The March/April edition of Mental Floss has an awesome article on parasites. One kind attaches itself to the tongue of a fish, feeding off the blood supply until the fish’s tongue drops off. Then the parasite serves as a surrogate tongue..
Just as good is the female Sacculina, which starts out as a sluglike thing floating around in the water. It finds a crab, and then stabs one of the crab’s joints with a dagger-like appendage. The Sacculina ooooozes into the crab through the hollow dagger, leaving an empty shell outside. Once inside the crab, the gooey parasite takes root, wrapping around the crab’s eyestalks and legs, growing until a little bit of it pops out of the crab’s shell. Then it begins to steer the crab wherever it wants to go. (“Sacculina! You’ve just successfully overtaken over the body of a crab, thereby ensuring propagation of your larvae! What are you doing next?”)
Sort of makes intestinal worms seem cuddly.