in single celled organisms, their whole body is in direct contact with the environment and all the necessary processes like taking in food, excreation etc. take place through their single celled body. they take in food through their single cell and excreate the waste through the same cell. so there is no need of special organs in such bodis. but in complex multicellular organisms, unlike single celled organisms, their whole body is not in direct contact with the environment. also their are so many cells that the exchange of gases can't be carried out by a single cell and can't meet the energy reqired .so they have special organs to exchange gases in and out of the body.
