DAILY MIGRATIONS OF COPEPODS. 
121 
experiments. A piece of glass tube about 2 centimeters long was filled with sea 
water, its ends were plugged with wet cotton, and it was wrapped in a little more 
than one fold of white filter paper. A thread was tied around the tube, thus holding 
the filter paper in place, and the apparatus prepared in this way was lowered into the 
surface water of a small aquarium containing five male Labidocerse. These swam 
freely to and fro, and now and then collided with the tube. They were watched 
accurately for thiily minutes, and the number and character of the collisions noted. 
In all, ten collisions occurred, after each of which the animal usually deserted the tube 
at once. Those animals that just missed the tube in passing went on by it without 
special reaction. The tube was now withdrawn from the aquarium and five females 
were introduced into it. Because of the filter paper and the plugs of cotton these 
were not visible from the outside. The tube was again placed in the aquarium, and 
after five minutes the actions of the males were again observed for thirty minutes. 
Under these conditions the males collided sixteen times with the tube instead of ten. 
Moreover they seldom passed near the tube without some characteristic reaction. 
Usually they made one or two quick circles as they swam by, or even a somersault- 
like motion; these were observed fifteen times when the females were in the tube, 
never when they were not. 
The experiment was now varied b} r dropping the small piece of tube to the 
bottom of the aquarium and recording the reactions of the males as before. During 
the half hour in which no females were in the tube, males collided with it thirteen 
times, but showed no inclination to play about it. After the introduction of the 
females eighteen collisions were observed and eight playing movements as males 
swam by. I am, therefore, thoroughly convinced that males respond to females even 
when they can not see them or come in contact with them. As the females are not 
known to have any power of producing sound, they probably give rise to some sub- 
stance that serves as a scent for the males; in other words, the males are probably 
positively chemotropic toward the females. 
Admitting this last conclusion, we have now all the facts needed for a satisfactory 
explanation of the daily migrations of Labidocene. The females come to the surface 
as daylight vanishes because they are positively phototropic to weak light and because 
they are negatively geotropic. They descend from the surface into deeper water at 
the appearance of daylight because they are so strongly negatively phototropic to 
bright light that they overcome their negative geotropism. The males follow the 
females because they are probably positively chemotropic toward them. 
THEORIES OF DAILY MIGRATIONS OF PELAGIC ANIMALS. 
It is now comparatively simple to discuss the more important theories advanced 
to explain the daily migrations of pelagic animals. Chun’s (1887) hypothesis that the 
migration depended primarily on temperature changes is absolutely without support 
from what has been learned about the daily movements of labidocera cestiva. Weis- 
mann’s (1877) opinion that light governs the migration is very close to the truth, 
though it requires modification in the direction pointed out by Loeb (1891, p. 67), 
who, after having shown the general importance of light in the daily migrations of 
pelagic animals, expresses his belief “that light is not the only physical influence 
