208 
PACIFIC SCIENCE, Vol. XV, April 1961 
FIG. 2. The activity index of a population of P. 
carmelensis plotted against light intensity. Curve fitted 
by eye. 
ground. Twenty animals were kept under con- 
stant light in a black plastic bowl for 96 hr. The 
animals were tested at 24, 72, and 96 hr. At 24 
hr., 12 animals oriented to white and 8 to black, 
showing no significant difference (chi square 
= .8, p < 0.5), 72 hours, 8 oriented to white 
and 12 to black, again showing no significant 
difference. At 96 hr., 5 oriented to white and 1 5 
to black, now showing a significant difference 
(chi square = 5.0, p < 0.05). The animals 
were tested again 2 days later. During the two 
days, they were kept on the black background 
but received light only from 0800 to 1800 hr. 
Several tests were run, with the animals being 
kept in darkness between tests. In the first test, 
5 animals stopped on the white background and 
15 on the black background. The marked orien- 
tation to the black background was significant 
(chi square = 5.0, p < 0.05). However, when 
the test was repeated using the same animals, 
two of the tests gave essentially the same re- 
sults, but the third showed no difference in 
background selection. 
A second series of tests for background choice 
was made with newly collected worms from 
Carmel Point. Forty worms were placed in black 
dishes and 40 were placed in white dishes. The 
worms were kept under constant light and tested 
after 96 hr. The testing extended over 2 days 
and the animals were kept in darkness during 
the period of testing. The animals were tested 
under 35 ft. c. of light. The animals kept on a 
white background went to the white side of the 
dish 27 times and to the black side 1 1 times. The 
selection of the white background is highly sig- 
nificant (chi square r=. 6.7, p = c. 0.01). The 
animals kept on the black background went to 
the white side of the dish 9 times and to the 
black side 29 times. The selection of the black 
background is highly significant (chi square = 
10.5, p < 0.01). Each of these tests was re- 
peated once using the same animals. Results 
were essentially identical. 
The animals were kept in their respective 
dishes and given 48 hr. of constant light. They 
were then placed in the dark and subsequently 
tested under 70 ft. c. of light. 
The animals kept on a white background 
went to the white side of the dish 2 1 times and 
to the black side 16 times. There is no indica- 
tion that background selection occurred (chi 
square = 0.66, p < 0.5 ) . However, the animals 
kept on a black background went to the black 
side of the dish 33 times and to the white side 7 
times. Selection of the black background was 
significant (chi square r= 16.0, p < 0.001). 
Each of these tests was repeated twice using the 
same animals. The worms acclimated to the 
black background showed about the same pat- 
tern, but with lowered chi-square values. The 
animals acclimated to the white background 
showed almost complete randomness in selection 
of background (chi square = 0.01). 
These is no ready explanation for the shift 
in background selection by the worms acclimated 
to the white background. The experiments con- 
cerning photokinesis demonstrated that the 
worms had a differential sensitivity to light, 
some reacting to a weak stimulus, others to a 
strong stimulus. Because of the variation in sen- 
sitivity to light, it seems reasonable to postulate 
that under the increased light intensity, photo- 
kinesis was stimulated more in the light-sensi- 
tive animals. These sensitive animals then ori- 
ented to the black background to reduce the 
amount of stimulation. The less sensitive ani- 
mals continued to orient to the white back- 
ground to which they were acclimated. That 
there is a threshold of sensitivity to light 
whereby the reaction to light stimulation is 
reversed is indirectly indicated. Costello and 
Costello (1938 b) reported that P. carmelensis 
may ”... be positively phototropic to moderate 
light intensities”; the positive phototropism was 
