any of the echinoid marine invertebrates of the order Clypeastroida (phylum Echinodermata), in which the body is flattened. The surface is covered with short spines (often furlike) and inconspicuous pedicellariae (pincerlike organs). In many species the hollow, slightly elongated test (internal skeleton), which accommodates the water-vascular system, is symmetrically notched on the edge or has narrow perforations (lunules). The upper surface of the test has a radial, flowerlike arrangement of five porous spaces, called petaloids; the pores permit the extension of tube feet modified for respiration. Tube feet on the underside of the body are used for taking food and may also assist the spines in locomotion.

Found worldwide in shallow seas, cake urchins burrow in sand and feed on organic particles wafted to the mouth on water currents. They are in turn the prey of other echinoderms and mollusks.

Cake urchins in which the body is a coinlike, thin-edged disk are called sand dollars (sand dollar), a name sometimes given to cake urchins generally. The test of the common sand dollar (Echinarachnius parma) is often washed up on beaches of North America and Japan. Species with lunules are generally called keyhole urchins. The largest and thinnest cake urchin is the yellow or purple sea pancake (Echinodiscus auritus) of the East African coast.