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Echinoderm, member of a large group, or phylum, of marine invertebrates with an external skeleton of plates just under the skin. Some live fixed on the ocean floor; others move slowly by means of arrays of tube-feet. Many classes of echinoderms are extinct, but 5 are still in existence and are very abundant: Crinoidea (sea lilies and feather stars), Asteroidea (starfishes), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), and Holothuroidea (sea cucumbers).

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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Dream to Eijkman, Christiaan