Sand dollars are the fused calcium-based shells of a type of sea urchin. These sea urchins are burrowers that feed on tiny organisms in ocean sediment. Their burrowing lifestyle means their spines must be very short, and their bodies are flattened and slightly elongated to enable them to move through sediment more easily.
The sand dollars found on beaches are generally the hollowed and dried skeletons of the sea urchins that grew them, and each of their features corresponds to missing parts of the whole. The short spines, which are not very useful for defense, are used for locomotion instead. Sand dollar skeletons bear five grooves that somewhat resemble flower petals. These grooves hold double rows of pores the sea urchins use to breathe. They also hold tube feet that are also used for respiration, not for locomotion as with many other echinoderms like sea stars. Their shells have a small hole in the center bottom where their mouth is located. Food in the sediment around them is coated in mucus and moved toward the mouth via the spines and other structures. The urchins have internal strong jaws that grind grains of sand and extract any organic matter before excreting the sand through the tops of their shells.