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While the majority of animal body plans are bilateral with a distinct head and tail, echinoderms do not follow this pattern. While many echinoderms begin life as a bilateral larva, later in life they take a radical change of course. They become radial with five-part symmetry and no central brain. Echinoderms move, feed and breathe with a unique water-vascular system ending in what are called tube feet. Sea stars use their tube feet to slowly pry open clams, mussels or other prey. Some sea stars can even evert their stomach between the two shells of a bivalve and digest the soft parts inside. The bodies of echinoderms are made of hard, calcium-based plates that are often spiny and covered by a thin skin. While most echinoderms are either stationary or slow-moving, methodical animals, they are nevertheless prominent members of the marine environment.
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