Describe the ventilation mechanism in fish

The buccal cavity of the fish opens and the buccal floor moves downwards. This creates a larger volume in the buccal cavity causing pressure to fall relative to the pressure in the external environment. Water will then run in through the fish's mouth making contact with the gill filaments, the fish's gas exchange surface, and gas exchange can take place. The buccal cavity closes and the buccal floor rises. This causes the volume in the buccal cavity to fall and pressure to rise relative to the external environment. This will cause the deoxygenated water to flow out through the operculum which opens as the buccal cavity closes