PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 287 
whether the outside margins of the siphons are sensitive. Solu-, 
tions applied to the outside rim of the oral siphon are drawn inside 
and the resulting reaction may be due to the stimulation of the 
latter region. Even a minute crystal of oxalic acid applied to the 
outside rim caused a local mechanical disturbance. Solutions 
applied to the atrial rim must be much stronger to produce an 
effect than those applied to the oral rim. This may mean that 
it is not the outside rim which is stimulated, but that to affect 
the interior of the atrial siphon, a more concentrated solution 
is required in order to diffuse in against the outgoing current; 
or it may signify that the outside oral rim is more sensitive than 
the atrial. On the basis of my experiments it is not possible to 
decide between these alternatives. 
The region of maximum chemical sensitivity is the inside 
of the oral siphon. A crystal of oxalic acid held on the end of 
a fine glass rod inside the siphon, at once produced a reaction, 
although, due to the incoming current, none of the dissolved acid 
touched anything but the interior of the siphon. There may be 
chemical receptors on the tentacles, but they are not of primary 
importance. Individuals with the tentacles removed were not 
qualitatively different from intact animals. 
In these experiments, the values recorded are all for the sensitiv- 
ity of the oral siphon. More accurate results could be obtained 
by using this siphon because the direction of the water current 
favored the passage of the stimulating solution. For the sake of 
comparison, however, the relative sensitivity of the two siphons 
to HCl was determined. One set of animals showed that the 
oral siphon responded vigorously to 0.002 N solution of HCl; it 
required, however, a 0.04 N solution of HCl to stimulate the 
atrial siphon. Hence the oral region is apparently twenty times 
as sensitive as the atrial. 
4. Osmotic pressure 
Ascidia did not respond when 1 cc. of seawater was discharged 
near its oral siphon. Seawater evaporated to twice its con- 
centration also failed to stimulate. Seawater evaporated to 
four times its concentration called forth a vigorous reaction. 
