I recently read this interesting but pretty revolting account describing a type of parasitic crustacean commonly known as the tongue-eating louse. Basically, the little thing gets into the mouth of a certain type of fish (Spotted Rose Snapper) and attaches itself to the base of the fish’s tongue.
There it lives and grows by sucking the blood from the artery feeding the tongue. Eventually, it uses up so much blood that the tongue ‘dies’ and shrivels up to a stub. The amazing thing is:
The louse, which grows to be about the same size and shape as the original tongue, remains connected to the stub of the tongue—in other words, it effectively replaces the fish’s tongue with itself. At this point, having lost its blood supply, it switches to a new food source: bits of whatever the fish happens to be eating. Other than having a lousy tongue, the fish appears to be unaffected by the parasite; it can still, in fact, manipulate the louse just as it would its natural tongue. No other parasite has been found to completely replace an organ in the host.
If you want to see a couple of pictures go here.