Certain land-dwelling animals have layers of skin thin enough for gases, such as oxygen, to pass directly through them. These skin types are usually referred to as permeable. The most common land animals with permeable skins are amphibians and earthworms. Although amphibians respire through their skins, some species possess lungs too. Other amphibians, such as certain species of salamander, have no lungs and process breathing functions entirely through their skin.
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Animals that breath through their skin have blood vessels very close to the skins’ surface. These blood vessels are used to move oxygen to muscle tissue and to ensure carbon dioxide can be shifted to the body’s surface. These blood vessels in skin-breathing animals are known as capillaries and can be observed under laboratory microscopes.
Toads and frogs
Land animals, such as toads and frogs, live near ponds and streams where humidity and precipitations levels are high. Amphibians also ingest water through their skin rather than their mouths, so they can absorb moisture from the atmosphere to keep hydrated. A significant amount of water is lost by animals who breathe through their skin. Due to these diminishing moisture levels, skin-breathing animals reside in climates where the air is humid to prevent their skin from drying out completely.
Earthworms live in soil where the air has very high humidity levels. When temperatures rise during the summer months, they burrow into the soil as the humidity levels above ground weaken. Earthworms have mucus on their skin which causes the air to dissolve when it comes into contact with the skin. They must remain moist at all times or they will no longer be able to breathe, leading to suffocation. When air is absorbed through the skin, oxygen is pulled into the worm’s blood circulation. The heart of the worm then pumps the blood toward the head. The movement of the worm cause the blood to flow back and forth through the length of its body, as the carbon dioxide passes out through the skin.
There are three varieties of salamander: the aquatic, that remain in water for their whole lives, the semi-aquatic, who only enter the water during breeding season and the terrestrial varieties, that live entirely on land. Some species of salamander keep their skin gills for their entire lives, whereas other varieties lose them after months or years of residing on land. A salamander takes in oxygen through the lining of its mouth and through tiny blood vessels situated close to the skin.
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