Theory:

Like human beings, various animals such as elephants, lions, cows, goats, snakes, and birds have a chest cavity in which the lungs are found. In this theory, we shall study the process of respiration in certain organisms, such as cockroach, earthworm, and fishes.
Breathing in cockroach:
Similar to other insects, cockroaches have small openings known as spiracles on the sides of their body. They also have an air tube-like structure called the tracheae for the exchange of gases.
When the air that is rich in the oxygen enters the body through the spiracles, it diffuses and reaches the body's cells via the tracheae. Similarly, the carbon dioxide from the cells enters the trachea and moves out of the body through spiracles.
 
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Important!
The tracheae or air tubes are found only in insects and not in any other group of animals.
Breathing in earthworms and frogs:
As you have studied in your Class VIearthworms are animals that breathe through their skin.
The gases can easily pass through the skin of the earthworms as they have soft, slimming, and moist skin.
 
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Important!
Similar to the earthworms, certain amphibians such as frogs and toads can breathe in water with the help of skin, on land with the help of their lungs like the humans.
Breathing in fishes:
Similar to the animals on the land, the animals which are found underwater also have particular respiratory organs called gills.
The gills are comb-like structures found on the skin of animals that lives underwater. These structure, help the fishes to use the oxygen dissolved in water.
 
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Gills help in breathing as they are abundantly supplied with blood vessels for the exchange of gases.