There are many orders of sea urchins. The ones in the Subclass Euechinoidea go like this: Superorder Atelostomata - Orders Cassiduloida and Spatangoida; Superorder Diadematacea - Orders Diadematoida, Echinothurioida, and Pedinoida; Superorder Echinacea - Orders Arbacioida, Echinoida, Phymosomatoida, Salenioida, and Temnopleurioda; Superorder Gnathostomata - Orders Clypeasterioda (sand dollars) and Holectypoida.