DAILY MIGRATIONS OF COPEPODS. 
113 
end of the experiment than at the beginning. The exposure of a similar tube to an 
actual shower of rain on a very dull day was followed by no observable change in the 
distribution of the animals. Rain, therefore, with the slight accompanying change 
in the density of the surface waters, plays no part worthy of consideration in the 
distribution of Labidocerse. 
Temperature changes were next tried. In testing the effects of these changes, 
animals were used that had been kept several days in a large glass aquarium, whose 
temperature was uniformly 23° C. Five male Labidocerse were transferred from 
the general aquarium to the large glass tube described on page 108, now filled with 
sea water at 23° C. to the 50-centimeter mark. The animals were soon scattered 
through the water, as characteristic of males. They were now carefully taken out, 
and the water changed for some at 15° C. On reintroducing the males, they again 
gradually scattered through the water. The same was true for watei at 10° C., as 
well as for that at 30° and 35°. These temperature changes, therefore, did not 
alter the distribution of male Labidocerse. 
Five females were then subjected to a series of corresponding experiments. At 
23° C. the females remained near the top, and the same was true at 15° and 10° C. 
At 30° C. and at 35° G. all the animals swam rapidly downward to the bottom. To 
ascertain whether this reaction was in any way connected with a possible change of 
the animals in reference to light, the experiments were repeated in the dark. Five 
females were introduced into the tube containing water at 30° C. and the tube 
placed in a light-proof chamber. After 10 minutes the tube was inspected, and 
all five animals were found at its bottom. There thus seems to be no question but 
that an increase of temperature over the normal converts the geotropism of female 
Labidocerse from negative to positive. 
Something of the nature of this change may be inferred from the following 
experiments. Five negatively geotropic females were introduced into a large vessel 
filled with sea water at 30° C. They immediately became positively geotropic and 
swam to the bottom. After having remained there some minutes they were trans- 
ferred by a pipette to the bottom of a second vessel filled with sea water at 21° C. 
They immediately became negatively geotropic and swam to the top. The change 
from one condition to the other, as these experiments showed, takes place almost 
instantly, and the condition lasts only as long as the appropriate stimulus is present. 
This may be illustrated by another experiment. The lower half of a large glass tube 
was filled with sea water at 21° C., the upper half with sea water at 30 C. The warmer 
water was poured into the tube so as to mingle as little as possible with the cooler 
water. A female Labidocera was now introduced at the top to ascertain whether she 
woidd swim to a point deeper than the separation between the waters of different 
temperatures. She swam rapidly downward, but stopped almost exactly at the plane 
of separation for the two temperatures. A second tube was prepared containing 
sea water at 21° C. in the lowest third, at 23° C. in the middle third, and at 35° C. in 
the uppermost third. In this tube a thermometer was hung by a thread and a female 
Labidocera introduced at the top. She swam down almost at once to a point near the 
middle, which, the thermometer having been adjusted, was found to have a temper- 
ature of 23° C. ; she then rose somewhat, and live minutes after introduction was in 
water of 26° C. Ten minutes later she had risen somewhat farther, but was still 
in water registering 26° C. After half an hour, when the water in the top of the 
F. C. B. 1901—8 
