78 PROCEEDINGS OF THE AMERICAN ACADEMY. 



by forward movement in a direction different from that in which it had 

 been moving, or by a backward contraction. When in a suspended 

 condition, however, escape from a stimulating solution can be effected 

 only by a contraction. It is highly probable, however, that the anterior 

 elongations and squirming of the worm observed in some slow reactions 

 corresponds to the effort of a crawling worm to escape an irritation by 

 moving forward in a new direction. This ineffective forward reaction 

 most likely delayed the backward one and so lengthened the reaction 

 time. Although the quick reaction is the more characteristic one, the 

 general average of slow and quick reactions may be taken as a fair 

 measure of the reaction-time. 



Assuming that these reaction-times are indicative of the degree of 

 stimulation, it is clear from the figures in Tables I to XVI that hydro- 

 chloric acid is more stimulating than nitric, sulphuric, or acetic acids, 

 but beyond this it is difficult to go. This method was therefore a-ban- 

 doned for one that gave through a greater number of worms a more 

 immediate comparison of the different acids. 



In this second set of observations, twenty-four instead of five worms 

 were tested in all four acids and their relative reaction times were 

 recorded in the manner previously described. The tests were so made 

 that each acid had first place with some worm. Thus the possibility 

 of error due to always passing from a stronger to a weaker acid with a 

 given worm was eliminated. The results are recorded in the following 

 table. 



By inspecting this table, it will be seen that the order of acids 

 arranged according to their stimulating value is hydrochloric, nitric, 

 sulphuric, and acetic. Furthermore, that the reaction times for hydro- 

 chloric and nitric are practically identical, while that for sulphuric is 

 nearly twice that for hydrochloric or nitric acids. The stimulating 

 value of acetic acid, according to these results, is about half that of 

 sulphuric and scarcely a third of that of hydrochloric and nitric acids. 



These last conclusions differ from those of Kahlenberg (98) for sen- 

 sations of taste in man, in that at j^,, he was unable to distinguish be- 

 tween hydrochloric, nitric, and sulphuric acids. As Table XVII shows, 

 the earthworm, on the contrary, distinguishes between these acids ; and 

 since in each solution the hydrogen ions are the stimulating elements, 

 it must follow that the difference in reaction is due to a difference in 

 concentration of these ions. 



As compared with other acids, solutions of acetic acid are peculiar 

 in that their sour taste is more intense than would be expected from 

 their degree of dissociation. Kahlenberg, for instance, found that a 

 5^j solution of acetic acid, being dissociated only about 6 per cent, has 



