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Cells cannot remain alive outside certain limits of temperature and much narrower limits mark the boundaries of effective functioning. Enzyme systems of mammals and birds are most efficient only with a narrow range around 37℃; a departure of a few degrees from this value serious impairs their functioning.(1)____(2) ____ Even though cells can survive wider fluctuations the integrated actions of bodily systems are impaired. Other animals have a wide tolerance for changes of bodily temperature.(4)____
For century it has been recognized that mammals and birds differ other animals in the way they regulate body temperature.(4)____(5)____ Ways of characterizing the difference have become more accurate and meaningful over time, but popular terminology still reflects an old division into " warm-blooded" and " cold-blooded" species; warm-blooded included mammals and birds whereas all other creatures were considered cold-blooded.(6)____ As more species were studied, it became evident that this classification was adequate.(7)____ A cold-blooded fence lizard usually has a body temperature only a degree or two below those of humans and so is not cold.(8)____ Therefore the next distinction was taken between animals that maintain a constant body temperature, called homeotherms, and those whose body temperature varies with their environments, called poikilotherms.(9)____ But this classification was also proved inadequate, because among mammals there are many that vary their body temperatures during hibernation.(10)____Furthermore, many invertebrates that live in the depths of the ocean never experience change in the depths of the ocean, and their body temperatures remain constant.
