MODIFICATION OF THE BEHAVIOR OF FISHES 25 



dioxide was the factor thus avoided. This is good evidence 

 that fish ascertain the condition of the water by peripheral sense 

 organs or otherwise. The modifications appearing after a number 

 of trials must be due to increased sensitiveness to the modified 

 water or to associative memory or to both. 



The physiological explanation which may be made for the 

 increased sensitiveness with increased exposure to water high in 

 carbon dioxide is simple, and is based largely upon the relations 

 of organisms to carbon dioxide. The arterial blood of dogs and 

 horses (Hill, '06) has been shown to contain 330-550 cc. per liter 

 of carbon dioxide, free and combined. The free carbon dioxide is 

 about 20 cc. per liter, so in most of the concentrations used 

 more carbon dioxide would be taken up (Hill, '06, p. 533) and 

 its removal from the blood and tissues was undoubtedly hindered 

 in all cases. Since carbon dioxide is constantly produced inside 

 the fish's body the effect of increased concentration on the out- 

 side would become greater with repeated trials of the carbon 

 dioxide water. Waller ('96) has shown that carbon dioxide in 

 small amounts increases the irritability of nerves. Hill ('09) 

 states that similar results have been obtained with micro "organ- 

 isms.* A stimulating concentration of carbon dioxide is generally 

 recognized among physiologists. Because of the increase in inter- 

 nal carbon dioxide brought about by entrance into the carbon 

 dioxide water, the fishes tended to become more sensitive with 

 repeated entrances and hence to turn back in weaker concentra- 

 tions. After spending some time in the weaker carbon dioxide 

 of the tap water end of the gradient tank, they partially recover 

 and tend to resume their usual movements. This brings them 

 again into the modified water and the process is repeated, hence 

 we have a rhythmic invasion of the carbon dioxide water as 

 shown in chart 1, Expt. 10. 



As was noted in table 3, the respiratory center is stimulated 



and the respiration movements increased in carbon dioxide and 



in low oxygen. This requires more time than the reflexes which 



follow the sensory impressions (Westerlund, '06). In carbon 



dioxide the increased respiratory movements occur within a few 



seconds after the reflex movements. The same is true in low 



*The sensitiveness of fishes to carbon dioxide probably increases with starvation. 

 A stock of fishes kept during the winter of 1911-12 without food showed markedly 

 low resistance in that they lost their equilibrium in from 30 to 50 cc. of carbon 

 dioxide per liter. 



