278 SELIG HECHT 
or by diffuse light, or even by direct sunlight, remained undis- 
turbed. Rays from an electric bulb, concentrated to a point and 
focussed on the individual ocelli failed to call forth any response 
from the animal. By means of a large lens whose focal length 
was 30 cm., direct sunlight was focussed upon each ocellus in 
turn by way of the outside and the inside of the siphon. In no 
case did this result in a reaction. I must, therefore, conclude 
that the so-called ocelli of Ascidia atra do not contain receptors 
for photic stimuli. 
Nevertheless, this species is sensitive to light of great intensity. 
If direct sunlight is allowed to pass through 36 cm. of seawater 
and is then concentrated by means of a lens, there is no evidence 
of the heat that is associated with it. Such a beam of light pro- 
duced no sensation of heat when focussed on a finger under water, 
nor a rise in temperature when directed upon the bulb of a ther- 
mometer. As was to be expected, when .this light was con- 
centrated on the ocelli, the animal again failed to respond. It 
also called forth no response when focussed on any portion of the 
test. If the beam was, however, directed into the oral siphon in 
the general region of the tentacles, about one or one and a half 
centimeters below the ocelli, the animal responded at once. The 
reaction was of the crossed kind in which the oral siphon closed 
only partially, the atrial completely, and the body contracted 
vigorously, forcing the water from the branchial sac out through 
the oral siphon. 
When Ascidia was kept in broad sunlight, even if the rays 
entered the oral siphon, the animal remained undisturbed. If 
the sunlight was suddenly intercepted by means of an opaque 
sereen, no reaction was observed. The same results were ob- 
tained when the pigment spots of the siphons were illuminated 
for a time and then suddenly darkened. These facts indicate 
that Ascidia is insensitive to a sudden diminution of the intensity 
of the light to which it is exposed under the normal conditions 
of its existence. 
uring the course of the summer, I repeated all of these experi- 
méhts with light on many occasions. The results as I have de- 
scribed them were identical every time. Therefore, although 
