• 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.
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