The Library of Evolution

This is the creepy Cymothoa exigua, or the tongue-eating louse. It is a crustacean that enters fish through the gills, and then attaches itself at the base of the fish's tongue. It extracts blood through the claws on its front, causing the tongue to atrophy from lack of blood. The parasite then replaces the fish's tongue by attaching its own body to the muscles of the tongue stub. Once it 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 functionally replacing a host organ.

It is currently believed that C. exigua are not harmful to humans unless picked up alive, in which case they can bite. So stay away!