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Article
Published: 2007-12-21

Phylum Echinodermata*

National Museum of Natural History, Mail Stop MRC163, Smithsonian Institution, Washington DC 20013-7012, USA
Echinodermata

Abstract

The Phylum Echinodermata, comprising approximately 7,000 living species, and 13,000 fossil species, is epitomized by the familiar sea star, a universal symbol of the marine realm.  This distinctive group of animals may be briefly defined as possessing a skeleton of calcium carbonate in the form of calcite; a unique water-vascular system which mediates feeding, locomotion, and other functions; and a more or less conspicuous five-part radial symmetry.  A closer look at some extant echinoderms will show that some taxa of sea cucumbers lack calcite in their body walls, some taxa of sea stars have “outgrown” five-part symmetry and may have 50 or more arms, and many echinoderms show a more or less conspicuous bilateral symmetry superimposed upon a radial pattern.  Fossil echinoderms can be even more puzzling, for some are decidedly asymmetrical, and others may lack evidence of a water-vascular system.  Perhaps the only truly reliable taxonomic character of the phylum is that its members today are restricted to the marine realm.

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