308 L. H. HYMAN. 



It seems probable that in this behavior of the worms is to be 

 found the explanation of the numerous puzzling variations in 

 the degree of depression obtained. 



(c) Acclimation. It was my intention at the beginning of this 

 work to determine whether the animals could recover from the 

 depression induced by acid if allowed to remain for some time 

 in the acidified solution. I found, however, that the experiment 

 is impractical, because of the carbonate content of the water 

 used. When the water is acidified it soon becomes alkaline and 

 unless acidified to a point which would be fatal to the worms, 

 returns to an alkaline condition within 24 hours. Consequently 

 if worms are placed in the acidified water at a certain pH, the 

 water does not remain at this pH, but the pH rises (the acidity 

 falls). If then the worms are tested at the new pH, the depression 

 is naturally less than it was at the beginning of the exposure. 

 If tested at the original pH, the depression is greater than at the 

 pH attained by the standing solution. In brief, it is impossible 

 to determine the effect of long continued exposure to a given pH, 

 unless the water is freshly acidified and changed every hour, 

 for two or three days. As this is physically impossible, for me 

 at least, the experiment had to be abandoned. I have, however, 

 a number of experiments, especially with HC1, in which the 

 oxygen consumption was tested during the first, third, and fifth 

 hours of continuous exposure to a given concentration. Some of 

 these data appear in Table IV. In general it was found that 

 there is very slight if any recovery during successive hours of 

 exposure. After this had been repeatedly determined, the tests 

 of the later exposures were abandoned. 



The reader will no doubt at once inquire why carbonate-free 

 water was not employed in a study of acclimation. The reason 

 for this as will appear later is that acidified carbonate-free water 

 does not affect the oxygen consumption of Planaria. 



10. General Results and Discussion. We may now state and 

 discuss the results obtained with acidified unaltered water. 



(a) Acidification of the water generally causes a depression of 

 the rate of oxygen consumption. This was true of the three 

 mineral acids, hydrochloric, sulphuric, and nitric, at all concen- 

 trations employed. Even a change from the normal pH 8.0 of 



