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Some fish or amphibians secrete mucus that retains water or oxygen particles and then it passes into their body.

Explanation:

Many amphibians and fish have thin and in many cases slippery skin.

This is because some species secrete slimy substances through the skin, keeping it wet.

Since some fish or amphibians cannot ingest water directly, they need a way to stay hydrated.

This viscosity or mucus makes possible for the animal to come into contact with water and oxygen, and then ingest them.

In addition to this in some amphibians, mucus disperses secretions from other glands, the poisonous glands, which serve as a shield for predators.