The tongue-eating louse is the only known parasite to completely replace an organ in another animal:
An unknown person working at Galveston Island Sate Park, Texas Parks and Wildlife, has posted a picture of a unique fish that was caught at the park on Facebook—it has no natural tongue. Instead, it has a tongue made up of a group of parasites known as a tongue-eating louse. In the picture, the fish is held up to the camera with its mouth wide open showing the strange foreign ‘tongue’ inside …
It makes its way into the fish’s mouth through its gills—only the female replaces the tongue, while the males remain in the gills. He also notes that until now, he had never seen it in an Atlantic croaker. He adds that the louse is the only known parasite to completely replace an organ in another creature. Oddly, the new tongue does not seem to harm the fish, or the people who may catch it, though it is not known what would happen if a person were to eat the parasitic tongue. Fisher notes that not enough research has been done on the parasite to understand how it pulls off such a feat. Prior research has shown that the parasite does not survive by eating the food taken in by the fish but instead consumes the mucus that forms on the inside of the fish’s mouth.Bob Yirka, “Parasite that replaces a fish’s tongue caught at Texas state park” at Phys.org (October 26, 2021)
The takehome point is that devolution (shedding independent characteristics in order to survive, perhaps symbiotically) leads the history of life forms down some strange paths.
You may also wish to read: Devolution: Getting back to the simple life.