CARL ZIMMER, NATIONAL GEOGRAPHIC CONTRIBUTER:

There's a parasite that lives in North America and Europe called leucochloridium paradoxum. When it's inside a snail's body, it crawls up into the snail's tentacles. Because the tentacles are translucent, you can actually see the parasite inside. And there, it twitches around so that it creates a strange kind of pulsating appearance to the tentacle. So what you see when you see this pulsing is actually the parasite inside the snail.

The parasite also causes the snail to crawl up plants. Snails usually stay away from the light but the parasite takes over its brain and make it go up. And the reason it does that is because the parasite needs get into a bird. Birds to actually like to eat snails, but when a snails goes up a plant and its tentacle look like a caterpillar instead of a snail, the birds gets very interested.

And they swoop down and they take a bite out of the snail tentacle. In that way, the parasite is able to get inside the bird. The parasite travels down through its gut, to its rectum. And there, it becomes an adult, produces its eggs, which is then passed out with the bird droppings where snails come along and feed on them. And now the parasite's eggs get into a new snail which then get into the tentacles, and so on and so on.