The most accurate illustration I know of what these parasites really look like, and a photo of what a brood sac looks like when it leaves the snail - something that was once assumed to happen just by accident to injured snails, but now we know the sacs routinely wander off as entire false caterpillars :)
Huh, they look like a cluster of roots instead of a worm as I expected :o
They’re related to flatworms (which includes tapeworms) but are unusual in that they branch out from a central point, kind of a colonial worm blob or a many-headed hydra!
Someone got an amazing video where you can see at least four broodsacs pulsating in one snail:
