In animals with a more three-dimensional body plan, most of the body cells are isolated from the environment and the respiratory surfaces are generally thin, moist epithelium with a rich blood supply. Usually only a single cell layer separates the respiratory medium from the blood or capillaries.
Earthworms have moist skin which overlays a dense network of capillaries.
Most other animals lack sufficient body surface area to exchange gases for the entire body. These animals possess a region of the body surface that is extensively branched or folded, thus providing a large enough respiratory surface area for gas exchange. In most aquatic animals, external gills are present and are in direct contact with the water. Terrestrial animals have internal respiratory surfaces that open to the atmosphere through narrow tubes. Air lacks the supportive density and moisturizing qualities of water.