FUNCTIONS OF SWIMBLADDER OF FISHES. 97 



entirely lacking. Both these factors make the bottom of a 

 lake less favorable for respiration than shallower regions. Of 

 course a high amount of carbon dioxide is toxic and the lack of 

 oxygen may lead to asphyxiation. In Fig. 6, the average carbon 



FIG. 4. A graph showing the volume change of a perch when placed in water 

 where dissolved free carbon dioxide was increasing. Time is recorded at two- 

 minute intervals. 



dioxide gradient of Lake Mendota is plotted for the month of 

 August from 1906 to 1921. The data were obtained from records 

 kept by President Birge and Professor Juday. It can be seen 

 that the carbon dioxide gradual^ increases with depth and 

 averages about five cubic centimeters per liter at the bottom. 



FIG. 5. A graph showing the temporary decrease in volume when a perch 

 was placed in water containing lactic acid (i : 1000). Time is recorded at two- 

 minute intervals. 



At times it may be two or three times as great as this average 

 figure for Lake Mendota. Such an amount of carbon dioxide 

 may be sufficient to produce an increased volume in the fish and 

 cause t to rise above regions that might be unfavorable for 

 respiration. Shelf ord (1913) has shown that dissolved carbon 

 dioxide may be taken as an index for the distribution of fishes. 

 He attributes distribution to the results of behavior-regions 



