120 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Although this explanation makes clear the movements of the females, it in 
no wise applies to the males, for, as we have seen, they are almost indifferent to the 
effects of light and gravity. The occasion of their migrations must be sought for in 
other factors. 
REACTIONS TO SUBSTANCES IN THE WATER (CHEMOTROPISM). 
The Labidocerge that collect at the surface of the water on the light side of an 
aquarium have already been shown to be some males and some females. A hundred 
or more of these were placed in an aquarium tilled with sea water and in a short 
time the majority of the newcomers were found at the top of the water near the 
light; others, however, were scattered throughout the whole body of water. These 
scattered individuals were then picked out, and, as might have been expected, were 
found to be almost exclusively males. An hour or so after this the aquarium was 
again examined and was apparently as full of scattered males as before. These, like 
their predecessors, were removed. After a time, however, they seemed to reappear, 
although in diminishing numbers. The process of picking out was continued until 
few or no scattered individuals could be found. An examination of the assemblage 
at the top now showed it to be made up almost exclusively of females, the males 
having escaped for the most part to the lower water, where they were picked out. 
Under ordinary conditions, then, males are continually escaping from the assemblage 
of males and females at the surface and swimming oil' to become scattered individuals 
in deeper water. 
The males not only are continually leaving such an assemblage; they are also 
continually entering it. At the bottom of a glass aquarium containing only female 
Labidocerse ten males were liberated. They swam about as scattered individuals for 
some time, but after half an hour only six could be seen; four, as a close inspection 
showed, had joined the assemblage of females at the top. The females must, there- 
fore, be admitted to have some power of attracting the males. 
It occurred to me that the possible phosphorescence of the female might, at least 
at night, be a means of attracting the males. Among the abundant phosphorescent 
organisms in the summer waters at Woods Hole small luminous points had often 
been shown me as phosphorescent copepods. By taking at random test tubefuls of sea 
water at night a single such phosphorescent point could be isolated. Such samples 
of sea water contained sometimes one, sometimes more copepods, but they were so 
frequently without copepods at all that I was forced to decide that the luminous 
points were something other than these animals. Moreover, when a single male or 
female Labidocera was put in a test tube containing sea water which on previous 
agitation showed no phosphorescence, and the whole vigorously shaken, not the least 
phosphorescence was ever observed. Labidocera eestiva is, in my opinion, not phos- 
phorescent, and therefore this factor can have no part in causing the males to be 
attracted hy the females. 
Since phosphorescence seems to play no part, and since the higher sense organs 
of the Labidocerge, the eyes, etc., are so very rudimentary, some form of scent would 
seem the most natural way by which the males might be led to the females. This 
would probably depend upon some substance given out by the females to which the 
males would react. That there probably is such a substance is seen from the following 
