An outpocketing of the gut has evolved into a dead-end sac that serves as a lung. This enables the lungfish to breathe gulped air.
One of the gill arches is modified to carry deoxygenated blood to the lung. In addition, two other gill arches have lost their gill filaments, and they deliver blood directly to the dorsal aorta.
The two bloodstreams stay mostly separate, so oxygenated blood blood goes to the dorsal aorta, and deoxygenated blood travels either through the gills or the lung. This enables the African Lungfish to breate either air or water.
While it is satisfactory, the African Lungfish's circulatory system could be vastly improved with only minor evolutionary tinkering. To prevent any mixing of blood at all, the ventricle is split in two.
The two modified gill arches separate and form an artery that merges with the dorsal aorta. In their place are functional gill arches. The left ventricle now pumps oxygenated blood directly to the systemic capillaries. Furthermore, one gill arch pumps blood directly to the lung.
These minor modifications offer tremedous advantages. The African Lungfish can now maintain normal metabolic rates when exposed to air, and to operate at much higher metabolic rates in water. An additional benefit is the pulmonary and systemic circuits can now operate at different pressures. Furthermore, this opens up the potential for it to become warm-blooded. Overall, it allows the African Lungfish to be a much more versatile animal.
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