A flatworm obtains oxygen needed for cellular respiration through simple diffusion. A human obtains oxygen by breathing it in from the air and into the lungs. Which system is the flatworm missing?

Flatworms are high-flattened organisms dependent on direct diffusion as means of oxygen intake and release of carbon. This small organisms lack "fully evolved respiratory tissues such as lungs or gills and complex circulatory system as means to transport oxygen through out the body.

>Direct diffusion of gases through the outer membranes can be used by organisms such as flatworms as a means of respiration due to their small size and simplicity.