I was just going to start a topic like this.
The Tongue-Eating Sea Louse: Was It Designed?
This sea louse makes it house inside the fish mouth and feeds on the blood from the tongue till the organ withers away. The parasite very cleverly attaches itself to the remaining stump and continues to feed on blood and mucus in the mouth. It may seem as a consolation that the fish can use the louse exactly as its own tongue.
There is a whole series of parasites: http://alltoptens.com/top-ten-incredibly-dangerous-parasites/