CHAPTER 7 



EXCRETION 



Les corps ammoniacaux sont toxiques, ils doivent etre elimines rapide- 

 ment, ou etre transformer en corps moins toxiques (uree, acide urique) 

 pour eviter leur accumulation dans 1'organisme. 



H. Delaunay, 1931 



INTRODUCTION 



Excretion, in general, refers to the elimination of waste or poisonous 

 substances from the organism. In this sense it obviously embraces a 

 multiplicity of processes and functions and requires closer definition. On 

 occasion the loose use of the term has resulted, at best, in loss of precision 

 and, at worst, in much confusion in zoological literature. This is apparent 

 in treatment both of excretory organs and excretory processes. As with 

 many other functions, excretion is best known among vertebrates, es- 

 pecially mammals. Among invertebrates our knowledge of excretion is 

 still fragmentary. Reviews of certain aspects of the subject are available, 

 notably those of Delaunay (8), Scheer (44), Prosser (39) and Baldwin 

 (2a). 



As the result of processes of growth, metamorphosis, tissue maintenance 

 and metabolism, an animal periodically or continually produces and 

 accumulates waste materials, obsolete tissue and metabolic end-products 

 which are eliminated in various ways. The nature and amount of these 

 products depend upon the animal's way of life, its diet, activities and 

 environmental conditions. Let us consider these first in terms of materials 

 which are eliminated. 



The foodstuffs taken in by the animal contain a certain amount of 

 indigestible material, and this is eliminated in the faeces, together with 

 mucus and any other products which may be discharged into the aliment- 

 ary canal. 



All animals, except possibly in some instances of suspended animation, 

 are engaged in metabolizing organic materials, either breaking them down 

 to provide energy, or building them up into specific products or tissues. 

 Of the foodstuffs which are metabolized by the animal, carbohydrates 

 and fats are oxidized to water and carbon dioxide, and are readily elimin- 

 ated as such. The nitrogenous compounds, predominantly proteins and 

 nucleic acids, are degraded into various nitrogenous end-products, which 

 are eliminated in several ways according to the species. A large part of the 

 nitrogen (about 90%) is derived from the a-amino-N of amino-acids 

 which are split off from proteins through the action of proteolytic enzymes. 

 Amino-acids are deaminated for the most part to ammonia. Other 



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