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The phylum Platyhelminthes, which includes parasitic and free-living forms, is made up of acoelomate organisms called flatworms. They are separated into the Trematoda, Turbellaria, Cestoda, and Monogenea groups.

The monogeneans are simple ectoparasites, primarily of fish, with free-swimming larvae that adhere to fish to start their transition into the parasitic adult form. The worms may create enzymes that break down the host tissues or they may just graze on skin and mucus surface fragments.

• Trematodes, often known as flukes, are internal parasites that affect humans as well as many other species, including molluscs.
• Cestodes, often known as tapeworms, are parasites that live inside of vertebrate animals.
• Although some species dwell in freshwater or moist terrestrial habitats, internal, free-living sea species make up the majority of the Turbellaria class.