Urea/uric acid Gaseous CO2; bicarbonate
Ammonia/urea Calcium carbonate; bicarbonate; gaseous CO2
Key: + = flux from environment to body; - = flux from body to environment; flux; PF = physiological flux.
= no flux; TFF = thermodynamically favored
Thus, physiology gives us a useful criterion for deciding whether an animal "belongs" in a terrestrial, marine, or freshwater aquatic habitat. If the water balance organs are designed to meet the challenges of living in fresh water—that is, if they support high filtration rates and selective reabsorption of salts but not water—the animal that possesses them is properly a freshwater animal. Conversely, an animal with kidneys that are equipped to deal with the different challenges of the terrestrial environment is physiologically terrestrial.
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