De SELIG HECHT 
Under these arrangements, a tap on the glass aquarium still 
resulted in a reaction. Therefore, there seems to be a reception 
of the stimulus by way of a solid medium too. 
It was possible to determine the relative effectiveness of stimuli 
transmitted by the two pathways, by measuring the quantity 
of energy which must be expended to produce the same reaction 
in the two cases. A pendulum bob weighing nearly 75 grams was 
suspended from a& string about 150 centimeters long, in such a 
position that when at rest it hung 5 millimeters from the wall of 
the aquarium. The string was made long so that the velocity 
of the pendulum bob would be practically the same when it 
swung from any distance within about 35 centimeters of the posi- 
tion of rest. In this way the amount of energy which the impact 
of the bob delivered against the side of the jar varied with the 
square of the distance from which it was released. In one 
experiment, to quote a typical example, a swing of 10 centimeters 
was enough to produce a complete siphon closure when the 
vibrations were transmitted through the water only; whereas 
it required a swing of 30 centimeters to produce the same reac- 
tion when only the solid transmission occurred. The vibration 
through the water was therefore nine times as effective as the 
vibration through the glass. 
If, now, we keep in mind that Ascidia is very sensitive to even 
slight vibrations, it seems clear that it is their occurrence in the 
water which furnishes the stimulus under normal conditions. 
Stimulation through the solid base of attachment is most likely 
a wholly secondary phenomenon. Hence, in looking for the 
structure where the reception of the vibration occurred, I had 
to consider only those parts fully exposed to the water. 
No statocysts have been described in adult ascidians; such 
sense organs occur in the larva, but disappear during the meta- 
morphosis (Herdman, ’04). There is a space between the test 
and the left side of the body, which usually contains some fluid. 
I, therefore, tested the possibility that there might be some 
equilibrium relation between the test and the body which would 
be influenced by the vibration. The animals were suspended in 
all sorts of positions calculated to disturb this relation, without, 
