288 W. J. Crozier, 



4. Discussion. 



From the standpoint of the present experiments there are two 

 aspects of reaction to chemical stimuli — 1. the reaction time, and 

 2. the extent or amplitude of the effect. Reaction-time measure- 

 ments take into account only one of these factors, while for the 

 other there is no convenient method of quantitative expression. In 

 general, they paralleled each other, a brief reaction time being 

 associated with a reflex of wide amplitude, but this was by no 

 means an invariable rule. For example, comparing the reactions of 

 the anterior and posterior ends to M/5 acetamid (Table XIX), the 

 mean reaction time of the tentacles (3,9 sec.) was slightly greater 

 than that of the posterior end (3,5), but the amplitude was much 

 greater with the anterior end. Taking all the aspects of the reac- 

 tions into consideration, the order of reactiveness of the various 

 parts of Holothuria is, however, the same whether reaction time or 

 amplitude be made the criterion, and is as follows : tentacles > an- 

 terior end ) posterior end ) tube-feet papillae > mid-body surface. 



It was difficult to stimulate the anterior part of the surface of 

 Holothuria without involving the tentacles, but the following ex- 

 periment with M/10 NH4CI will show the relative sensitivity of the 

 anterior end: 



a) Tentacles expanded; reaction time 0,8 sec. 



b) Tentacles retracted (i. e. not stimulated); 

 reaction time 1,8 sec. 



Five observations were made on each condition, in the same 

 individual. 



Considering (1) the reaction-time, (2) the amplitude of the effect, 

 and (3) the limiting concentrations effective as stimuli, for all the 

 parts stimulated, the order of decreasing stimulus intensity of re- 

 presentative substances was: — 



hydrochloric acid ) atropine sulphate > acetic acid > potassium 

 hydroxide > potassium chloride > maltose) acetamid) glycerin. 

 "Within the several groups of substances studied, the relative 

 efficiency of stimulation, at the concentrations noted, was: — 

 Acids (Table XV), 



hydrochloric ) acetic ) (picric) ) tannic ) mallic ) lactic (M/10). 

 Salts (Table XIII), 



potassium chloride ) sodium chloride ) lithium chloride ) pot- 



