A parasitic organism which belongs to the flatworm family, and lives by infecting snails.
The parasite causes the eyestalks to become swollen, colorful, and to pulsate, a combination which resembles the appearance and motion of many insect larvae. These swollen eyestalks cannot be withdrawn into the protective shell. Our parasitic friend also causes the snail to radically change its behavior. Formally seeking moist and dimly secluded areas, it now seeks out exposed areas, preferably in the sun. This conspicuous unsnail-like appearance and behavior draws the attention of birds, which rip off the eyestalks and eat them. This is rather hard on the snail, who then must regenerate a new set of eyestalks which become infected by the same little beasty, renewing the cycle. The eyestalks which were avian appetizers contain eggs of the parasite, which are passed through the digestive system and excreted, creating a biohazard for any snail which comes oozing along. Snails seem to find bird droppings an item of haute cuisine.
I don't know whether to feel sorry for the snail or exclaim the poetic justice inflicted on a creature which dines on bird dung. The hapless snail is a little better off than Prometheus, who gets his liver eaten by birds, a penalty exacted by the gods for giving humanity the gift of fire. The snail can eventually die and break the cycle. One can only wonder what largesse the snail bestowed. Perhaps it was giving the French the idea of escargot.