Gas Exchange Media, Respiratory States, and Environments
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Regarding the part of the biosphere they occupy, animal life is classified into aquatic, terrestrial, and aerial groups. Among vertebrates, fish are predominantly aquatic, amphibians are transitional, and reptiles, birds, and mammals are fundamentally terrestrial. Overlaps in occupation of various ecosystems occur. Among mammals, the cetaceans have reinvaded water while some amphibians live in highly desiccating deserts (McClanahan et al. 1994). The insects, the now extinct pterosaurs, the birds, and the bats, chronologically in that order, are the only groups which have evolved powered flight. The assortment of animals such as the flying squirrels, lemurs, snakes, lizards, and flying fish which can momentarily remain air-borne are essentially gliders. They use a part of their body to delay the fall and did not have to grapple with the aerodynamic and aerobic challenges which beset the active flyers.