Respiration and Metaholism 273 



a good deal of conflicting data. Many obligatory anaerobes are harmed by high 

 oxygen tensions, as in the classic defaunation experiments on termites^'' ^-^ It 

 has been claimed that the period of survival of Ascaris megalocephala in cul- 

 ture is shortened by increased oxygen. ''^'^ Most ascarids and tapeworms utilize 

 oxygen whenever it is made available, thereby actually increasing metabolic 

 efficiency.-'^- «»• -'i' 



Elimination of carbon dioxide occurs during anaerobiosis,^^'' -**^' ^^^ but 

 how much of this actually represents a production of carbon dioxide and how 

 much represents its release by the acid metabolites formed is still subject to 

 further experimentation and interpretation. Certainly much of the carbon 

 dioxide, having already been formed prior to the experimental anaerobiosis, 

 is merely released from the tissues. The carbon dioxide recovered post-anaero- 

 bically from Termopsis is less than would be expected in oxygen debt payment 

 and may represent a shift in alkali reserve necessary to buffer the large amounts 

 of acid metabolites produced. An interesting buffer safeguard against these 

 acids is found in bivalve molluscs, which utilize calcium carbonate from the 

 shell during long periods of anaerobiosis.^''^' 



Anaerobiosis among invertebrates has been extensively surveyed by von 

 Brand in a monograph based largely on his own investigations."*'' Under experi- 

 mental conditions certain organisms from all major invertebrate groups can 

 survive oxygen lack or its near depletion. The relative resistance to anaerobio- 

 sis may vary with activity, nutrition, size, and stage in the life cycle, eggs and 

 cysts, for example, being considerably more tolerant to oxygen lack than free- 

 living adult stages. 



True anaerobiosis is not uncommon among Protozoa, although it is not easy 

 to rule out the possibility of last traces of oxygen. The resistance to anaerobio- 

 sis among Protozoa is known to be greater under natural conditions than under 

 experimental oxygen lack, possibly owing to the gradualness with which the 

 organisms in nature become subjected to anaerobiosis, thus allowing for a 

 certain degree of adaptation not seen in the laboratory. 



All degrees of oxygen tolerance are found among the intestinal worms— 

 ascarids and tapeworms— depending on the worm in question and on the 

 particular site of its location in the intestinal tract. According to von Brand^^ 

 "the old controversy 'aerobiosis or anaerobiosis,' with respect to intestinal 

 worms, should be abandoned, and ... it should be recognized that one animal 

 may, depending on its organization, live a predominantly anaerobic life in the 

 same surroundings in which another worm is capable of leading a chiefly 

 aerobic life." 



Among the molluscs the lamellibranchs show the greatest anaerobic adapta- 

 bility, cephalopods the least. Anaerobiosis among lamellibranchs must be of 

 considerable importance to those marine forms which live in the tidal zone and 

 are unable to obtain oxygen during intervals of low tide. 



Anaerobic conditions have been demonstrated in many crustaceans, in- 

 cluding copepods, cladocerans, ostracods, and cirripidians, as well as in certain 

 insect larvae. 



In conclusion, anaerobiosis as a physiological adaptation seems to have 

 developed in a number of forms, and is particularly refined to permit certain 

 organisms to survive conditions of oxygen want as endoparasites. The pre- 

 vailing anaerobic glycolysis throughout the animal kingdom already had pro- 



