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Animals which Breathe through their Skin
Some animals which live on land have a skin which is so thin that gases can easily pass through it. We say that they have a permeable skin. Earthworms and amphibians have a skin which is permeable to gases. Amphibians also have a pair of simple lungs but they are not sufficient on their own for breathing.
Large animals which breathe through their skin also use blood to transport oxygen to their tissues and to bring carbon dioxide to the surface of the body. This means that the blood vessels must come very close to the skin. With a microscope it is possible to see the tiny blood vessels called capillaries.
The main disadvantage of breathing through the skin is that the animal loses a lot of water in this way. The skin of these animals is always wet. To stop their bodies from drying out they must always live where the air is humid.
Earthworms live in the soil where the air is humid. In the summer, when the soil dries out, they burrow deeper to find damp soil. Amphibians are also found in wet places near ponds and streams.
The animals which have lungs or tracheae do not have to live in a humid atmosphere. Lungs and tracheae are found inside the body, so they are protected.