These structures can be considered primitive kidneys. They are unique to the phylum. Planarians are placed in the Class Turbellaria, and these were discussed extensively above. The animals generally have a sucker around the mouth and one on the ventral surface. These suckers allow the animal to cling to the body organs in which they live.
Many flukes have immature stages that live in other animals.
These are released from snails, and a second immature stage then lives within fish. When humans ingest raw or undercooked fish, they also ingest immature stages of the fluke. The immature stages migrate to the bile ducts of the liver where they mature. The mature stages may live within the bile ducts for 15-20 years. While there they cause damage of the liver and may cause death of the human host.