Introductionsea star, also called starfish, echinoderm of the class Asteroidae, common in tide pools. Sea stars vary in size from under 1/2 in. (1.3 cm) to over 3 ft (90 cm) in diameter. They are commonly dull shades of yellow or orange, but there are many brightly colored ones as well. There are about 2,000 species distributed throughout the world, mostly in shallow water along rocky coasts.
The body of most species consists of a central disk from which radiate a number of tapering arms—usually five, but up to 25 in some species. Some sea stars are pentagonal, the points of the disk not extending into arms. Each arm contains an extension of the body cavity and body organs. A network of calcareous plates located beneath the skin forms an external skeleton; the plates are joined by connective tissue and muscle, giving the apparently rigid sea star considerable flexibility. Calcareous spines, some of them movable, project from the skin.
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