114 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
tube was at 26.5° C., she was still some inches from that level and in water at 26° C. 
It would therefore seem that 26° C. is the temperature limit above which this femaie 
Labidocera became positively geotropic. 
That there should be no question about the difference between the males and the 
females in their capacity to be transformed geotropically, the following experiment 
was tried. Ten animals taken at random from the top of the general aquarium were 
suddenly introduced with as little water as possible into a tube tilled with sea water 
at 30° C. They quickly separated into two sets, one going to the bottom, the other 
remaining near the top. The six that remained near the top were picked out and 
all proved males; the four that swam to the bottom were then recovered and found 
to be females. These observations show that the temperature of the water has no 
obvious effect on the slight geotropism of the males. The geotropism of the female 
is negative in cold water and positive in warm, and for normal animals the critical 
temperature seems to be about 26° C. 
It thus appears that of the three means Suggested whereby the sense of the 
geotropism of Labidocera? might be changed, only one, namely, temperature, has 
proved really effective. The critical point in the temperature change is, however, 
so high, compared with any temperature range the animal is likely to meet with 
naturally, that the assumption that the daily migrations of the females depend on 
geotropism changed from negative to positive and back again by temperature dif- 
ferences is wholly unwarranted. So far as one can see, the only part played by 
geotropism in the daily migrations affects the females; they rise to the surface and 
stay there because of their strong negative geotropism. 
REACTIONS TO LIGHT (PHOTOTROPISM). 
Light, like gravity, influences bodies of water, as a rule, from one side only, and, 
since it increases and decreases during the day, it has naturally been regarded as a potent 
factor in determining the daily migrations of pelagic animals. That it plays a part 
in the distribution ot copepods can be seen from the fact that in an aquarium standing 
near a window many Labidocera? congregate near the surface of the water on the 
side next the window; i. e., they are positively phototropic. 
If a considerable number of male Labidocera? are placed in a large glass jar filled 
with sea water and illuminated from one side, they will soon be found scattered through 
the water. A close inspection, however, will show that rather more are in the half 
of the jar away from the light than in the half toward it. To determine with greater 
accuracy what this difference amounted to, a nearly cubical glass jar tilled with sea 
water was so arranged that a temporary glass partition could be slipped into it 
vertically, thus dividing its contents into a front and a back half. One meter in front 
of the jar was placed an incandescent electric lamp of about 14-candle power. The 
light from this fell perpendicularly on the front face of the jar and the partition was 
so placed that of the two chambers formed by it one was toward the lamp, the other 
away from the lamp. The partition having been withdrawn, ten male Labidocera? 
were liberated in the. middle of the jar and after ten minutes exposure to the light the 
partition was inserted and the numbers in the chamber toward the light and in that 
away from it were counted. The experiment was then repeated, the Labidocera? being 
each time liberated at the center of the jar and allowed ten minutes in which to 
become distributed. Table 7 gives the result of ten such trials. 
