PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 273 
however, affecting the sensitivity in the slightest. The equilib- 
rium idea was therefore incorrect. 
The ring of very sensitive oral tentacles is in direct communica- 
tion with the surrounding water. The tentacles are thin-walled 
protrusions of the branchial sac, and remain in a more or less 
rigid extension due to the pressure of the blood which circulates 
freely within them. It might be imagined that there existed 
an arrangement similar to the hair-crowns of mammals, which 
would make them. receptors for the vibrations in the water. 
. Cutting away all the tentacles was a simple matter, except that 
it involved the use of an anesthetic (chloral hydrated). Recovery, 
however, was very rapid and in an hour the animals were normal 
again. After this procedure, they were as sensitive to vibrat‘on 
as before the operation. Moreover, as will berecalled, touching 
of the individual tentacles produces a crossed reaction; this is 
not the kind of response which the animal gives to vibration. 
Therefore the tentacles are not concerned with the reception of 
vibratory stimulation. 
The only other regions acutely sensitive to touch are the lobes 
of the siphon rims. In the expanded animal these thin flaps 
protrude prominently into the surrounding water. If a speci- 
men is suspended with the siphons hanging down, the water in 
the aquarium may be withdrawn so as to leave only the rim of the 
oral siphon immersed in the water. When this is done carefully 
the animal does not close its siphons. Under these conditions 
the oral siphon will react when the aquarium is tapped. Removal 
of the lobes from the rims of both siphons does away completely 
with the sensitivity of the animal to vibrational stimuli. The 
conclusion, therefore, seems inevitable that the thin, projecting 
lobes of the siphon rims contain the receptors for this kind of 
environmental disturbance. 
3. Repeated stimulation 
The continued application of vibratory stimuli at regular 
intervals results in a cessation of response on the part of the 
animal. Whether this failure to react is the result of the fatigue 
of the musculature, or of the sense organs, or whether it is an 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 25, No. 1 
