A member of the Class Echinoidea from the Phylum Echinodermata. Echinoids are subdivided into two subgroups: regular echinoids, with nearly perfect pentameral (five-part) symmetry; and irregular echinoids with altered symmetry. Examples of regular echinoids are modern sea urchins and pencil urchins; irregular echinoids include the heart urchins, sea biscuits, and the sand dollars.
