KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY
Sand dollar. The upper surface of the dried test (shell) of an unidentified species of sand dollar. Sand dollars belong to the same class as sea urchins (Echinoidea), but are flattened and covered with short spines. They move by means of the spines and small tube feet on both surfaces of the body. There are large tube feet which protrude from five double rows of holes on the upper surface; these have a respiratory function. Sand dollars live on the sea bed and swallow sand, digesting the organic matter contained in it. They are dioecious, that is, eggs and sperm are produced by different individuals. Fertilization occurs in the sea water.
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