The only parasite that functionally replaces an organ. The tongue-eating louse (Cymothoa exigua) is one of the strangest and most disturbing parasites in the animal kingdom. It enters through the gills of a fish and attaches itself to the base of the tongue. It severs the blood vessels, causing the tongue to wither and fall off. Then it replaces the tongue, attaching itself to the stump and functioning like a prosthetic tongue. The fish can still eat and live normally—with the louse acting as its new tongue.

I can name a few more disturbing than this one.