NITROGENOUS EXCRETION 59 



yet untraced. Some facts are known about the concentrations of 

 substances within the coelom (Table 9) but in general the higher 

 molecular weight organic compounds such as glucose, proteins and 

 amino-acids are at a very low concentration compared with the 

 blood values of the same materials. On the other hand there is less 

 discrepancy between the quantities of nitrogenous substances in 

 blood and coelomic fluid. This may mean that the nutrient materials 

 are speedily removed from solution by the tissues bathed by the 

 coelomic fluid, and consequently the concentration of these 

 substances always tends to be low. Alternatively the chemicals may 

 be re-absorbed into the blood system which then carries the 

 necessary raw materials to the tissues. The blood system then, is 

 our third possibility as a transport mechanism. Since the intestinal 

 wall is very well provided with a plexal system of blood vessels it is 

 easy, but unproven, to visualize the likelihood of food substances 

 being absorbed from the gut into the blood system and distributed 

 by this system as it wends its way round the body. Perhaps the 

 chloragogen is indeed liver-like, acting as a homeostatic device to 

 maintain a constant level of circulating substances. It has the 

 added advantage of being a mobile liver and able to migrate to areas 

 of great need. 



The excretory function of the chloragogen cells for a long while 

 rested upon a report by Willem and Minne (1900) who considered 

 that the granules contained within the cells are composed of 

 guanine, an end product of nitrogenous metabolism. This has 

 been brought into doubt by Peschen (1939), Abdel-Fattah (1955), 

 van Gansen (1956) and Roots (1957). Roots (1957) doubted the 

 importance of chloragogen tissue as a site for nitrogenous metabo- 

 lism. She isolated the granules of the cells, and analysed them to 

 find a composition of C, 43%; H2, 6%; N, 4%; P, 3-5% and 

 S, 1%; as guanine contains 60% N it cannot be present, and it 

 seems unlikely that other nitrogen excretory compounds are to be 

 found as granular inclusions in the cell. But although the more 

 complex granules may not be present there is adequate evidence 

 that other simpler nitrogen compounds are present, and these 

 would have been overlooked in Roots' (1957) work, since she was 

 not concerned with materials in the liquid phase. A report that 

 guanine is also found in the walls of the nephridia has been 

 discounted by Bahl (1947b). 



