298 L. H. HYMAN. 



by Dr. E. B. Powers that he was already engaged in a similar 

 investigation on fish, and I was quite willing to relinquish this 

 particular task to him. 



i. General Procedure. There is little to add to the procedure 

 already outlined. Large stocks of Planaria dorotocephala are 

 always at hand in our laboratory. Only stocks which had 

 sojourned in the laboratory for at least one or two months were 

 employed, as the basal respiratory rate of freshly collected 

 material is variable. Only worms which had starved from four 

 days to two weeks were used, since during this period the respir- 

 atory rate is constant. Each lot of worms to be used for an 

 experiment consisted of a number of individuals sufficient to 

 consume a readily measurable amount of oxygen in an hour. 

 Such a lot was placed in a 500 cc. Erlenmeyer flask at least 

 several hours before the experiment because when the worms are 

 placed in any clean strange container they are apt to travel 

 about restlessly for some time. By placing the worms in advance 

 in the container in which they are to be tested and by darkening 

 them during the experiment, movement can be practically 

 entirely eliminated. It is certain that movement does not play 

 any role in the results. Three such lots of worms were carried 

 throughout the work in order that three tests of each strength 

 of each acid could be performed simultaneously. This procedure 

 shortens considerably the amount of labor involved. Each lot 

 of worms was kept in a particular flask for nearly two weeks and 

 used several times during that period. If desired such lots could 

 then be fed once or twice in the flasks and after four days again 

 utilized for experiments. This procedure was found most suit- 

 able because different lots of worms are apt to yield different 

 percentages of depression with the same concentration of the 

 same acid. To obtain a graded set of results showing a graded 

 increase in the percentage of depression with increasing concen- 

 tration of acid, it is almost necessary to test all of the concentra- 

 tions of any one acid on the same lot of worms. If different lots 

 are interpolated in such a series, the relation between depression 

 and concentration is less regular, although always exhibiting the 

 same general trend. 



The remainder of the procedure was as already described. It 



