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.
Flatworms: The First Hunter
Next Generation Science Standards for this Video
The body plan of the flatworm is bilateral, with a head that leads—a plan that continues in most animals living today. The structure of this bilateral plan is ideal for an active hunter. Flatworms have some organ systems like reproductive organs. They have a nervous system, but they lack a circulatory or respiratory system.
Flatworms are the first animals to transmit signals from sensory receptors along nerve cells to the brain. The head has eyes—the first in the animal world. With this combination, flatworms were the first active hunters.
The body plan animation visualizes the complex flatworm structures and how they function:
The combination of flatworm traits are:
- Bilaterally symmetrical with a hard and a tail
- Centralized nervous system
- No hard skeleton