Cymothoa exigua, or the tongue eating louse, is a parisitic isopod that attaches itself to a fish and crawls inside it through it's gills to the mouth. It then attaches it's front claws to the fishes tongue and draws blood through them until the tongue atrophies and shrivels away. It then attaches it's legs to the fish's tongue ligaments and replaces the tongue as a working organ. In some cases, it lives with the fish for the rest of it's life; feeding off more blood or fish mucus.