PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 293 
provided for. The application of the stimulating substance to 
the sense organ merely starts a prearranged process of response. 
In order to understand the nature of the process set up in the 
receptor, it will be necessary to consider more closely the phys- 
iological effects of the substances used in the stimulation of 
Ascidia. It was on the basis of the action of the salts of the alkali 
metals that Héber (14) first pointed out the relation between 
irritability and colloidal constitution of the plasma membrane. 
Since then the ubiquity of the cation and anion series has been 
demonstrated for such diverse processes ‘as melanophore con- 
traction (Spaeth, *13), hemolysis (Héber, 714) and rhythmic 
pulsation (Crozier, ’16a). The presence of these ionic series in 
the sensory stimulation of Ascidia indicates that the significant 
process which underlies it, resembles, if it is not identical with, 
the determining reactions of the other physiological phenomena. 
The acids have already received attention in regard to their © 
sensory effects (Richards, ’00; Kahlbaum, ’00). The anomalies 
which are exhibited by the acid taste in man are typified in the 
behavior of the three acids which were used in these experiments. 
Although HCl is more effective than formic acid, the difference 
between them is not great. They both, however, are much more 
powerful than acetic. In the penetration of cells by acids 
(Crozier, ’16b), we find the same order of effectiveness. The 
anomalies which were referred to are as follows. When the dis- 
sociation constants of the acids are taken into account, it is found 
that the same effect is produced by acetic acid with a lesser 
quantity of hydrogen ions than by formic acid; and less in turn 
by formie than by hydrochloric acid. In Ascidia the liminal 
concentrations of the acids contain the following quantities of 
hydrogen ions: acetic, 4.1x10-4N; formic, 6.0x10-‘N; and 
hydrochloric, 1.6 10-3N. 
An analogous difficulty exists in the effects of NaOH and 
NH,OH. Experiments on penetration have shown that NH:OH 
enters tissue rapidly, whereas NaOH may hardly be said to pene- 
trate living tissue at all. Still, NaOH is more toxic than NH,OH 
(Harvey, 13). Similarly it is a more effective sensory stimulant 
than NH,OH. 
