268 SELIG HECHT 
II. MECHANICAL STIMULATION 
1. Touch 
Ascidia atra is an animal that under normal conditions is 
stimulated preéminently by mechanical means. This is the only 
variety of stimulus which is capable of calling forth all the possible 
responses of the species. The selection of its food—if mere ex- 
clusion may be called selection—is made on the basis of size, and 
rejection depends on the mechanical stimulation by the larger 
particles. ‘The remarkable sensitivity to touch was known to 
even the oldest zodlogists who concerned themselves with the 
study of the large monascidians. Its very delicacy in Ascidia 
atra was a stumbling block to locating precisely the sensitive 
regions. 
The presence of a heavy cellulose test would suggest an in- 
sensitivity of the exterior to any stimulation. Yet, even a gentle 
touch on the surface of the body results in a reaction of the direct 
type. Careful experimentation has convinced me that this is 
not due mainly to an irritability of the test to mechanical stimula- 
tion. An individual normally attached to a rock, and removed 
to the laboratory with its attachment intact, serves best for this 
type of experimentation. Moreover, if the substrate be securely 
clamped in the aquarium, the accidental jarring of the animal 
may be almost completely eliminated. Under these conditions 
a gentle touch with a glass rod on the test surface leaves the 
animal undisturbed. A coarser application at once stimulates. 
I am not prepared to deny the presence of touch receptors on 
the surface of the test. But I am convinced that most of the 
results of mechanical stimulation of the test are not due to 
sense organs within it, but to the passage of the stimulus through 
the elastic material to the more sensitive region of the siphon rim. 
In favor of this view is the lack of any demonstrable nerve con- 
nection between the test and the tissue underneath it. More- 
over, sources of stimulation, such as light, heat, and chemicals, 
which cannot easily be transmitted along the test substance, fail 
to be effective when applied to the outside of the test; whereas 
in all other regions, they are just as effective as touch. 
