REACTIONS OF FISHES TO ATMOSPHERIC GASES 231 



Those with a difference of about 65 cc. between the two ends, 

 the whole tank being regarded as gradient because the carbon 

 dioxide drifted across to the tap water end so as to raise the 

 content from 5 to 13 cc. The amount of carbon dioxide was 

 markedly less at the surface, due to loss into the atmosphere. 



Occasionally the fishes, when put into the center, moved into 

 the tap water end and came to rest without encountering the 

 high carbon dioxide. In such cases they were driven to the 

 center with the hand or were roused by jarring the tank and a 

 corresponding number of drivings or tappings was effected in 

 the control. The reactions of the fishes to the carbon dioxide 

 gradient fall into the following three main classes: (a) The}' 

 entered the carbon dioxide water with little perceptible hesitation 

 and gave some definite reaction while there, (b) They gave an 

 avoiding reaction upon reaching the increased dioxide, (c) They 

 reacted by rising to the surface. 



The reactions of the first type were given by most of the 

 swinmiing species usually at the beginning of the experiment in 

 which carbon dioxide exceeded 20 cc. per liter. With the excepr 

 tion of the Abramis, the fishes gave a coughing reaction in which 

 the mouth was thrown wide open and the jaws slightly protruded 

 with a sudden jerk. The same reaction sometimes took place 

 more slowly and may be characterized as a yawn. In all cases 

 observed, the gill movements were increased. The second type 

 of reaction usually occurred after the fishes had tried the highest 

 carbon dioxide and had given one of the reactions just mentioned. 

 The commonest of these reactions may be characterized as a 

 testing or hacking-starting reaction. The fish mo\'ed forward and 

 acted as though it encountered a sheet-rubber wall which it 

 carried forward for a short distance but which in turn through 

 its elasticity, caused the fish to rebound for an equal distance, 

 probably 2 to 10 mm. In other words, the fish suddenly stopped, 

 backed a very short distance and immediately moved forward 

 the same distance again, usually repeating several times. The 

 fishes sometimes turned back after giving this reaction, and 

 sometimes turned back without giving it, and without any other 

 characteristic movement. The third type of reaction was some- 

 times given following the other two but was especially charac- 



