272 Comparative Animal Physiology 



anaerobiosis vary, of course, from individual to individual. The larva of the 

 helminth, Eiistrongylides, for example, can tolerate anaerobiosis for 18 hours 

 and then recover locomotion in 30 minutes. Some worms, molluscs, and crusta- 

 ceans can survive anaerobic conditions for periods exceeding three months."*" 

 Generally speaking, terrestrial organisms are less resistant to anaerobiosis than 

 are aquatic forms. Insects, particularly adults, are sensitive to oxygen deficiency. 

 Likevi'ise, the recovery times of insects are relatively long. An important factor 

 in the survival time is always the degree of activity of the organism in ques- 

 tion.^"** The ability of certain organisms to survive complete oxygen lack- 

 true anaerobiosis— is rather rarely encountered. An even rarer occurrence pre- 

 vails when organisms are injured by the addition of oxygen.'^ We find, then, 

 animals exhibiting all degrees of oxygen-deficient metabolism, ranging from 

 (1) those anaerobes which find oxygen harmful (e.g., intestinal ciliates of 

 termites); (2) those which usually live in an oxygen-free or practically oxygen- 

 free environment but can utilize oxygen if it is supplied (e.g., tapeworms); 

 (3) those which ordinarily use oxygen but may get along for a while in an 

 environment devoid of oxygen (e.g., frog); to (4) those which may be con- 

 sidered essentially aerobic and depend on oxygen (e.g., man), but in which 

 anaerobic processes such as muscle glycolysis occur, and, during exercise, 

 an oxygen debt is contracted. Anaerobic muscle metabolism is particularly 

 pronounced in the diving birds and mammals (see above). 



It is worth noting that under experimental conditions many animals, in- 

 vertebrates particularly, may be shifted in favor of or against an oxygen-poor 

 environment, causing some organisms to become more, and others less, an- 

 aerobic than is their tendency in nature. Such experimental modification is 

 not without significance in consideration of the variations in data that have 

 been reported by some investigators for low oxygen consumers. For conven- 

 ience as well as for accuracy, experimental anaerobiosis should be clearly 

 distinguished from natural anaerobiosis. Anaerobic demonstrations under 

 experimental conditions in the frog,-^* cockroach, •^-^' '-^-^ earthworm, ^^-" Para- 

 mecium,'^'^'-^ Planaria and Tenehrio^'^'' are all valid cases of the adaptive 

 ability of animals to withstand oxygen lack for various periods of time, gener- 

 ally by way of reducing metabolism and enduring considerable oxygen debt. 

 They tell us, however, little else, and it is unlikely that oxygen-free conditions 

 in nature are confronted by any of these animals. 



The oxidative accomplishment by which true anaerobiosis is attained in 

 endoparasitic ascarid worms, for instance, rests on two factors: the dependence 

 on anaerobic glycolysis, which, although not a high energy-yielding process, 

 nevertheless is adequate for their needs, and further, the unusual tolerance to 

 acid metabolites, as lactic, valeric, caproic, butyric, and propionic acids, which 

 accumulate to a considerably degree prior to excretion, rather than being 

 oxidized, as would occur if oxygen were present.'-'^- -^^' "^''"' "^'^^ 



The fact that greater amounts of glycogen may be consumed under anaero- 

 bic conditions than during aerobiosis was shown by the work of von Brand""* 

 on Spirographis and Halla, and of Dausend'*'"' on Tubifex. Concerning the 

 importance of fat and protein as energy sources under anaerobic conditions, 

 von Brand regards fat as of little consequence but suggests that in Hirndo and 

 Ascaris protein catabolism may play a significant role. 



The utilization of oxygen when supplied to normal anaerobes has presented 



