Wager : Movements of Aquatic Micro-Organisms. 213 
into the liquid and gradually becoming disseminated through 
it. Being thus brought out of the shadow of the mass of 
Euglenae at the edge of the water and into the path of the 
light rays, they begin to ascend again and to seek the illumin- 
ated side of the vessel. A continuous circulation is thus kept 
up and the Euglenae are prevented from too dense an aggre- 
gation which would interfere with their assimulating and 
respiratory functions. If the vessel is shaded by the hand or 
by a sheet of card, the downward movement becomes more 
decided, the Euglenae disappear from the surface and the 
upper layer of the water becomes clear. If the vessel is kept in 
the dark the Euglenae remain in constant movement and become 
disseminated through the liquid in the form of an irregular 
network (Fig. VI.), or under certain conditions may become 
aggregated into evenly spaced circular groups each with a 
dark-green central spot (Fig. VII.). In a narrow' tube placed 
horizontally in the dark a similar aggregation takes place, and 
the Euglenae form a series of vertical bands (Fig. IX. as seen 
from the side, Fig. VIII. from above). Under a low power of 
the microscope each band or group is seen to consist of a central 
dark mass of cells sinking with their hinder ends downwards ; 
the lighter coloured peripheral area consists of cells swimming 
upwards. These regular groupings and the cyclic up and 
dowm movements are apparently controlled mechanically 
by the action of gravity and cohesive or molecular forces. 
‘ The upward movement which always regularly follow's the 
dov'mvard one is caused by the active movements of the organ- 
isms themselves. As soon as the congested streaming mass of 
cells approaches the bottom of the vessel, it begins to spread 
out and the Euglenae becomes diffused in the water. The 
crow'ding becomes diminished and the result is that the 
Eulgenae are now' once more free to move. But they are still 
too crowded for entire freedom of motion ; their axial orienta- 
tion is still more or less vertical with the anterior ends upwards ; 
consequently they are compelled to move upwards. That they 
are not able to move in any other direction is due to the action 
of gravity. Freedom of motion in all directions is only 
possible w'hen the Euglenae are few in number and more or 
less isolated from each other; The force of gravity alw'ays 
tends to bring them into a vertical position and anything 
which impedes, even to a slight degree, their movements 
allow's this action of gravity to be brought into play.'* 
In the dark the movements of Euglenae are apparently 
controlled by gravity alone ; in a bright light the action of 
* Wager, ‘ On the Effect of Gravity upon the Movements and Aggre- 
gation of Euglena viridis, Ehrb. and other Micro-organisms.’ Phil. 
Trans. B., vol. 201, 1911. 
1911 July 1. 
