Fish first evolved gills and generally had no lungs. Fish are variable temperature aquatic vertebrates that swim by Gill breathing, swing of tail and trunk and coordination of fins and feed on upper and lower jaws. When the fish is in the water, each gill piece, gill filament and gill piece are fully opened, expanding the contact area between gills and water and increasing the opportunity to absorb dissolved oxygen in the water. Fish evolved gills or lungs first. Gills are the respiratory organs of fish. Water enters from the fish mouth and flows out after passing through the gills. The water flow direction is opposite to the blood flow direction in the gill plate, forming a countercurrent exchange system, which can efficiently bring oxygen and take away carbon dioxide.
Lungs can directly exchange oxygen in the air, and biological evolution always ranges from simple to complex, from low to high, from aquatic to terrestrial. Fish are mostly aquatic animals and usually breathe with gills. （YuXianJinHuaChuSaiHaiShiFei）