206 
PACIFIC SCIENCE, Vol. XV, April 1961 
During low tides when bright sunlight was 
present, Polychoerus was found under rocks and 
gravel, indicating that the genus might be posi- 
tively geotaxic under bright light. 
Twenty-one animals were distributed in six 
25 X 75 mm. plastic vials so that there were 
at least 3 worms in each vial. The worms were 
allowed to come to rest after being added to the 
vials. The number of worms on the side of the 
vials was determined. Then the worms were 
stimulated for 3 min. with various intensities 
of light. All activity was recorded. The worms 
were kept in total darkness for 1 hr. between 
subsequent tests. The same worms were used 
throughout. The results are summarized in Table 
2. Low light intensities had essentially no effect 
on the animals. Higher light intensities resulted 
in an increase in over-all activity with all an- 
imals becoming active at the highest intensity 
of light. Animals never crawled upward. There 
was an increasing percentage of animals on the 
sides of the vials that crawled downward with 
the increase in light. This experiment indicates 
that P. carmelensis becomes more active with 
increasing light intensity, and where directed 
movement is possible the animal crawls away 
from the source of light. Thus the absence of 
animals on the upper surface of rock and gravel 
during low tide and bright sunlight can be at- 
tributed to the reaction to light demonstrated 
above. 
REACTIONS TO LIGHT 
In the previous section some of the reactions 
of P. carmelensis to light were described. That 
experiment was designed to explain part of the 
upward and downward movement of the worms 
in the intertidal substrate in the absence of a 
diurnal rhythm. The reactions to light led to 
some further exploration of the behavior of the 
worms in relation to light. 
The increase in the number of animals show- 
ing locomotion on the bottom of the vials at 
higher intensities of light (Table 2) indicated 
a photokinetic response. Photokinesis is usually 
defined as a change in the rate of undirected 
locomotion resulting from a change in the in- 
tensity of light. The photokinetic response in 
P. carmelensis was measured in two experiments. 
The first experiment was designed to meas- 
ure the rate of crawling under varied intensities 
of light. Two narrow strips of plastic were fas- 
tened 6 mm. apart in the bottom of a petri dish 
with a diameter of 14 cm. The plastic strips 
and the bottom of the dish were covered with 
black friction tape. A 5 cm. course was marked 
off between the plastic strips. The course was 
illuminated from one end. A worm was dropped 
into the dish at one end of the course with the 
light turned on, and the time that elapsed until 
the worm reached the end of the course was 
determined. The behavior of the worms was 
highly erratic. Some of them spent considerable 
time in turning the head from side to side, others 
ceased crawling before reaching the end of the 
course, and some crawled directly down the 
course. All of the worms were photonegative 
at the intensities used. Because of the variability 
in behavior, the experiment was discontinued 
after a small series of determinations were made. 
The rates of crawling for worms that crawled 
directly down the course illuminated by means 
of 5 ft. c. and 37 ft. c. were analysed by means 
of an analysis of variance and the between groups 
variance was statistically significant. The mean 
rate of travel was 0.86 mm/sec at 5 ft. c. and 
1.34 mm/sec at 37 ft. c. The rate of crawling 
increased about 55 per cent when the light in- 
tensity was increased about 640 per cent. Sim- 
ilar slight increases in the rate of crawling with 
large increases in the intensity of light were 
found for Dugesia gonocephala and Plagiosto- 
mum sp. (Carthy, 1958: 37). 
A second experiment attempted to measure 
photokinesis by determining the amount of ac- 
tivity initiated in a population of quiescent 
worms illuminated at various intensities of light. 
Five worms were placed in each of five petri 
dishes filled with sea water. After 1 hr., the 
worms were illuminated dorsally at various light 
intensities for 3 min. The time in seconds for 
a worm to respond was determined, as well as 
the nature of the response. Responses were of 
three types, ( 1 ) head raising; ( 2 ) body move- 
ment in which the animal might swing the an- 
terior end side to side several times or show 
other changes in body form, but remaining es- 
sentially in the same location in the dish; (3) 
locomotion, in which the animal actively crawled 
about the dish. An activity index was determined 
