256 
BULLETIN OF THE BUREAU OF FISHERIES 
while that of one NaOH curve (B) is even less, and of the other (A) is approximately 
45°. If it be assumed, as seems reasonable, that the small angle of the curves is due 
to the time required for the stimulating substance to penetrate a layer of mucus on 
the tentacles, a possible explanation of these curves may be obtained. It has been 
pointed out that the potassium salts are much more efficient than sodium salts in the 
process of stimulation. 
While the threshold for KC1 is 0.01 M., that for NaCl is 0.10 M., or 10 times as 
high. It may be that the two ions in solutions of KC1 or KOH stimulate separately, 
and that the observed latent period is that due to the ion which initiates the reaction 
first. Then, if it be assumed that the curve for KOH (figs. 8 and 9) is a record of the 
stimulation by potassium ions, it is probable that the NaOH curve B represents the 
response to sodium ions, and curve A, OH ions. If this is the case, it would appear 
that the OH ions penetrate the layer of mucus much more rapidly than the ions of 
Na or K. Consequently the concentration of OH ions X the latent period is a con- 
stant, for the logarithmic curve has the angle of 45°. However, the time required 
for penetration of the mucus by Na and K ions must be subtracted from the latent 
period in order for this relationship to obtain. 
CANE SUGAR 
Parker (1908) stated that it appears that cane sugar does not stimulate the 
sensory receptors of Amphioxus. Similar results were obtained with the oyster. 
Only when a very high concentration of sugar was employed did the tentacles retract, 
and then the reaction was similar to that due to diluted or concentrated sea water. 
The effect obviously was due to osmosis by extraction of water. Crozier and Arey 
(1919) obtained similar results with Chromodoris, but found (Arey and Crozier (1919)) 
Chiton to be sensitive to 0.5 M. sucrose and lactose. 
Negative evidence such as this is, however, of doubtful significance. The reac- 
tion of the oyster to the solutions employed is negative. The retraction of the ten- 
tacles or mantle appears to be of a protective nature and results in the withdrawal 
of these organs from the source of irritation. It is not impossible that substances 
such as sugar may stimulate the receptors positively and that the reaction as to irrita- 
tion does not take place. 
DISCUSSION 
A source of constant difficulty in the experiments described in the foregoing 
pages is the occasional sudden change in the latent period values from a low to a 
high level, or vice versa. One example was given (figs. 5 and 6) in which two distinct 
curves were obtained. It was thought possible that frequent stimulation might 
result in fatigue of the receptors or in sensitization like that observed by Irwin (1918) 
to follow successive stimulation with a strong potassium salt. It seemed also possible 
that changes in light intensity or temperature might cause variations in the threshold 
(Kribs, 1910). However, no evidence has been obtained that any of these sugges- 
tions is the case. There appear to be definite levels at which the response to any 
concentration of a salt takes place. It has frequently happened that two separate 
and distinct responses may be observed, and measured, following stimulation. At 
other times either the lower or the higher reaction time may be measured. It is as 
if both such reactions always occur and that in some cases one is distinct and the other 
too weak to be observed. 
