Lungs are internal structures found in most terrestrial animals in which gas exchange occurs. The lungs are lined with moist epithelium (outer cells) to avoid becoming dried out. Some animals, including certain lungfish, amphibians, reptiles, birds, and mammals, have special muscles to help move air in and out of the lungs; other animals have lungs connected to the outside surface with special openings and do not require special muscles to move air in and out of their lungs.