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The contractile vacuole allows free-living protists to expel extra water. <b>The contractile vacuole allows free-living protists to expel extra water <br /></b>.
It includes sponges, coelenterates and echinoderms. There are flame cells which help in excretion and occur in flat worms. The excretory system is unique in round worms which contains canals and tube. Annelids have nephridia as their excretory organ. Arthropods have malphigian tubules as their excretory organ.