The worm in question is Leucochloridium, and it’s a parasitic worm that invades a snail’s eyestalks, where it pulsates to imitate a caterpillar (in biology circles this is known as aggressive mimicry—an organism pretending to be another to lure prey or get itself eaten). The worm then mind-controls its host out into the open for hungry birds to pluck out its eyes. The worm breeds in the bird’s guts, releasing its eggs in the bird’s feces, which are happily eaten up by another snail to complete the whole bizarre life cycle.
It’s an existence that’s as brilliant as it is strange. But while science has known about Leucochloridium for more than a century, it was only in 2013 that biologist Tomasz Wesołowski of Poland’s Wrocław University confirmed the worm is indeed capable of manipulating its snail hosts. (Specifically, amber snails—like many other mind-controlling parasites, it’s highly species-specific, that is, it’s unable to manipulate the behavior of more than one species.)
As if it weren’t enough of a meany-head, Leucochloridiumalso castrates its host. This makes good evolutionary sense: Energy normally spent producing eggs and sperm (snails are hermaphrodites) goes toward sustaining the worm. So, pumped full of the requisite nutrition, the sporocyst sends out branches that tunnel through the snail’s body and into its eyestalks, also known as tentacles, where it forms a brood sac full of larvae.