PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 283 
which the sense organ was subjected les between the recorded 
temperature and the temperature of the water in the pipet, prob- 
ably nearer the former than the latter. 
The test and siphon rims have already been shown not to 
contain the receptors for temperature changes. Insensitivity 
to these stimuli extends for a few millimeters into the siphons. 
Below this, the interior of the oral siphon is sensitive to heat and 
‘cold.’ Although the oral tentacles are probably concerned they 
are not the only regions which receive the stimulation. When 
the tentacles were removed, reactions to hot and cold seawater 
were still obtained. Therefore the wall of the siphon also con- 
tains regions sensitive to temperature changes. 
V. CHEMICAL STIMULATION 
1. Presence of chemical sense 
The earliest statement of the chemical sensitivity of ascidians 
is to be found in Nagel’s (94a, p. 553) words that Ciona and 
“probably most sessile ascidians lack completely any chemical 
sense.’’ Nagel found that the siphons of Ciona were retracted 
not only on the application of acids, but of pure water as well 
(94b, p. 173). Since it was his idea that fresh water stimulated 
because of the mechanical disturbance set up by its discharge 
from a pipet, he explained the action of the acids on similar 
grounds. 
Nagel, however, was incorrect in the interpretation of his 
data. This is shown by an incidental experiment of Magnus 
(02, p. 485), in which the mechanical stimulation was avoided. 
He found that Ciona will respond to the presence of a crystal of 
sodium chloride held near the oral siphon. This sensitivity of 
Ciona to salt constitutes, as far as I am aware, the only known 
fact about the chemical sense of ascidians. 
It was an easy matter to determine whether Ascidia atra was 
responsive to substances in solution. When a crystal of aso uble 
substance was held near the oral siphon, the incoming current 
dissolved enough of the material to stimulate the an’‘mal. If the 
substance was a liquid or was non-crystalline, a wad of absorbent 
