More than a hundred hooks, aligned in tidy formation, serve as the strike force of an adult thorny-headed worm. The parasite drills into the intestinal wall of its rat host. Once anchored, the worm lolls in the rat’s gut, simply absorbing nutrients without even needing to digest them. Females bestir themselves now and then to lay eggs. AVERAGE SIZE: up to 4.3 inches (11 centimeters).