ABSTRACT
This chapter examines the various gas exchange structures that have evolved in fish including gills, lungs, swim bladders and other accessory air-breathing organs and relate their use as respiratory structures to the different environments occupied by fish. The survival of fish depends largely on its morphological, physiological and biochemical adaptations to the environment. The evolution of a counter system of gas exchange whereby blood in the gill secondary lamellae and water flow in opposite directions enables the fish to obtain the maximal possible amount of oxygen from the water. Accessory respiratory organs have enabled bony fishes belonging to more than 50 genera access oxygen directly from the air. The swim bladder in most fish is thought to have evolved from the lungs of ancestral bony fishes. The pharyngeal chambers have undergone diverse modifications in various species of fish. Evolution of the fish respiratory system has undergone several changes in extant fish.
