. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. CHAPTER 2 Water. INTRODUCTION ATER IS AN ESSENTIAL Constituent of ajl living things; it is the universal biological solvent, the diffuse phase in which most of the cellular reactions of metabolism occur, and the most necessary to life of all environmental constituents. Life undoubt- edly began in a watery medium. Numerous exits from water to land have been made in the course of evolution, but only a few groups of animals have been successful in maintaining themselves out of water. Each group which has made the e
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. CHAPTER 2 Water. INTRODUCTION ATER IS AN ESSENTIAL Constituent of ajl living things; it is the universal biological solvent, the diffuse phase in which most of the cellular reactions of metabolism occur, and the most necessary to life of all environmental constituents. Life undoubt- edly began in a watery medium. Numerous exits from water to land have been made in the course of evolution, but only a few groups of animals have been successful in maintaining themselves out of water. Each group which has made the exit from water has used its own set of adaptations to life in air, some being more successful than others. Only the insects have made the exit complete, and some of them return to water for at least part of their life cycle. All other animals, including birds and mammals, return to a watery medium at least for embryonic life. One problem of animal life is to maintain inside the organism just the proper amount of water—not too much, not too little. Terrestrial animals must retain and use what water is available; fresh-water animals must exclude water to prevent self-dilution; some marine and parasitic forms are in osmotic equilibrium with their medium, whereas others are more dilute and have the problem of taking in enough water while living in a plenitude of it. Nearly all fresh-water and terrestrial plants, by virtue of their cellulose walls and active plasma membranes, maintain their cellular constituents, particularly their vacuolar sap, at concentrations higher than those of the fluids which bathe their tissues. The cells are continually more concentrated than the tissue fluids and hence turgid. In animals, however, cellulose walls are absent, and the effective intracellular concentration equals, or slightly exceeds, the concentration of body fluids. Regulation of osmotic concentration in animals takes place, then, not in single cells, as in plants, but in the organism as
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