Introduction to the Platyhelminthes Introduction to the Platyhelminthes Life in two dimensions. . . Because of the lack of any other body cavity, in larger flatworms the gut is often very highly branched in order to transport food to all parts of the body. Flukes, like other parasitic flatworms, have complex life cycles often involving two or more host organisms.
Planarians are free-living flatworms, and have a much simpler life history.
Most are less than a centimeter long. Flatworms were once divided into three groups. The Trematoda, or flukes, are all parasitic, and have complex life cycles specialized for parasitism in animal tissues. The Cestoda, or tapeworms, are intestinal parasites in vertebrates, and they also show anatomical and life history modifications for parasitism. Recent molecular studies suggest that the Platyhelminthes as a whole may even be polyphyletic , having arisen as two independent groups from different ancestral groups.