CHAPTER 5 



EXCRETION AND WATER 

 BALANCE 



1 . Excretion 



The greater part of the food of leeches consists of proteins which 

 during digestion are converted to amino-acids and absorbed into 

 the body. A proportion of these amino-acids*"%fe synthesized 

 again into proteins for use in body building but most are deamin- 

 ated and used as a source of energy. Deamination results in the 

 production of ammonia, a substance which is poisonous and readily 

 diffusible, so it has to be excreted quickly or changed into some- 

 thing less toxic, such as urea or uric acid. Another source of 

 nitrogen in the food is nucleic acid which on digestion yields 

 purines such as adenine or guanine. These are likewise amino 

 compounds and on deamination give hypoxan thine or xanthine, 

 which in the presence of xanthine oxidase may be changed into 

 uric acid. In vertebrates most of these metabolic processes take 

 place in the liver and the end products, ammonia, urea or uric acid, 

 are then excreted by way of the kidneys. In annelids these pro- 

 cesses are particularly associated with the chloragogenous tissue, 

 while the organs of excretion are the nephridia. 



Chloragogen cells are modified coelomic epithelial cells which 

 become loaded with yellow or brown material in their cytoplasm 

 and project freely into the coelom. When fully loaded they break 

 free and float about in the coelomic fluid. In oligochaetes and 

 polychaetes they are concentrated in the vicinity of blood vessels, 

 particularly those of the gut. In leeches, where blood vessels are 

 often lacking, they line the haemo-coelomic channels and may be 

 especially concentrated in a dense network of coelomic capillaries 

 known as the botryoidal and vaso-fibrous tissue (see p. 20). 

 Nitrogenous excretion has *^BbeQ^ most closely studied in 



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