A respiratory surface is covered with thin, moist epithelial cells that allow oxygen and carbon dioxide to exchange. Those gases can only cross cell membranes when they are dissolved in water or an aqueous solution, thus respiratory surfaces must be moist. Flatworms and annelids use their outer surfaces as gas exchange surfaces. Images from W.H. Freeman and Sinauer Associates, used by permission. The Body Surface Flatworms and annelids use their outer surfaces as gas exchange surfaces.
Earthworms have a series of thin-walled blood vessels known as capillaries.
Gas exchange occurs at capillaries located throughout the body as well as those in the respiratory surface. Amphibians use their skin as a respiratory surface. Frogs eliminate carbon dioxide 2.5 times as fast through their skin as they do through their lungs. Constraints of water loss dictate that terrestrial animals must develop more efficient lungs. Gills Gills greatly increase the surface area for gas exchange.