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PACIFIC SCIENCE, Vol. XV, April 1961 
74, 100, 125 per cent s.w. All animals in these 
concentrations showed normal activity for 2 
weeks. After 16 days, 3 animals in 125 per cent 
s.w. died. 
150 per cent s.w. Most animals curled up 
shortly after being placed in the dish. One hr. 
later, 3 responded to the light stimulus and 7 
responded upon being touched with a glass rod. 
The low level of activity was maintained with- 
out detectable change for the next 14 hr. By 
the 15 th through the 24th hr. movement was 
limited to 4 worms who could expand and con- 
tract their anterior ends, but could not move 
their posterior ends. Most animals were in the 
U-shaped position, but some curled into an oval 
position. At 42 hr. a few showed feeble move- 
ments, 4 were disintegrating, and the remainder 
were in the U-shaped position. Twenty-four hr. 
later all of the animals except 3 were disinte- 
grating and only 1 was capable of any move- 
ment. 
The second experiment was set up to deter- 
mine if the worms are capable of acclimating 
to levels of salinity. The experiment was not 
designed to determine the range of acclimation 
to salinity, but to demonstrate that acclimation 
was possible. Fifty animals were placed in 75 
per cent s.w. 96 hr. prior to testing. Survival 
times in 30, 40, 50, 60, and 70 per cent s.w. were 
determined for worms acclimated to 7 5 per cent 
s.w. and 100 per cent s.w. Ten animals were 
placed in each solution. 
30 per cent s.w. All worms were immobile. 
In 2 hr., 7 of the group acclimated to 100 per 
cent s.w. and 3 acclimated to 75 per cent s.w. 
were sloughing epidermal cells. At 8 hr., all of 
both groups were disintegrating. 
40 per cent s.w. All animals of both groups 
were immobile and failed to respond to normal 
stimuli. However, at 8 hr. 7 of the group ac- 
climated to 75 per cent s.w. responded to a light 
stimulus from a no. 2 photoflood. Six of these 
were still active at 12 hr. and 5 showed feeble 
movements at 24 hr. None of the group accli- 
mated to 100 per cent s.w. showed any response 
at any time. 
50, 60, 70 per cent s.w. The number of an- 
imals surviving in each of these three concen- 
trations is plotted against time in Figure 1. The 
curves for the animals tested in 50 per cent s.w. 
FIG. 1. Number of active animals plotted against 
time for P. carmelensis acclimated to 75 per cent s.w. 
and 100 per cent s.w. and tested in 50, 60, and 70 
per cent s.w. 
indicate that acclimation has taken place and 
has acted to increase survival time in the accli- 
mated animals. The curve also indicates that 
the difference in survival time between the two 
acclimated groups increases with increasing time 
in the test environment. The curves also illus- 
trate the variability in tolerance to salinity 
among individuals. This is particularly evident 
in the curves for the animals tested in 60 per 
cent s.w. and in 70 per cent s.w. After the least 
tolerant individuals of the worms acclimated to 
100 per cent s.w. died, all the groups reached 
an equilibrium with essentially all the remain- 
ing animals surviving. 
DIURNAL RHYTHM 
The only known case of a persistent rhythm 
in the phylum Platyhelminthes has been reported 
for the acoel Convoluta roscoffensis (Harker, 
1958). The worm comes to the surface of the 
sand at low tide and disappears into the sand 
when the tide returns. The rhythmic behavior 
is maintained in the laboratory for about 1 week 
in vessels of still water and is independent of 
day or night. P . carmelensis is found on the up- 
per surfaces of algae or gravel during low tide 
and disappears into the gravel as the tide re- 
turns. The similarity between the two species 
makes desirable a study to determine if a diurnal 
or tidal rhythm occurs in P. carmelensis. 
About 150 worms were collected near Pt. 
Pinos and brought into the laboratory. Ten an- 
imals along with a few pieces of rock were 
