Sea Urchins and Sand Dollars (Echinoidea)

Echinoids are the echinoderms that include the sea urchins and sand dollars. Their skeleton is almost always a rigid test of tightly interlocking plates, in contrast with the more flexible skeletons of starfish, brittle stars and sea cucumbers, and test shapes range from nearly globular in sea urchins to highly flattened in sand dollars. Living echinoids are covered with movable spines anchored in sockets in the test, and the mouth of most is fitted with five hard teeth in a circlet called Aristotle's lantern. Regular echinoids, with nearly perfect five-part symmetry, include the pencil urchins and sea urchins, while irregular echinoids include the sand dollars, sea biscuits and heart urchins. The roe of some sea urchins is eaten, notably in Japanese sushi.