262 VICTOR E. SHELFORD AND W. C. ALLEE 



required for the fishes to become affected by the lack of oxygen, 

 carbon dioxide, et cetera, and that when the system has once been 

 affected, for example, by carbon dioxide, a slight increase may 

 have a more pronounced effect than at first when the blood 

 supply is relatively free from this substance. 



VII. SUMMARY 



1. In the experimental control of gases, several and not one 

 factor are commonly varied; the varying of single factors is 

 unusually difficult, being essentially impossible, when gas is 

 bubbled through water (p. 210). 



2. Fishes are clearly affected by lack of oxygen; species usually 

 die in the order of their relation to these factors in nature (p. 223). 



3. Carbon dioxide, in concentrations probably used to produce 

 reversals of reaction in some of the invertebrates, is poisonous 

 to fishes, producing death very quickly (p. 224). 



4. Fishes are not seriously affected by high nitrogen except 

 when gas is in excess under one atmosphere pressure and comes 

 off in bubbles on rough and warm objects; under these conditions 

 gas bubble disease occurs (p. 222). 



5. Fishes react negatively to a gradient of decreasing salts, 

 nitrogen, and 1.5 cc. per liter of carbon dioxide, in combination; 

 to a decrease in oxygen and other effects of boiling in combina- 

 tion; to carbon dioxide, to carbon dioxide in combination with 

 the effects of boiling, and to carbon dioxide in boiled water at 

 both ends of the experimental tank. The precision and definite- 

 ness of the reaction is indicated by the order in which the factor 

 and combinations are given the most definite reaction being to 

 carbon dioxide (p. 229). 



6. The negative reaction of the fishes is evident through longer 

 stays in the tap water end of the gradient tanks, by tm'nings 

 back from the center, by risings to the surface, or by any combi- 

 nation of the three (p. 231). 



7. Such reactions are accompanied by backing-starting reac- 

 tions, coughing, gasping and gulping, directly proportional to the 

 degree of avoidance of the treated water (p. 231). 



