed, tiny, dark purple spines cover live sand dollars and hide the star design.
In their sandy seafloor habitat, sand dollars use their fuzzy spines, aided by tiny hairs (cilia), to ferry food particles along their bodies to a central mouth on their bottom side. They capture plankton with spines and pincers (pedicellariae) on their body surfaces.
The sand dollar's mouth has a jaw with five teethlike sections to grind up tiny plants and animals. Sometimes a sand dollar "chews" its food for fifteen minutes before swallowing. It can take two days for the food to digest.
Scientists can age a sand dollar by counting the growth rings on the plates of the exoskeleton. Sand dollars usually live six to 10 years.