There is a strange relationship that exist in the sea between toungue-eating louse and fish.  The louse is a parasitic crustacen.  The female attaches itself to the toungue, while the male attaches itself to the gills on the outside.  The female extracts blood from the tounge of the fish causing the tounge to retract and eventually shrink away.  The louse then attaches its own body directly to the toungue muscle and the toungue continues its functionality without harming the fish.  This is the only known case of a parasite replacing a host organ.