The fishes belong to class Pisces under vertebrates.
2. In fishes, the respiratory system consists of mouth, pharynx, internal branchial apertures, branchial pouches and external branchial apertures.
3. The branchial (gill) pouches are covered by mucous membrane which is thrown into folds called branchial (gill) lamellae.
4. Fish keep its mouth open and lower the floor of the oral cavity. Then the water (with dissolved oxygen) enters oral cavity.
5. While the mouth is closed, the floor of oral cavity is raised and the water from oral cavity flows into pharynx then into internal branchial apertures.
6. The gill lamellae are now bathed with water and gaseous exchange takes place.
7. Oxygen diffuses from water into the blood and 2 CO diffuses out from the blood into the water.
8. Then the water flows out through external branchial apertures.
9. The continuous opening and closing of mouth by a fish in water is to draw water into the oral cavity.