PHYSIOLOGY OF ASCIDIA ATRA LESUEUR pyipsl 
stimulus. In the laboratory an animal in a battery jar on a 
table will react when a person walks vigorously across the floor. 
A sharp tap on the glass walls of the aquarium results in an 
extreme reaction of the animal as a whole. 
It is hardly possible to exaggerate the sensitivity of Ascidia 
to this kind of stimulus. Its very delicacy was at first a source 
of interference in the study of the sensory physiology of the 
species. The plan was, therefore, adopted of working in an 
isolated house the concrete floor of which rested on the bed rock 
of the laboratory island.2 Only in this way was it possible to 
avoid the constant unintentional stimulation of the animal 
under investigation. 
To determine the special region concerned with the reception 
of the vibrational stimuli, I had first to discover along which 
medium the stimulus was carried to the animal. There could be 
no doubt at all about the existence of a water transmission. A 
drop of water allowed to fall on the surface of the water in the 
aquarium sets up a series of waves which are very effective in 
stimulating the animal. Moreover, it was easy to cause a re- 
action by dropping a solid into the water near an individual 
in its natural surroundings. When an animal, submerged in 
the water of a large battery jar, is suspended from the ceiling in 
such a way as to prevent its touching any portion of the jar, the 
stimulus from a tap on the side of the jar will be transmitted 
to the animal by way of the water only. Under such conditions, 
a sharp tap called forth an immediate reaction. Therefore the 
vibration in the water serves as a stimulus. 
The question whether Ascidia is stimulated by vibrations 
transmitted through its solid base of attachment was answered 
in the affirmative by an experiment like the following. An 
animal was placed: upright on the bottom of an aquarium. Over 
it was then inverted a battery jar which was a centimeter or so 
greater in diameter than the width of the animal. The jar 
rested on the glass bottom of the aquarium, and sufficient cotton 
was put under its rim to remove any direct transmission of vibra- 
tions to the animal by means of the water in the aquarium. 
3 To those familiar with Agar’s Island:—the cook-house. 
