GENERAL ZOOLOGY 



course, wide variations in the tolerances of different species to changes in 

 the medium. These differences are most striking when we compare the 

 adaptabihty of animals inhabiting tidal waters, such as the estuaries of 

 rivers, with that of related marine and fresh-water forms in localities where 

 they are not subjected to periodic tidal fluctuations in salinity. The broadly 

 distributed estuarine species are tolerant of large variations in salt concen- 

 tration. The essentially fresh-water forms at the upper end of the estuary are 

 very often excluded from more brackish waters by increases in salinity of as 

 little as one or two parts per thousand. At the other extreme, truly marine 

 species are excluded by their inability to regulate their water balance in the 

 face of periodic dilution of estuarine waters by fresh water from upstream. 



The most striking and best-known examples of unusual adaptability are 

 furnished by certain migratory forms, such as salmon and eels. For example, 

 young salmon hatch in the headwaters of certain rivers, where the salt concen- 

 tration is very low. As juveniles, they migrate to the ocean and then pass 

 several years as marine fish. At maturity, they again enter the rivers of their 

 origin and make their way to the headwaters for breeding. Eels exhibit 

 essentially the reverse pattern, hatching in the sea, spending their maturing 

 years in rivers, and returning to the ocean for breeding. The exceptional 

 physiological adjustments involved in maintaining the water balance in the 

 face of such extreme variations in external conditions must be far-reaching 

 indeed. 



For terrestrial organisms, the availability of water is of the utmost signifi- 

 cance as an environmental factor. By definition, terrestrial animals are more 

 or less independent of water as an environmental medium; nevertheless, the 

 maintenance of a favorable water balance is no less important for these than 

 for aquatic forms. There are great variations in the tolerance of different 

 species to differences in the availability of water. At one extreme are such 

 forms as the isopod crustaceans, earthworms, onychophores, and amphibians, 

 which depend, at least in part, on a moist skin for respiration. These may be 

 considered as only imperfectly adapted to terrestrial life; they require very 

 moist environments, and as we have seen, most amphibia and some insects 

 depend on water as an environmental medium for their developmental stages. 

 At the other extreme are found the many species of insects, mammals, and 

 reptiles, in particular, that are adapted to life under arid desert conditions. 

 Between these extremes lie the great majority of terrestrial animals, with 

 widely varying degrees of adaptation to abundance or scarcity of available 

 moisture. 



The water relations of terrestrial animals involve balancing gain against 

 loss. Water may be obtained by drinking, by eating succulent food, by 

 utilizing "water of metabolism" yielded in the metabolic breakdown of even 

 dry foods, or by absorption from a moist environment. Water is lost chiefly 

 by evaporation, through the skin generally or in the organs of respiratory 

 exchange, or by its use as a vehicle for the elimination of egesta and excreta. 

 Successful adaptation to dry conditions is correlated with extreme economy 



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