Sand Dollar, invertebrate having a flat, circular outer shell. The common sand dollar of the northeastern United States is found on sandy bottoms in comparatively shallow water. The shell is about 8 cm (about 3 in) in diameter and has many small perforations that form a symmetrical petal-like design. The entire shell is penetrated by many small, brown spines that give the shell a velvety appearance and enable the animal to move about. Sand dollars obtain food by engulfing sand from which they extract minute plants and animals.