FUNCTIONS OF SWIMBLADDER OF FISHES. 



8 7 



TABLE III. 



THE RESULTS OF ANALYSES OF THE SWIMBLADDER GASES OF VARIOUS FISHES 



KEPT IN LABORATORY TANKS. 



The temperature varied from 20 to 23 degrees centigrade. August and 

 September. 



THE EFFECTS OF OXYGEN DEFICIENCY. 



Pearse (1920) showed that perch, even though they are able 

 to recognize the proportion of oxygen and carbon dioxide in 

 water, enter regions where conditions are unfavorable for respira- 

 tion and may remain there in oxygen-free water for as much as 

 two hours without dying. He also showed that the oxygen 

 supply of the swimbladder was depleted when the fishes were 

 subjected to waters of low oxygen content. On his suggestion 

 further investigations were carried on during the summer of 

 1921. 



A search was made, in Lake Mendota, for a region of low 

 oxygen. Such an one was found at the mouth of University 

 Creek, in University Bay. This creek empties into the southern 

 end of the bay in shallow water behind a bar. During the sum- 

 mer months a thick covering of duckweed (Lemna and Wolffia) 

 may be found there. The position of this plant covering is 

 governed by the direction of the wind, but whenever a wind 

 from the North, Northwest, or Northeast is blowing, or when 

 there is little or no wind, one may prophecy with a fair degree 



