379 THE AMERICAN NATURALIST. [Vor. XXXV. 
The exact purpose that is served by this method of swimming 
is a point deserving of further emphasis and fuller discus- 
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Fic. 1, — Diagram of spi- 
ral course of Loxodes 
and d below this plane. 
sion. The Flagellata and Ciliata are as a rule 
unsymmetrical in form. One of these organ- 
isms, as, for example, Loxodes (Fig. 2), or 
Paramecium (Fig. 3), when it leaves the bot- 
tom and starts to swim freely through the 
water, cannot go in a straight line, but owing 
to its lack of symmetry continually swerves 
toward one side, so that it tends to describe 
a circle. If no method is taken of compen- 
sating this deflection, the circles described 
are frequently very small, and of course the 
animal makes no progress by swimming in 
this way. Paramecium and Loxodes thus 
tend to circle toward the aboral side, Chilo- 
monas (Fig. 4) toward its “lower lip,” all the 
Hypotricha to the right, etc. 
To obviate this difficulty, revolution on the 
long axis is combined with the forward move- 
ment of the organisms. By this means the 
continual swerving toward one side is com- 
pensated, since this side is continually turned 
in a new direction. Thus, if Loxodes is 
swimming (freely through the water) away 
from the observer, and the aboral side is at 
first to the observer's left (Fig. 1, 4), the 
organism at first swerves to the left; but as 
it revolves the aboral side soon comes to be 
the upper side, and the animal now swerves 
up (č). By continued revolution the aboral 
side is brought to the right (c), so that the 
animal swerves to the right. Next, of course, 
it swerves down and the process is continued, 
the animal swerving successively to the left, 
* up, right, down, etc. These movements, 9 
course, compensate each other, so that only 
the forward component of the motion 1S 
