Any echinoderm (order Clypeastroida, class Echinoidea) that has a coinlike, thin-edged body. Five petals spread out from the center of the upper body. It burrows in sand, feeding on organic particles wafted to the mouth, located in the center of the body's underside. Small spines covering the body are used for digging and crawling. Tests (external skeletons) of the common sand dollar (Echinarachnius parma), which often wash up on beaches in North America and Japan, are 2–4 in. (5–10 cm) in diameter.