Gut-worms live inside their parents like nesting dolls

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…"])

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(via JWZ)