Through Trachea: - Insects and other land arthropods use tracheal tubes for exchange of gases. This is very simple and effective method. These animals have network of tracheal tubes. These tubes on reaching near the cells divide into very minute tubes called tracheoles. A fluid is present in these tracheoles. The diameter of these tubes is less than one micrometer. Air enters into the tracheal tubes directly then oxygen diffuses in fluid present in tracheoles. The CO2 produced diffuses out in fluid and passed out alongwith out going air.
Through Lungs: - In large land animals exchange of gases takes place through lungs.
In Invertebrates: - In some invertebrates lungs are also present but in them there is no proper ventilating mechanism.
In Amphibians: - In amphibians simple and sac like lungs are found. The air is forced into the lungs through buccal cavity by the pumping action.
In Other Vertebrates: - The lungs in reptiles, birds and mammals have become more efficient. In them the respiratory system consists of alveoli. Birds have air sacs in addition to lungs.