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The pulmonates have lost their gills [1] and adapted the mantle cavity into a pallial lung. The lung has a single opening on the right side, called the pneumostome, which either remains permanently open, or opens and closes as the animal breathes. The roof of the lung is highly vascularised, and it is through this surface that gas exchange occurs.
Rather than using lungs, "[g]aseous exchange takes place across the surface of highly vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism. The density of the water prevents the gills from collapsing and lying on top of each other; [such collapse] happens when a fish is taken out of water." [4]
In this case, the lung is more suited to the absorption of oxygen from air, rather than water. Instead of branchiostegal lungs, some terrestrial hermit crabs ( Coenobita ) possess multiple gills and small lungs , with other varieties of gas diffusion methods supporting the transition from aquatic to terrestrial dwelling.
The lungs develop early and are used as accessory breathing organs, the tadpoles rising to the water surface to gulp air. Some species complete their development inside the egg and hatch directly into small frogs. These larvae do not have gills but instead have specialised areas of skin through which respiration takes place.
External gills are used for respiration, although buccal pumping (gulping air from the surface) may also be used to provide oxygen to their lungs. [15] Buccal pumping can occur in a two-stroke manner that pumps air from the mouth to the lungs, and with four-stroke that reverses this pathway with compression forces.
External gills are the gills of an animal, most typically an amphibian, that are exposed to the environment, rather than set inside the pharynx and covered by gill slits, as they are in most fishes. Instead, the respiratory organs are set on a frill of stalks protruding from the sides of an animal's head. The axolotl has three pairs of external ...
Cutaneous respiration, or cutaneous gas exchange (sometimes called skin breathing), [1] is a form of respiration in which gas exchange occurs across the skin or outer integument of an organism rather than gills or lungs. Cutaneous respiration may be the sole method of gas exchange, or may accompany other forms, such as ventilation.
Posteriorly from the mouth, the alimentary system extends into a wider and dorso-ventrally flattened region called the pharynx, from which both the gills and lungs arise. The esophagus , which is also characterized as a dorso-ventrally flattened tube, extends from the mouth into the stomach and leads to a flattened ellipsoidal structure.