Amoeba : We know that amoeba is a unicellular organism which lives in fresh water. This organism does not have a specialized organ to do the process of respiration. So in amoeba respiration takes place through its cell membrane also calledplasma membrane. Amoeba gets oxygen gas dissolved in surrounding water through its plasma membrane by the process ofdiffusion. The oxygen gas diffused inside the body is used up by amoeba. In the body the oxygen gas absorbed is used to break down the complex food material into simple molecules. During these metabolic reactions in the body of amoeba the oxygen gas is converted into carbon dioxide gas. The carbon dioxide gas is also liberated in the surrounding water through the same process of diffusion.

Hydra :

Well hydra has no specialized organs forrespiration but exchange of gases is carried ou' by entire bodyy surface. Oxygen and carbon dioxide diffuse from the body digestive system. Thus an efficient exchange is carried ou by enteron..