Which major classes belong to the phylum Platyhelminthes?

The phylum Platyhelminthes, commonly known as flatworms, consists of four major classes:1

• Turbellaria: These are primarily free-living organisms, such as planarians.1
• Trematoda: This class includes parasitic organisms known as flukes.1
• Cestoda: This class consists of parasitic tapeworms.1
• Monogenea: These are also parasitic flatworms.1

It is important to note that while these four are generally recognized as the primary classes, some scientific authorities classify Monogenea as a subclass within Trematoda. Unlike the free-living turbellarians, members of the other three classes are parasitic during all or part of their life cycles.1