The phylum, Platyhelminthes, is divided into four classes:
Class Turbellaria, free-living, non-parasitic, marine, freshwater, and terrestrial flatworms
Class Trematoda, parasitic internal flukes
Class Cestoda, parasitic tapeworms
Class Monogenea, parasitic external flukes
Turbellarian includes over 4,500 known species, mainly marine, divided into twelve orders. They are the most primitive of the Platyhelminthes. Scientists believe the other three classes of Platyhelminthes all evolved from Turbellarians.Turbellarians are mostly carnivorous, preying on tiny invertebrates.
The majority of freshwater turbellarians are non-parasitic although a few European freshwater species are ectoparasitic on crustaceans. Ectoparasitic is a parasite that lives on the exterior of its host. Some freshwater forms like European and Australian triclads are commensal on crustaceans and turtles. Commensal is where one lives with, on, or in another without damage to either.