374 THE AMERICAN NATURALIST. [VoL. XXXV. 
supposedly accounts for the sinuous path of these organisms. 
It should be clearly stated that the actual movements of these 
creatures lend no support to this account, but are, on the 
contrary, quite incompatible with it. The 
organisms swerve always toward the same side, 
not first to one side, then to the other. 
But it is well known that it is not only 
unsymmetrical organisms that swim in a spiral, 
but that the same is true for many bilateral 
organisms also, —as, for example, the Rotifera. 
Since the two sides are alike in these animals 
(see Fig. 6), there is no reason for swerving 
to the right rather than to the left, and the 
spiral path calls for some further explanation. 
The significance of the spiral path in such cases 
c.6.—Dormal view 1$ clearly seen when the movements of these 
parachi animals are carefully studied. When creeping 
Weber) to show the On the bottom or the surface film, there is no 
sadanefthetwo rotation. Here the only possibilities of devia- 
tion from the straight line are either to the 
right or to the left, and since the two sides are alike there is no 
reason for swerving in either direction. But the dorsal and 
ventral sides are not alike (see Fig. 7), and in swimming freely 
through the water the animal 
might err by turning toward the 
dorsal or toward the ventral side, 
or in any intermediate direction. 
As a matter of fact, careful ob- 
servation shows that most roti- 
fers do swerve toward the dorsal =Z 
side as they swim freely through — 
the water. This tendency seems fic. 7. — Rotifer (Brachionus pala, mon 
traceable to the fact that the fos fom Hoi c eet 
rotifers are primitively creepers side when rising from the bottom. 4 
on the bottom, and most of them 9?! 54e: ® peas a 
still retain this habit. In order to rise from the bottom into the 
free water, the animal must necessarily move toward the do 
side (as in Fig. 7). The cilia which bring about the free 
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