“The adult body,” wrote scientist Janice Moore, “is essentially a little bag of reproductive organs....” Sometimes reaching 12 inches (30 centimeters) in length, the thorny-headed worm wastes no energy on eating. In fact, it lacks intestines and directly absorbs any nutrients it needs from the rat’s innards.

Because parasites reproduce so prolifically and have a short life span, they evolve more rapidly. This evolutionary edge has given them a profound influence on the behavior of all creatures—indeed on all life—great and small. AVERAGE SIZE: up to 4.3 inches (11 centimeters).