We've all heard about human zombies, but what about snail zombies? There is a dastardly devious worm that hijacks a snail's brain and leads that brain to sacrifice itself. Link
The parasitic worm is called "Leucocholordium Paradoxum" and lives primarily in north america and Europe.
Once it infiltrates the snail's body, it crawls not the snail's tentacles. These tentacles, for added spookiness, are translucent and you can actually see the worm working its will from within.
Snails typically have the instinct not to crawl up plants, because it is safer to stay away from the light. But this worm is so intense that it turns the snail against its instincts in order to get the snail visible to birds.
Want one more layer to this complicated plan? Birds don't really like snails -but they do like caterpillars. It's tentacles, and the worm inside, resembles a caterpillar on a leaf and the bird becomes interested. Typically, they snatch one of the tentacles, allowing the worm to enter its guy and travel through it its rectum. In the bird's rectum, it becomes an adult and produces.
Once the worm is 'passed' with the birds droppings, snails come across and feed on them.
And then the parasite's eggs get into a new snail...and so on and so forth.
The above picture is not infected with Leucocholordium Paradoxum and is from Flickr User Andy Powell and is licensed under Creative Commons.