AEROBIC FERMENTATIONS 163 



released through complete oxidations. This holds true 

 even in a parasite like Ascaris where the reactions of the 

 incomplete type are very pronounced. Von Brand (1934a) 

 determined the oxygen consumption of 100 g. of Ascaris 

 at the oxygen tension of atmospheric air and found that 

 0.21 g. were consumed in 24 hours; he likewise showed 

 that during this time 0.86 g. of glycogen were decom- 

 posed through fermentative processes. The energy yield 

 of the latter is estimated to lie between 6 and 12 per 

 cent of that liberated by the complete oxidation of the 

 carbohydrate (von Brand and Jahn, 1942). Taking a 

 mean value of 9 per cent, 0.077 g. of glycogen would 

 have to be decomposed through complete oxidations to 

 yield the same amount of energy. This would require 

 0.092 g. of oxj^gen, or an additional intake of 44 per 

 cent over the amount actually consumed. It is likely 

 that in free-living organisms, like the ciliates mentioned 

 in the preceding section, the i^ercentage increase w^ould 

 be much smaller, but the available data do not allow 

 detailed calculations. Thus, the sum total of the evi- 

 dence indicates rather strongly that it is not the in- 

 ability of the animals to secure sufficient oxygen which 

 compels them to leave their oxidations incomplete. 



IncomjDlete oxidations are very uneconomical because 

 the energy bound up in the excreted end products is sim- 

 ply lost. Free-living animals have rarely an over-sup- 

 ply of food; the occurrence of these processes in such 

 organisms is therefore especially surprising. It is easier 

 to see, from the nutritional viewpoint, why wasteful proc- 

 esses are no handicap to parasites; these animals nor- 

 mally live in surroundings in which an abundance of 

 food is always available. 



As was stated previously, many intestinal parasites 

 are forced to resort to a predominantly anaerobic me- 

 tabolism in their normal habitat because of the low 

 oxygen content of the latter. One may assume that they 



