RHEOTAXIS IN ISOPODA 313 



asphyxia, partially due to a lack of oxygen and partially to a 

 direct depressing action upon the central nervous system. When 

 mixed with sufficient oxygen the specific effects of the gas may be 

 observed without asphyxia. Under these conditions transient 

 stimulation occurs, followed by subsequent depression of the 

 central nervous system and heart. In well diluted vapor, only the 

 exaltation occurs as the anesthesia does not follow. This would 

 mean that the results shown in the first parts of tables 15 and 16 

 are due to the action of carbon dioxide as a stimulant, and this 

 may even retard the depressing effect of a decreased oxygen sup- 

 ply (table 11, part 4). But when over 200 cc. of carbon dioxide 

 per liter are present, it acts as a Strong depressant. Regarding 

 the effect of large quantities, Cushny (I.e., p. 588) says that 

 in mammals a large amount of carbon dioxide probably acts as a 

 poison to protoplasm, for it lessens the amount of oxygen absorbed, 

 so that in the final analysis it would seem that the depressing 

 effect of carbon dioxide is directly due to increased oxidations and 

 thus it acts in the same way as when the supply of oxygen is 

 decreased. 



It will be remembered that the free carbon dioxide present in 

 the streams is about 2 cc. per liter, while that of the ponds may 

 run as high as 40 cc. But since in the experiments 45 cc. of car- 

 bon dioxide acts as a stimulant, and since small amounts of the 

 gas are known to be mammalian stimulants, it is improbable 

 that the low positive response of pond isopods is correlated with 

 the increased carbon dioxide content of the water. This then 

 leaves the decreased oxygen supply as the main environmental 

 factor with which the lowered positiveness seems correlated. 



c. Chloretone 



Chloretone (acetone chloroform) belongs to the class of sub- 

 stances commonly known as anaesthetics. Authorities generally 

 agree that these anaesthetics inhibit to some extent certain of the 

 fundamental metabohc reactions (Child, '10, p. 173). Different 

 strengths of chloretone were used in these experiments but 0.005 

 per cent was found to ,work best for experiments that were to run 

 some time. Preliminary tests showed that the animals collect 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 13, NO. 2 



