From Kottke, a video of the Leucochloridium parasite infecting a snail:
The worm is consumed by the snail, and begins its development in the snail digestive tract. Once it grows and matures, it moves into the snail’s optical tentacles, where it will pulsate and writhe as an example of aggressive mimicry, turning the tentacle into a dead-ringer for a caterpillar larvae, and making the snail a visible snack to a passing bird. The worm’s dance is also deadly because it renders the snail insensitive to light, making it incapable of shielding itself from predators. After the bird eats the infected snail, the worm matures fully inside the bird’s digestive tract, there it reproduces and lays eggs. Once the bird excretes the Leucochloridium larva, it is consumed by snails, thus continuing its life cycle.
Reminds me of that Star Trek episode where all the Star Fleet personnel on some planet were infected by parasites. And then Will Riker came and saved them.
Because he’s Will Riker. And awesome.