Virginia Land Snail

"Eastern Forest Snails are sometimes infected by a parasite called a Brainworm. Brainworms are a type of microscopic animal called nematodes. Eastern Forest Snails will eat deer poop because of the plant material in it. When they do, Brainworm larvae (baby nematodes) develop inside the snail's body. The Brainworm larvae live inside the snail, continuing to grow, until a new deer accidentally eats the snail when it is munching on grass. The larvae then grow into adult Brainworms inside the deer and lay new eggs which come out in the deer poop. The Brainworm nematode cannot live without both the snail and the deer." Source: http://www.fcps.edu/islandcreekes/ecology/eastern_forest_snail.htm