306 L. H. HYMAN. 



selected as an example of a dibasic, the latter of a tribasic acid. 

 The results were not of particular interest and are summarized 

 in the general table, Table VI. The action of tartaric acid was 

 very similar to that of the mineral acids. Citric acid was less 

 effective than the inorganic acids at concentrations lower than 

 pH 7.0. 



9. Consideration of Other Factors. (a) Size. It was found 

 long ago by Child that young (small) individuals are more 

 susceptible to lethal concentrations of various substances than 

 are old (large) individuals of a given species. MacArthur ('20) 

 studied the relation between age (size) and susceptibility to 

 acids in Planaria dorotocephala. He found that in concentrations 

 of hydrochloric and other acids which are quickly lethal, between 

 pH 2.0 and 4.5, the young (small) individuals die sooner than 

 the old (large) ones. At slightly lower concentrations, pH 4.7 

 to 4.9 for HC1, the relation is reversed, the larger individuals 

 succumbing first. In still weaker solutions, planarians live 

 indefinitely. These results have been repeatedly confirmed in 

 this laboratory in class work, although of course the precise 

 concentrations required to yield the results mentioned are subject 

 to variation, owing chiefly to differences in the physiological 

 condition of the worms themselves. 



Since according to these results, small worms are more sus- 

 ceptible to acids than large worms at concentrations greater 

 than pH 4.5, it seems probable that the percentage of depression 

 induced by acids should bear some relation to size. This was 

 tested in a number of experiments, in which the depressing action 

 of HC1 on large and on small worms was compared. Only 

 recently collected material was used as the metabolic differences 

 between worms of different sizes are greater in such material. 

 The heads of all worms were removed before the test, in order to 

 eliminate movement, as small worms are more active than large 

 ones. Decapitated worms are very inactive. The small worms 

 used were under 10 mm. in length, the large ones over 20 mm. 

 Twelve experiments were performed at concentrations of HC1 

 ranging from pH 3.6 to 4.6. In ten of these the percentage of 

 depression was greater in the small than in the large worms. 

 The difference ranged from 3 to 20 per cent. In two cases the 



