60 THE PHYSIOLOGY OF EARTHWORMS 



The discrediting of guanine as an excretory product has been 

 followed by the demonstration of another purine base in these 

 cells. Van Gansen (1956, 1957) working on the chloragogen of ^. 

 caliginosa found that a purine derivative, having an absorption 

 spectrum different from that of guanine, but similar to that of 

 heteroxanthine, a product commonly found in mammalian urine, 

 is present in the chloragogen (Fig. 19). Heteroxanthine, or 7- 

 methyl-xanthine, may thus prove to be of importance in the 

 removal of one class of chemical substance from the body. This 

 suggestion has been criticized, however, by Abdel-Fattah (1955), 

 and also by Roots (1960) who found no trace of purine in the 

 chloragogen of L. terrestris. 



Though these observations are interesting they cannot be the 

 major concern when considering the excretory role of chloragogen 

 since these are at best minor fractions of the urine produced, and 

 may account for the so-called creatinine fraction of Bahl (1947b). 



HN=--=CO 



I ! /CH 



OC C N-^^ 



HN — c — rg^ 

 Fig. 19. Formula of heteroxanthine. 



But urea and ammonia, as we have seen, are the most important 

 excretory products, and these must be represented in the chlora- 

 gogen if it is the site of origin of these substances. 



In fresh chloragocytes obtained from A. caliginosa ammonia is 

 found in large quantities, amounting to 0-60-1 -36 /itM/lO mg dry 

 wt., whereas the urea content is negligible. After starvation the 

 ammonia content is unaffected but urea now appears within the 

 chloragogen at a level of 0-07-0-35 jitM/lO mg dry wt. At the same 

 time the nephridia hold 0- 12-0-57 /xM/10 mg dry wt., and the 

 intestinal wall 0-12-0-18 /xM/10 mg dry wt. (van Gansen, 1958). 

 The transition to urea as a major excretory product after starvation 

 has already been noted but the complete absence of urea from 

 chloragocytes in fresh animals is not easily explained, although the 

 amounts within the cells must be very small. 



As mentioned previously, doubt has been cast upon the function- 

 ing of the Krebs-Henseleit cycle by Heidermanns (1937) as a result 



