Answer: Respiratory system is missing in flatworms.
Animals and complex multicellular organisms have a complex mechanism of supplying nutrients and oxygen to all parts of the body and removal of carbon dioxide and other waste materials. The exchange of gas between the blood and tissues is done by the circulatory system; the blood first absorbs oxygen and then releases carbon-di-oxide in lungs. The circulatory and respiratory systems have the function of obtaining oxygen and releasing carbon dioxide work tandem.
In small organisms with highly flattened body like flatworms; diffusion acts as a means of obtaining oxygen and releasing carbon dioxide. The flat shape increases the surface area for the process of diffusion to ensure that all cells inside the body has access to the oxygen being close to the outer membrane. They lack specialized organs or tissues like lungs, gills or other respiratory passages along with the circulatory system for transportation of nutrients and oxygen throughout the body.
