In the hydra, respiration is accomplished via diffusion of molecules of oxygen and carbon dioxide through the skin. The hydra's simple cellular structure ensures that its body's skin layers are thin enough for diffusion to occur.
In animals, three main types of respiratory systems exist: respiration through the skin, respiration through the gills or respiration through internal air structures. Respiration through the skin, as in the hydra, involves the passing of molecules into and out of the body via diffusion. Aquatic animals use gills, which are structures composed of tissues and blood vessels that extend from the body, for their respiration needs. Most terrestrial animals use internal air structures, such as lungs or tracheae, to respire.