PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 285 
In order to avoid the mechanical stimulation of the current 
produced by the application of a substance, recourse was again 
had to the muffled pipet used in the experiments on the tempera- 
ture sense. By this means the solution to be tested could be dis- 
charged into the siphons without producing any effect other than 
that due to the dissolved material. The muffled pipet was, there- 
fore, used in all the experiments. 
The outflow of one cubic centimeter of solution from the pipet 
took place at the following rate. The first quarter was discharged 
into the seawater in 1.2 seconds; the next quarter required 1.8 
seconds; the remaining half took 5.0 seconds. The discharge 
of the entire cubic centimeter was accomplished therefore in 8.0 
seconds. In actual practice any solution which stimulated at 
all, did so during the discharge of the first half cubic centimeter. 
There is an inherent weakness in this method of applying a 
stimulus that must never be lost sight of. It is that the con- 
centrations in the pipet do not represent the solutions which 
reach the sense organs, because of the diffusion of the stimulating 
solution into the surrounding seawater. It is difficult to form 
an exact estimate of the amount of this dilution. A con- 
servative statement would be that the material from the pipet 
is mixed with about two or three times its volume of seawater 
before it reaches the inside of the siphon. The matter is simpli- 
fied in Ascidia by the existence of a continuous current entering 
the oral siphon. The solution thus diffuses less than if it were 
not at once sucked into the branchial sac. In order to avoid 
still more the effects of the dilution, the tip of the pipet was kept 
within a centimeter of the opening of the oral siphon. 
As a quantitative measure of the stimulating strength of the 
chemicals tested, I determined the lowest concentrations which 
would produce a reaction. The reaction time—that is, the 
time elapsing from the moment of application of the stimulus to 
the beginning of the reaction—cannot be used with advantage 
in this type of experiment. It is complicated by so many fac- 
tors, such as diffusion rate and speed of penetration, that it gives 
little direct information. To measure the amplitude of the 
effects of equal concentrations of different substances requires a 
