Flat worms are very advanced than Porifera and Cnidaria because of the development of mesoderm as third layer, development of brain, well organised reproductive system and bilateral symmetry.Â
Phylum Platyhelminthes (Flat worms) is divided into three classes.
Class: Turbellaria, Class: Trematoda, Class: Cestoda

  i. Class: TurbellariaÂ
- This class includes mostly freee living flat wormsÂ
- Example: Planaria
ii. Class: Trematoda

- Â This class includes a group of parasites that infect various animals including man. Body is highly specialixzed for parasite mode of life.
- Example: Fasciola (Liver Fluke) and Schistosoma (Blood Fluke)
