Different animals breathe in different ways. All land living animals do not have the same type of respiratory organs. For example, man, cockroach, earthworm etc., are land living animals. In man, respiration is carried on by lungs, in cockroach by air, tubes called tracheae and in earthworm by skin. In the same way, all water living animals do not have the same type of respiratory organs. Fish take in the oxygen dissolved in water through gills. Animals like frogs carry on respiration through skin and lungs. When a frog is in water, it takes in oxygen dissolved in water through its skin. While on land, it breathes through its lungs.
In man, air enters the nasal cavity through the nostrils. From there, it passes in to the wind-pipe and then into the lungs. Lungs are spongy and elastic. Respiration occurs by contractions and relaxations of the lungs. As in all animals, respiration in man also occurs in two stages. They are: 1) inspiration and 2) expiration
Take some lime water in a beaker. Blow air into it with a glass tube. What do you observe? The lime water turns milky. This is because carbon dioxide you have breathed out turns lime water milky.
In cockroaches, the respiratory system is well developed. They have small openings on their either side of their bodies called spiracles. These openings lead into the air tubes called tracheae which from a network inside the body.
In earthworm, gaseous exchange takes place through the moist skin by diffusion.
In scorpion, book lungs are the respiratory organs.
In fish, water enters through the mouth and passes out through the gill pouches. The oxygen dissolved in water enters the blood through the gills by diffusion. Blood passes out into the water as it flows over the gills. This water goes out of the gill chamber.