RESPIRATORY METABOLISM 387 



(especially if a considerable amount of decaying organic matter, and 

 consequently hydrogen sulphide, is present), those which are found in 

 sewage-disposal plants, those which appear near the bottom of putrid 

 laboratory cultures, and those which inhabit the lumen of the lower in- 

 testine of Metazoa. These organisms, because of the characteristics of 

 their environment, are deprived of one of the chief sources of energy 

 available to other animals — the reduction of molecular oxygen, and they 

 must be able to obtain energy by other methods, such as molecular re- 

 arrangements (e.g., glucose to lactic acid) or oxido-reductions (e.g., 

 glucose to CO2 and alcohol). A summary of the early theories of an- 

 aerobic fermentations is given by Slater (1928), and a review of the 

 data pertaining to anaerobic life of Protozoa and other invertebrates is 

 given by von Brand ( 1934) . Some of the anaerobic Protozoa seem to be 

 obligatory anaerobes and are quickly killed by aeration (e.g., Trepomonas 

 agilis, Lackey, 1932). Therefore one might expect them to have a type 

 of metabolism comparable to those of the anaerobic bacteria. 



Other organisms, such as certain intestinal forms, are certainly not 

 strict anaerobes, but are facultative, or amphibiotic. Measurements of the 

 intestinal gases (reviewed by von Brand and Jahn, 1940) and of the oxi- 

 dation-reduction potential of the digestive tract (Jahn, 1933a) indicate 

 that the lumen of the intestinal tract is largely devoid of oxygen. How- 

 ever, organisms which live at the surface of the epithelium (e.g., Giar- 

 dia) and within the villi, and especially those such as Endamoeha histo- 

 lytica and Balantidium coli which invade the tissue, do have access to 

 molecular oxygen. The O. tension of the environment of the rumen in- 

 fusoria of ruminants must be extremely variable, but, for considerable pe- 

 riods of time, almost devoid of oxygen. The question then arises as to 

 what kind or kinds of respiratory mechanisms are present in these faculta- 

 tive organisms. The same question arises with such organisms as Para- 

 meciuni, which are normally aerobic but can withstand lack of oxygen 

 for a relatively long period of time. Do the facultative anaerobes of the 

 phylum Protozoa have respiratory mechanisms comparable to those of 

 bacterial facultative anaerobes? This question, although interesting and 

 suggestive, is unanswerable at present because we know nothing about 

 the respiratory mechanisms of anaerobic Protozoa, and not very much 

 about those of bacteria. However, recent investigations indicate that 

 among bacteria the respiratory mechanism of the strict anaerobes is 



