The factors which set the metabolic potentials of animals - the physicochemistry of the respiratory medium utilized, the phylogenetic level of development, the habitat occupied, and the life-style pursued -, have been the most decisive determinants of the constructions of the gas exchangers. After molding the most appropriate respiratory devices and adjusting them in accord with the individual needs for molecular oxygen, animals occupy different ecological niches and even share common environments. From the perspective of respiration, they have very few choices: there are only two fluid respiratory media and essentially two habitable spaces - water and air. They must adapt to either and in rarer cases to both.