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Why are echinoderms more advanced than arthropods and mollusks?

User Firnas
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They have internal skeletons which cover their skin. The body of an echinoderm seems little more than a skeleton of tiny plates and water. They don't use large muscles working on large body parts like many other animals. Instead they move, feed and breathe with a unique water-vascular system ending in hundreds of water-filled tube feet.
User VladH
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