 Sea Urchins |
 Sand Dollars (Echinarachnius parma) courtesy Marine Biological Laboratory |
The final class of echinoderms is the class Echinoidea, comprised of the sea urchins and sand dollars. Sea urchins lack arms but have five rows of tube feet that protrude through the hard outer body and allow the sea urchin to move by using a pulling motion. Certain muscles also allow sea urchins to walk on their spine, an alternative way of moving. Although their spines, which are attached to muscles and can be moved quickly, look ferocious, sea urchins are herbivores, feeding primarily on algae. There are approximately 1,000 species of sea urchins and sand dollars making up class Echinoidea.
Information provided by: http://www.sidwell.edu