ENVIRONMENTAL RELATIONSHIPS 



ins; altitude at an average rate of 6°C. for every 3280 feet, to a minimum 

 of -55°C. near 36,000 feet. 



Water and Salts. Water, containing many salts and other substances 

 in solution, is the most abundant compound in the bodies of organisms. The 

 ability of an animal to maintain a favorable water balance between its 

 internal and external environments is of great significance in determining its 

 survival. The interchange of water between the internal and external media 

 involves its passage through living membranes, and such movements of water 

 obev the physical laws of diffusion and osmosis. Therefore, in aquatic 

 organisms, the maintenance of a proper water balance depends to a great 

 extent on the relative concentrations of materials in solution in the body 

 fluids and in the surrounding water; and the question of water as an environ- 

 mental factor cannot be considered apart from that of the materials which 

 the water holds in solution. We may think of a concentrated aqueous 

 solution of salts as containing relatively less water than a more dilute solu- 

 tion. Hence, the ocean is a relatively drier environment than fresh water, 

 and marine and fresh-water animals face very different problems in their 

 water relations with the environment. Sea water is of rather constant com- 

 position, its salinity averaging about 35 parts per 1000. The body fluids 

 of many marine animals, notably vertebrates, are much more watery than sea 

 water, and adaptation to survival in a marine environment must involve 

 perfection of mechanisms to imbibe water and eliminate salts, and to counter- 

 act the tendency of the body to lose water to the environment. Fresh waters 

 vary widely in salinity; they are never free of salt, but in general animals 

 inhabiting fresh water must be adjusted to a range of salt concentration far 

 lower than that of their body fluids. The tendency here, which must be 

 constantly counteracted, is for water to enter the body and dilute its fluids. 



In addition to their effects on osmotic relations, the salts in the external 

 medium are the source of specific ions necessary for the survival of aquatic 

 organisms. The vital functions of organisms require that certain substances 

 be available in minimum concentrations, to maintain the chemical constitu- 

 tion of the body fluids. Aside from its unfavorable osmotic effects, distilled 

 water is unsuitable as a medium for aquatic life because it lacks the necessary 

 materials in solution. Specific examples of individual requirements are fur- 

 nished by crustaceans and mollusks, which are excluded from waters deficient 

 in calcium carbonate; without this material the organisms are unable to 

 secrete and maintain their exoskeletons. 



Aquatic organisms are commonly adjusted to life within a specific range of 

 salt concentration, and thus to particular ranges of water conditions. 

 Adaptations of common occurrence involve modifications of the body wall to 

 minimize general gain or loss of water, and special activities of the gills, 

 excretory organs, and intestinal epithelium to conserve or eliminate water 

 and salts. In simpler animals contractile vacuoles and protonephridial 

 systems come into play. Usually the limitations of these adaptive mecha- 

 nisms restrict the animal to either fresh or salt waters, but there are, of 



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