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CHAPTER VI 



WATER RELATIONS 



The provision and maintenance of a sufficient quantity of water 

 within the body, either as lymph, blood, tissue water, mucus, 

 etc., is a problem that besets all animals. Each species has its own 

 particular problems with regard to dehydration, or flooding by 

 excess water, and each habitat has its own set of difficulties. Three 

 broad divisions into three habitat types may be mentioned, those 

 of the sea, fresh water and the land. Within each type there is 

 considerable variation, of course, as for example in the gradual 

 transition from fresh water to salt water found in the estuarine 

 reaches of rivers, and in the passage from swampy marshland to 

 dry arid desert found on the land surface. 



Marine animals, living in a medium of high osmotic pressure, 

 are always tending to lose water in response to the osmotic 

 gradient that exists between their bodies and the external medium. 

 Many marine animals remove their excretory products as simple, 

 highly soluble nitrogenous substances such as trimethylamine 

 oxide or ammonia, which are released into the sea. Salt-secreting 

 cells are found in the gills of marine fish which aid in keeping the 

 internal environment constant by removing excess mineral ions 

 taken in through the mouth in the sea water necessary to avoid 

 desiccation. 



Fresh-water animals, however, are faced with a diff"erent 

 problem. In their case the osmotic pressure of the body fluids 

 within the animal is greater than the osmotic pressure of the 

 medium and consequently water passes inwards through the 

 body wall and incessantly dilutes the internal fluids. This situation 

 is countered by the production of copious quantities of hypotonic 

 urine by the animals which maintain a constant internal environ- 

 ment in this manner. 



Terrestrial animals are faced with a similar problem to that of 



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