290 W. J. Crozier, 



paring the effects of hydrochloric and acetic acids, it will be seen 

 that the acetic acid has a far higher stimulating power than would 

 be predicted from its dissociation, on the assumption that the free 

 hydrogen ion alone is concerned. This is likewise true of all 

 the other organic acids, and is comparable to the effect in man 

 (EiCHAEDS 1898) and in the earthworm (Huewitz 1910). There is^ 

 I believe, a possibility that Holothuria's muscles are stimulated 

 directly by such irritants as tannic acid, etc. One other point in 

 the reaction-time results is the greater stimulating effect of potas- 

 sium chloride (Table XIV) in dilute solutions. The relation of con- 

 centration to stimulating effect was studied especially with potassium 

 chloride, hydrochloric acid, acetic acid, and potassium hydroxide. 

 In general, reaction time increases with decreasing concentration,, 

 but with (Table XIV) potassium chloride and (Table XVI) hydro- 

 chloric acid there is a quite perceptible decrease in reaction time 



M M 



in the neighborhood of -rrrr^^ -tttt, except in the case of the 



podia. This drop in reaction-time was accompanied by an increase 

 in the amplitude of reaction, and was therefore a real phenomenon, 

 paralleled by the results of Paekee & Metcalp (1906) on the 

 earthworm. 



It has been shown in this paper that the reaction to chemical 

 stimuli is a general integumentary function in Holothuria, though 

 the exact chemo-receptors have not been anatomically distinguished. 

 In all probability, however, they are of the usual invertebrate type, 

 comparable with the chemo-receptors of the earthworm (cf. Bovaed 

 1904) and the olfactory cells of vertebrates. According to a con- 

 ception of the evolution of organs of chemical sense in vertebrates 

 which has been developed by Paekee (1912) , the end organs of 

 smell, the common chemical sense, and of taste are related in a 

 genetic series, of which the olfactory receptor is phylogenetically 

 the oldest. Now, if we may legitimately run this series back to its 

 invertebrate source, several things might reasonably be predicted, 

 namely, 1) that the olfactory epithelium of some lower vertebrates 

 would retain traces of its sensitivity to dissolved materials which 

 ordinarily constitute homologous stimuli for taste, and 2) that the 

 chemo-receptors of invertebrates would show, with reference to the 

 range of concentrations which we associate with taste, a mechanism 

 of stimulation different in some essential particulars from that ex- 

 hibited by vertebrate taste buds, for the latter are receptor cells in 



