228 AX AEROBIC METMiOLI^M 



the solutions used in the experiments averaged only 5 

 c.mm., while the animals actually produced from 20 to 

 40 c.mm. 



In general, not enough attention has been paid so far 

 to the origin of the carbon dioxide produced during fatty 

 acid or mixed fermentations. The frequent practice of 

 studying chemically only the main anaerobic process, 

 namely, the carbohydrate fermentation, and of ascribing 

 to it all the carbon dioxide recovered, has, in several cases, 

 resulted in data that may well be changed by subse- 

 quent investigations. 



C. Production of combustible gases. Combustible gases, 

 especially hydrogen and, to a much lesser extent, me- 

 thane, are frequently formed during the fermentations 

 of micro-organisms. A variety of mother-substances 

 serve as their source. In invertebrates only a few cases 

 of hydrogen formation have been described and methane 

 has so far never been identified. Although some of the 

 cases have already been mentioned in other sections, a 

 brief survey of all available investigations will be pre- 

 sented here. 



It is a well-established and repeatedly studied fact that 

 one of the end products of the cellulose fermentation of 

 the intestinal protozoa of termites is hydrogen (Cook, 

 1932 and 1943; Cook and Smith, 1942; Hungate, 1939 

 and 1943a — cf. Sect. 1, C of this chapter). 



The symbiotic protozoa of the roach Cryptocercus 

 punctulatus also produce hydrogen, and in addition, 

 small amounts of a gas that gave rise to carbon dioxide 

 upon combustion (Gilmour, 1940a). 



A gas which is not absorbed by KOH and which is com- 

 bustible is developed in bacteria-free cultures by Tri- 

 chamonas foetus (Andrews and von Brand, 1938), but 

 its nature has not yet been determined. 



The only metazoon which produces large amounts of 

 hydrogen during anaerobiosis is the pupa of Calliphora 



