38 FRESH-WATER BIOLOGY 



insects, etc., can live for a long time without free oxygen, and in 

 fact occur in the putrescible organic muds of the bottoms of lakes 

 and ponds and the hypolimnion of thermocline lakes in summer. 

 They evidently obtain oxygen from some chemical compounds. 

 Carbohydrates are present in the sea in solution in minute 

 quantity and there is every reason to believe them present in fresh 

 water. Packard found that marine Fundulus embryos live in lack 

 of oxygen from 73 to 141 per cent longer in the presence of glucose, 

 maltose, levulose, and cane sugar, the amount of increase in resist- 

 ance differing with the different sugars. Lactose has no such effect, 

 probably because it cannot be absorbed or digested. 



According to Mathews' depolarization theory oxygen is obtained 

 from the water in a manner analogous to the oxidation of alcohol 

 to acetic acid. In the presence of O 2 the reaction is as follows: 

 C 2 H 5 OH + 2 = CH 3 COOH + H 2 O. 



In the absence of oxygen and the presence of levulose 

 C 2 H 5 OH + H 2 = CH 3 COOH + 2 H 2 

 2 H 2 + 2 C 6 Hi 2 O 6 = 2 C 6 H 14 O 6 . 



The levulose unites with the hydrogen and thus permits the 

 protoplasm to use the oxygen. The protoplasm is thus a strong 

 reducing agent. 



High respiratory quotients of various animals are further evi- 

 dence of anaerobic respiration. The respiratory quotient is 



5-^ 2 ^ . In aerobic animals this value is less than i 



Vol. O 2 absorbed 



because oxides other than C0 2 are given off and C0 2 does not rep- 

 resent all the oxygen used. Thus when the quotient is more than 

 i it indicates that oxygen is obtained from some source other than 

 free oxygen. The respiratory quotient of the medical leech is 

 usually near or a little more than i while that of a sea cucumber 

 (Cucumaria) and a sea sponge (Suberites) is over 2.5 (Putter). A 

 large number of -aquatic animals are probably able to secure oxygen 

 from compounds containing it and they are therefore facultative 

 anaerobes to a considerable degree. 



Distribution of organisms in water is not clearly correlated with 

 oxygen content. The minimum for most animals is comparatively 



