478 LESLIE B, AREY AND W. J. CROZIER 
than did the same amount of LiCl. NaCl was more effective, 
also, on animals previously fatigued by repeated stimulation. 
The foot is the part of Onchidium least responsive to ionic 
excitation; therefore it is in some respects a region advantageous 
for comparing the relative effectiveness of different substances. 
NaCl and LiCl induced pronounced puckering contractions 
when applied to any part of the foot, the animal being also induced 
to curl up, pill-bug fashion. KCl and NH4CI were very much 
milder in their effects; with KCl, tiny surface puckerings were 
induced, local wrinklings, but no deep local retractions such as 
were occasioned by NaCl; the efficiency of NH4CI was in this 
respect invariably less than that of KCl. 
We arrive therefore at the following series, which expresses 
the relative efficiency of these cations at uniform concentration: 
Na > Li > K > NH4 
The halides of potassium were used to obtain a corresponding 
series for the anions. KI and KCl were more effective than 
KNO3 or KBr in stimulating the foot of Onchidium. While 
these two groups were sufficiently distinct, the further separa- 
tion of the anion effects was a matter of some difficulty. The 
series eventually chosen as best expressing the results, in terms 
of the relative magnitudes of the reactions induced, was this: 
I > CI > NO3 > Br 
The cation series is practically the inverse of that usually 
found in such cases, and of that which we have found for the 
sensory activation of several other mollusks (Arey and Crozier, 
'19; Crozier and Arey, '19b), employing comparable criteria. 
The anion series is more like that commonly reported. The 
relatively great stimulating power of NaCl may be related to the 
fact that Onchidium creeps upon rock surfaces wet with sea- 
water, but exposed to the evaporating power of the sun. In 
this connection it should be noted that the mouth region is 
decidedly the most sensitive part, and also that in an actively 
creeping animal it is not possible, as a rule, to obtain sensory 
responses from activating substances at concentrations so dilute 
