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Gills are what fish, amphibians, and some other animals use to breathe in water. They have feathery parts which cause water to move across the animal's body, which contains dissolved oxygen, after the animal has swallowed the water. The oxygen is absorbed into the animal's blood, causing carbon dioxide moves out of the animal's blood and into the water through the gills.
Some insects that live in water have a plastron, which is a kind of gill. It is a patch of special hairs that keeps the water away from the insect's spiracles. This allows them to continue to take in oxygen and remove carbon dioxide while they are under water.