In unicellular organisms such as amoeba and paramecium exchange of gases takes place through plasma membrane(also known as the cell membrane, the plasma membrane is found in all cells and separates the cell's interior from the outside environment) by simple diffusion. In earthworm, leech and frogs exchange of gases takes place through their moist skin.
Flatworms are small, simple organisms that lack specialized circulatory and respiratory systems. Their flat shape and moist cells allow for quick diffusion of gases across their outer membrane(the outer membrane is made up of phospholipids, proteins). This process is called direct diffusion.
Gaseous exchange in earthworms takes place through their moist skin and capillaries, not through the plasma membrane. Earthworms don't have respiratory organs, so they exchange gases through their skin in a process called cutaneous respiration. The skin is covered with a thin layer of mucus that keeps the body moist and creates an environment for gas exchange.