372 THE AMERICAN NATURALIST. [Vor. XXXV. 
the form of the body is such as to oppose this reversal The 
unsymmetrical form seems rather an adaptation to this method 
of swimming, — a consequence of it. Many of these organisms 
are so shaped that the body forms part of a spiral; this is to a 
certain degree the case, for example, in Paramecium (Fig. 3). 
In some others this is much more marked. Phacus, for example, 
is frequently strongly spiral. Some of the bacteria swim in 
this same manner, and among these, Spirillum forms, as is well 
known, a sort of animated corkscrew. The prevailing asym- 
metry in the unicellular organisms is closely correlated with 
this method of swimming. 
When creeping along the bottom (as Loxodes usually does), 
or when in contact with any solid object, these same organisms 
exhibit no such rotation. When moving along a surface there 
are, of course, only two chances to err from the straight line, 
either to the right or to the left. When swimming freely 
through the water, on the other hand, the chances of devia- 
tion are indefinitely numerous, since the organism may swerve 
to the right or left, or up or down, or in any intermediate 
direction. Moreover, when in contact with a surface, this 
usually presents numerous stimuli, which serve as directives 
of motion, while in the free water such stimuli are lacking. 
Hence the necessity of some special device for keeping the 
straight course in the latter case. The movements and reactions - 
of organisms differ greatly when they are moving along a gui 
face from those. exhibited when passing freely through the 
water. (Pütter! has recently published a valuable paper on 
this subject.) Both flagellates (e2., Peranema) and ciliates 
move without rotation when in contact with a surface. Yet 
even then they usually cannot travel in a straight line; Col- 
pidium and Oxytricha, for example, follow a much curved 
course. 
As the present writer has fully set forth in his * Studies on 
Reactions to Stimuli” (doc. cit.), this method of locomotion is 
closely related with the usual method of reaction to a stimulus. 
In addition to swerving toward a structurally defined side 1n 
| Pütter, August. Studien über Thigmotaxis bei Protisten, Archiv f. Har 
Phys., Physiol. Abth., Supplement Band (1900), pp. 243-302- 
