Summary:
- Flatworms range in length from about 1 millimeter (0.04 inches) to more than 20 meters (66 feet) They have a flat body because they do not have a coelom or even a pseudocoelom.
- Instead, their cells exchange gases by diffusion directly with the environment.
- Flatworms reflect several major evolutionary advances in invertebrates.
- They have three embryonic cell layers, including mesoderm.
- They also show cephalization and bilateral symmetry.
- The sheep liver fluke has a complicated life cycle with two hosts.