Respiration 59 



Most animals require a more or less continuous supply of oxygen 

 and give off carbon dioxide as a result of their metabolic activities. 

 Carbon dioxide may accumulate and be present in such quantities 

 as to be injurious to animals. This seldom occurs in the atmosphere 

 but often does in small, shallow bodies of fresh water, especially if 

 the bottom contains organic debris which by decaying uses up 

 oxygen and produces carbon dioxide. Such animals as rotifers, nema- 

 todes, crustaceans, insects, and fishes may be poisoned by large 

 amounts of carbon dioxide in water (Nikitinsky, 1928) . On the 

 other hand, many aerobic animals have remarkable ability to live 

 in the absence of oxygen (Dakin, 1925). Fishes are very sensitive 

 to small variations in the gases (Oo, COo, HoS) dissolved in water 

 (Shelford & Powers, 1915). The ability of fishes to extract oxygen 

 from water depends upon the carbon dioxide tension in the water, 

 the alkali reserve in their blood, and other factors (Powers, 1923, 

 1932). 



Respiration is in some respects more difficult in water than air. 

 The atmosphere always contains about 22% oxygen, and poisonous 

 gases are rare. The amount of oxygen in the ocean is quite constant 

 at all depths, but in small bodies of water it may vary from none to 

 several cubic centimeters per liter. Furthermore, as water grows 

 warmer, its power to hold gases in solution becomes progressively 

 less. In the tropics, therefore, shallow bodies of water often lack 

 sufficient oxygen for the respiratory needs of animals. Bog ponds 

 in temperate or cool regions of the earth usually contain very little 

 animal life. This is not only on account of the acids and other sub- 

 stances present, some of which are toxic, but also because of the lack 

 of oxygen in winter when contact with the atmosphere is cut off by 

 a coating of ice (Rigg, Thompson, Lorah, & Williams, 1927; Jewell 

 dC Brown, 1929) . In the shallow klongs in Siam (Pearse, 1932) and 

 in the tanks in India (Pruthi, 1933) there is little oxygen at night, 

 and many fishes, snails, and other aquatic animals come to the 



