This long article on the history of tapeworms has approximately three squicks per paragraph (dinosaurs had incredibly long tapeworms!), but this bit takes the cake:
It appears the monogeneans move from fish to fish, each species of parasite living on a single species of fish host. (Here’s a digression but a good one: some monogeneans give birth to offspring without releasing them from their bodies. Their offspring mature inside them and give birth as well. Like a hideous Russian doll, a monogenean may contain twenty generations of descendents inside its body! [“Kids, it’s time you found a place of your own…”])
(via JWZ)