PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 269 
The reaction to mechanical stimulation of the test is not due 
to an irritation of the underlying mantle tissue. Individuals 
whose tests have been removed from a portion of the body show 
that the mantle is insensitive to touch. It is interesting to ex- 
plore the sensitivity of such an animal. Even vigorous poking 
of the mantle (the animal must be rigidly clamped, of course) 
is followed by no effect. One may approach to within one 
millimeter of the cut test and produce no stimulation. But 
once the test is touched, the animal immediately gives its char- 
acteristic response. An animal wholly denuded of its test is 
insensitive to touch on the outside except near the rims of the 
siphons. 
In the normal animal as one approaches the. region of the 
siphons the sensitivity to mechanical stimulation rises rapidly, 
and at the rim of the siphons the irritability is very great. The 
rim of the oral siphon is usually divided into eight lobes, and the 
atrial into six lobes. These thin lobes are the most sensitive 
portions of the outside of the body. By using large animals 
that have been a few days in the laboratory, and stimulating 
the individual lobes with a fine glass rod, I have secured local 
contractions of the portion of the rim contiguous to the stimu- 
lated lobe. The folds between the lobes are only slightly less 
sensitive than the lobes themselves. 
On theinside of the siphons below the lobes, a similar degree of 
sensitivity exists. Inside the atrial siphon the irritability is 
greatest near the rim, but the entire atrial cavity is also sensitive 
to touch. Within the oral siphon the surface is extremely sen- 
sitive, and remains so as far down as the ring of oral tentacles. 
Beyond this the sensitivity falls off rapidly. 
Of the surfaces which produce the group of crossed reactions, 
the tentacles are probably the most sensitive. The prettiest 
automatic response of Ascidia results from their stimulation. 
By illuminating the inside of the oral siphon it is possible to touch 
a single tentacle with a fine glass rod. If a delicate stimulus be 
applied carefully, it is most interesting to see the atrial rim close 
quietly while the oral siphon remains undisturbed. If the stim- 
ulus is more intense the ‘Ejektionsreflex’ is produced. When a 
