Cymothoa exigua

Cymothoa exigua, or the tongue-eating louse, is a crustacean, a parasitic isopod. 
This parasite enters fish through the gills, and then attaches itself. The female attaches to the fish's tongue, and the male attaches on the gill arches beneath and behind the female. The female parasite destroys the fish's tongue. Then it attaches itself to the stub of what was once the tongue and becomes the fish's new tongue.
Behavior.
"Cymothoa exigua" gets blood through the claws on its front, causing the tongue to atrophy (wither) from lack of blood. The parasite then replaces the fish's tongue by attaching its own body to the muscles of the tongue stub. 
The fish is able to use the parasite just like a normal tongue. It appears that the parasite does not cause any other damage to the host fish. Once "C. exigua" replaces the tongue, some feed on the host's blood and many others feed on fish mucus. This is the only known case of a parasite replacing the function of a host organ. There are many species of "Cymothoa", but only "C. exigua" is known to consume and replace its host's tongue.