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.
Lungs and insect tracheae are two variations. Lungs are the respiratory adaptation of most terrestrial vertebrates Lungs are highly vascularized invaginations of the body surface that are restricted to one location. The circulatory system must transport oxygen from the lungs to the rest of the body. Land snails use an internal mantle as a lung. Spiders possess booklungs.