Water gets into the water vascular system through an opening in the echinoderm's body. When the animal contracts its muscles, the water is forced through the system and into the tube feet. The tube feet act as suction cups, gripping the surface below. When the echinoderm retracts its muscles, the tube feet retract and let go of the surface. This series of contractions and retractions allows the animal to move along the ocean floor.
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