DAILY MIGRATIONS OF COPEPODS. 
105 
one species may not in another. So far as the very few copepods that have been 
studied are concerned, the chief factor seems to be light, though reaction to gravity 
has not been shown to be without influence, and heat and density of the sea water 
may play subordinate parts. 
In attempting to ascertain the factors that determine the migration of copepods, 
1 have carried out a series of experiments on one of the commonest summer species 
from the region about Woods Hole, Mass., namely, Labidocera cestiva Wheeler (1900, 
p. 178, tig. 16). This is an inconspicuous copepod of usual proportions and about 2 
millimeters in length, i. e., a dozen of them placed end to end would measure very 
nearly an inch. The males differ from the females in three conspicuous features: 
First, the dorsal eye lenses in the male are larger than those in the female; secondly, 
the right anterior antenna of the male is partly modified to form a grasping organ, 
while the antenna? of the female are unmodified and symmetrical ; and, thirdly, the first 
abdominal segment in the male is simple, and not swollen and covered with hairs, as in 
the female. This species was found to predominate in the tow taken during the whole 
of August, 1901. and served in every way as satisfactory material for experimentation. 
My work was carried on in the laboratory of the United States Fish Commission 
at W oods Hole, and 1 take pleasure in expressing my indebtedness to Dr. H. M. Smith, 
director of the laboratory, as well as to the station corps, for many courtesies shown me. 
MOVEMENTS. 
At the outset I put several hundred Labidocera? in a large, cylindrical, glass aqua- 
rium nearly filled with sea water and placed on a table near a window. In a short 
time the Labidocera; had distributed themselves in a very characteristic way. One 
set formed a densely packed row at the top of the water on the side of the aquarium 
next the window; the other was formed of individuals rather uniformly scattered 
through the whole aquarium. The following enumerations will suffice to show the 
composition of these two sets: Of 25 individuals taken from the row next the light, 11 
were females and 11 males; of 25 caught swimming at large, 23 were males and 2 
were females. The almost complete absence of females from the latter set makes it 
obvious that in experimenting on this species the material should not be considered 
as homogeneous, but males and females should be dealt with separately. 
The movements of the Labidocera? are in part due to the animal’s activities and 
in part to its weight. If a number of females are isolated in a small aquarium and 
watched, they will be seen sporting about close to the upper surface of the water. 
From time to time one or other will sink through the water for a distance of an inch 
or more, and then with a few sudden leaps will regain its former place. The sinking 
is apparently due entirely to the animal’s weight, for it is unaccompanied, so far as 
can be seen, by the movement of any locomotor organs. The upward leaps are due 
to vigorous muscular action. 
The sinking of the animal through the sea water implies that its specific gravity 
is greater than that of the water, and such, in fact, is the case. The specific gravity 
of the sea water in which this species was ordinarily taken proved on determination 
to be 1.025, and that of a solution of common salt, which just kept slightly etherized 
individuals from sinking, was found to be 1.109; the specific gravity of Labidocera?, 
in reference to sea water, is, therefore, ^ or 1.082 — . In consequence of this the 
