Flatworms, the first animal to hunt, are found in the ocean, freshwater, on land, and even inside other animals. The ancient flatworms were the first animals to develop a central nervous system and a head with a brain. The head had eyes—the first in the animal world. The flatworm body is bilateral—the first body plan with that design. As hunters today, flatworms hunt prey with their head leading the way. Flatworms are hermaphrodites: an individual flatworm is both male and female. When flatworms mate, the worm that first receives sperm, carries the fertilized eggs. They were the first animals with internal fertilization.