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Description: (a) Hydrostatic skeleton. By alternately contracting and relaxing circular and longitudinal muscles, earthworms use hydrostatic pressure to achieve locomotion. Clinging bristles along the body surface help prevent backsliding. (b) Exoskeleton. An arthropod's skeleton covers and protects its body, but it must be periodically shed and replaced to allow growth. (c) Endoskeleton. Echinoderms, such as the sea star (starfish), have endoskeletons of bony plates made of calcium carbonate (shown in inset).
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