j£-. PARASITISM 



Other animals, for it cannot be expected that the amino-acids of 

 their food will exactly fit their own tissues. Fasciola, TcBuia, and 

 Ascaris have all been shown to produce much ammonia, which 

 is the normal nitrogenous excretory product of aquatic species. 



Contrary to common belief parasites do not generally live in 

 an atmosphere devoid of free oxygen. It is obvious that those 

 living in arterial blood have plenty, and even in venous blood 

 and in the tissues there is no mean amount. The partial pressure 

 of oxygen in vertebrate tissue ranges from lo to 45 mm. of 

 mercury (that of the atmosphere is about 150 mm.). As this is 

 largely produced by the dissociation of oxyhaemoglobin, which 

 acts as an oxygen store, it represents a quantity of oxygen far 

 greater than would be available in the same volume of most 

 natural waters. There is no reason to expect that parasites would 

 respire in a different manner from other simple animals or the 

 tissues of the host, and experiments have confirmed the similarity. 

 Parasites use oxygen when they can get it, and most of them 

 have plenty of opportunity of doing so ; when it is not available 

 they respire anaerobically, just as does vertebrate muscle, by a 

 breakdown of carbohydrate, especially glycogen. Lactic acid is often 

 produced, valeric (=valerianic) acid (CH3CH2CH2CH2COOH) is 

 even commoner, and Trichinella rather oddly produces carbon 

 dioxide anaerobically without any organic acids at all. This 

 worm is also peculiar in that the adult has no glycogen. The 

 difficulties of culturing most parasites outside their hosts are 

 great, but it appears from a good many experiments that absence 

 of oxygen lowers activity and reduces survival time. The lumen 

 of the gut differs from all other parasitic habitats in sometimes 

 having a very low oxygen concentration, though it is no lower 

 than is often found in other natural habitats, such as the mud of 

 ponds. It is here, if anywhere, that we should expect to find 

 parasites that can live without oxygen, and Ascaris, for example, 

 seems to need and use only a small quantity ; the Protozoa 

 which live in the rectum of many vertebrates probably get very 

 little oxygen, and probably come as near as any animals to being 

 anarobic. There is, however, no reason to think that they are 

 completely without oxygen, or that they cannot use what they 

 do get ; since so aerobic a tissue as mammahan skin epithelium 

 can survive under strictly anaerobic conditions for a week, there 

 is no need to postulate any pecuhar type of Hfe for the parasites. 



A feature of their life which parasites share with the inhabitants 



