NITROGENOUS EXCRETION 55 



activity, but the concentration increases by a factor of 10 when 

 animals are starved for up to 4 weeks. This change in enzyme 

 activity can be correlated with the previously described alteration 

 in the importance of urea as a metabolic product during inanition. 



Cohen and Lewis (1950) also utilized an injection technique to 

 study the method of formation of urea within the earthworm. They 

 inserted a fine hypodermic syringe into the gizzard of anaesthetized 

 animals and injected amine compounds such as ornithine, arginine, 

 citruUine, glycine, glutamine, alanine, histidine and hydantoin into 

 the gut. The excretion of urea is affected only by arginine and 

 citrulline when used alone. But if citrulline is injected simultaneously 

 with another source of nitrogen such as alanine or glutamic acid, 

 urea is formed in even greater amounts. 



Therefore, although Heidermanns found no evidence for the 

 presumption that the ornithine cycle is active in earthworms it now 

 appears that such a cycle does in fact exist in these animals. 

 Citrulline and arginine are both intermediates in this system, and 

 arginase is the enzyme necessary for the breakdown of arginine to 

 form urea and ornithine. These three steps now seem fairly well 

 established, and further evidence on this point will be presented 

 later. The observation that the major part of the arginase activity 

 is located in the intestinal wall rather than the body wall is sugges- 

 tive of the role of chloragogen cells in excretion. Heidermanns 

 (1937) termed the chloragogen tissue "the central organ of urea 

 metaboUsm". Needham (1960) has provided confirmation of the 

 presence of arginase in Liimbriciis terrestris and E, foetida tissues, 

 and in the greater activity of gut preparations in this respect 

 compared with body wall preparations. The activity of the enzyme 

 varies in direct relationship to the urea excreted under feeding and 

 fasting conditions. 



The Chloragogen Tissue 



The function of the chloragogen tissue in the metabolic economy 

 of the earthworm has been a vexed question for many years. 

 Opinion has wavered between two extremes. First, it has been 

 argued that the position of these cells, closely applied to the 

 coelomic surface of the intestinal wall, is ideal for the development 

 of a liver-like function. Food substances absorbed through the 

 gut wall can be transferred into the chloragocytes, which by 



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