Liver flukes

Liver flukes are a polyphyletic group of parasitic trematodes (phylum Platyhelminthes). Adults of liver flukes are localized in the liver of various mammals, including humans.These flatworms can occur in bile ducts, gallbladder, and liver parenchyma. They feed on blood. Adult flukes produce eggs which are passed into the intestine. It depends on two intermediate hosts (a snail and a fish) to complete its life cycle.

Examples include:

...
Wikipedia