All Free-Living Flatworms

Free-living flatworms, traditionally the Turbellaria, live in the oceans, in fresh water and in moist terrestrial habitats, and a few are parasitic. The planarian Dugesia is a freshwater example, a carnivore or scavenger usually under a centimeter long with a much simpler life history than the parasitic flukes. The marine polyclads are the largest free-living flatworms, sometimes reaching 15 centimeters, and take their name from a highly branched digestive cavity. Flatworms have no body cavity and must respire by diffusion, so no cell can be far from the outside and the body has to stay flat.