CHAPTER V 



NITROGENOUS EXCRETION 



The diet of any animal contains a multitude of organic com- 

 pounds, carbohydrates, fats, amines and proteins among them. 

 From these the animal builds its own structure and obtains 

 energy for the many enzyme systems necessary to it. The protein 

 of the cells and secretions is manufactured from the amines formed 

 by hydrolysis and degradation of the proteins in the food. These 

 are absorbed, transported to the sites utilizing them and the 

 characteristic proteins of the body are then formed. This is not the 

 end of the story, however, for as was shown by the classic studies 

 of Schoenheimer and Rittenberg the body proteins are in a 

 continual state of flux, changing constantly. The materials no 

 longer required are removed, broken down to non-toxic units and 

 excreted. In some cases they may be excreted little altered e.g. 

 creatinine from creatine in vertebrates, but in the majority of 

 cases the nitrogen fragment is converted to a non-toxic substance 

 and removed in the urine. If the animal has access to voluminous 

 quantities of water ammonia is the most common and most easily 

 voided excretory product e.g. in amphibians, fish and marine 

 invertebrates. In land-dwelling species water conservation is of 

 great importance and the production of small quantities of urine 

 containing high concentrations of nitrogenous materials is the rule. 

 The less toxic products urea and uric acid make their appearance 

 in these forms. 



Nitrogen is also lost in many cases in the form of muco-proteins, 

 secreted by epidermal cells on to the surface of the body for the 

 purposes of lubrication, maintaining a clean body surface and 

 providing a buffer against the external environment. Oligochaetes 

 are no exception to these generalizations, and once again the vast 

 majority of the published work has been concerned with earth- 

 worms. 



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