PHYSIOLOGY OF ASCIDIA ATRA LESUEUR ZG. 
the same quality and intensity of stimulus is applied at regular 
intervals to a given region, Ascidia ceases to react, most prob- 
ably because the irritated sense organs have become so changed ° 
that they are incapable of receiving the stimulus. 
iT LIGH® 
The dark-blue pigment in the test of Ascidia atra is undoubt- 
edly an efficient light screen. Not only is the test well supplied 
with this pigment, but the inner surfaces of the siphons are pig- 
mented well into the branchial sac and atrial cavity. The out- 
side faces of the oral tentacles are also pigmented. Individuals 
found in exposed situations possessed a tough test, which was so 
colored as to justify unreservedly the name of the species. Speci- 
mens collected from under stones, and hence screened from the 
glare of the sun and the action of the waves, had a partially 
translucent, softer test not heavily supplied with pigment. These 
facts would seem to indicate a sensitivity of this species to light. 
Many ascidians are described as possessing red or orange pig- 
ment spots near the rims of the siphons (Seeliger, ’93—'11, p. 319). 
A. atra has eight of these ‘ocelli’ on the oral siphon and six on the 
atrial siphon; they are situated in the regions of the less pigmented 
folds of the test between the lobes of the siphon rims. In this 
species their color is due to an aggregation of pigmented cells 
which resemble, if indeed they are not identical with, the orange 
blood corpuscles found in the branchial sac. Although no 
evidence has ever been adduced for it, the idea has been prevalent 
that these ocelli are photic sense organs. 
A sensitivity of the body surface to photic stimulation has been 
ascribed to Ciona by Kinoshita (’10) on the basis of its reactions 
when carried from a dark room into diffuse light. It is difficult, 
however, to accept this interpretation with much confidence, 
because the author records no precautions to avoid the jarring 
of this highly sensitive animal (Frohlich, ’03) during the trans- 
fer. However, Nagel (96) has also stated that ‘“Ciona reacts 
powerfully to sudden illumination.” 
Ascidia is insensitive to light of ordinary intensities. Animals 
kept in the dark and suddenly illuminated by an electric light, 
