286 SELIG HECHT 
nicety in the grading of a response of which Ascidia was not 
capable. The value of the liminal concentration, however, has 
the important advantage that it gives the quantities of material 
which are required to produce the same physiologic result. 
No other measure of stimulating strength possesses this property. 
The sensitivity of individuals varied according to their size: 
the larger animals were less sensitive than the smaller. This is 
to be expected from what has been shown for the relation between 
certain activities and body weight; for example, the intensity 
of the water current and the frequency of the heart beat. An 
instance of this variation is the following. In one experiment I 
used three animals whose weight averaged nearly 30 grams. 
The liminal concentration for KCl was 0.075 N. In a parallel 
test on animals whose weight averaged 65 grams, the liminal 
concentration of KCl was 0.20 N. 
Ascidia atra was not sufficiently plentiful for me to employ 
animals of one size throughout. An experiment, therefore, con- 
sisted of the testing of a group of substances on each of three 
- similar sized individuals. The results secured in this way were 
uniform, and were serviceable for the desired comparisons within 
the groups. 
The procedure was as follows. Each animal was placed, rest- 
ing on its left side, in a finger bow] containing 300 cc. of seawater. 
Before applying a stimulus, I always waited for the culmination 
of a rhythmic contraction. This simulates the reaction to an 
irritant in so many ways, that it was deemed necessary to include 
it as a source of error. After each of the three individuals had 
been tested with 1 ec. of a solution, they were removed into bowls 
of fresh seawater and allowed to remain undisturbed for at least 
five minutes. A complete experiment involved ten or a dozen 
reactions of the same animals, and usually lasted about two 
hours. 
3. Distribution of sensitivity 
The test of Ascidia is insensitive to chemical stimulation. If 
have discharged concentrated HCl on various portions of the 
test without securing any response whatsoever. It is problematic 
