AuausT 23, 1918] 
acts vigorously when exposed to light. The 
pigmented ocelli on the oral and atrial siphons 
are not the sense organs concerned. Stimula- 
tion of the ocelli does not result in a reaction, 
and their removal in no way interferes with 
the sensitivity of Ciona. The receptors respon- 
sible for the light sensitivity are localized in 
the inter-siphonal region in an area corre- 
sponding to the neural complex of ascidians. 
When this spot alone is exposed to light, the 
resulting effect is identical with the one fol- 
lowing total body exposure. 
0 
The reaction time of Ciona to light is com- 
posed of two portions. The first is a sensitiza- 
tion period, during which Ciona must remain 
exposed to the light. The second is a latent 
period during which Ciona need not be, exposed 
to the light. At the end of this period it gives 
its characteristic reaction, though at the mo- 
ment it is no longer subjected to the source of 
stimulation. This latent period as found by 
averaging many determinations on a number 
of animals at different intensities, is 1.76 sec- 
onds. 
I 
The sensitization period (or roughly speak- 
ing, the reaction time) varies inversely as the 
intensity of the stimulating light. Moreover, 
the duration of the sensitization period multi- 
plied by the intensity of the light is constant. 
This means that the amount of energy (time 
intensity) required by Ciona for a reaction 
to light is the same for all intensities. This 
is a familiar phenomenon in the chemical 
effect of light (Roscoe-Bunsen rule) and sig- 
nifies that the light decomposes a constant 
quantity of photosensitive substance before 
Ciona reacts to light. 
IV 
When kept in diffuse daylight, this species 
does not respond to a lower intensity of light. 
It does react to sunlight. However, if Ciona 
is placed in the dark, it will become “ dark 
adapted” after a time and will respond to an 
artificial light of as low as 500 candle meters 
SCIENCE 
199 
intensity. The investigation of the rate at 
which it becomes “dark adapted” is of con- 
siderable interest. This is found by determin- 
ing the reaction time of an animal to a light 
of constant intensity at 15-minute intervals in 
the dark-room. The following is found to be 
true. At first the reaction time is long, then it 
shortens rapidly, then slowly, and finally it be- 
comes constant. 
The duration of the exposure time multiplied 
by the intensity of the light gives the amount 
of energy received. The amount of energy de- 
termines the quantity of photosensitive sub- 
stance decomposed. Therefore, the extent to 
which the photosensitive material requires to 
be is changed in order to produce the same re- 
action during “dark adaptation,” is at first 
large, then it decreases rapidly, then slowly, 
and finally it becomes constant. 
The significance of this rate of change will 
become apparent when we shall have considered 
the nature of the photosensitive substance and 
its mode of formation. 
vi 
Decomposition of the photosensitive mate- 
rial by light, presupposes the formation of this 
substance within the sense organ. It will 
simplify matters to assume that the action of 
the light results in the conversion of the photo- 
sensitive material into its precursor. Thus 
normally, and of course in the dark, the pre- 
eursor (P) forms the substance (S) sensitive 
to light. In the light, however, S is converted 
back into P. 
The rate of formation of the precursor from 
the photosensitive substance in the presence 
of light, has been shown to be a direct 
function of the amount of energy supplied by 
the stimulating light. The occurrence of the 
reaction in the opposite direction, however, 
must be considered in terms of the velocities of 
ordinary chemical reactions. The formation 
of the photosensitive material from its precur- 
sor is probably a reaction of the first order. 
For our purposes, however, it may be a reac- 
tion of even a higher order. Practically all 
chemical reactions have this in common: the 
velocity of the reaction is at first rapid, then 
