Fic. 9. — Diagram showing the course 
of Plæsoma as soon as it begins to 
revolve on the long axi 
higher and lower, respectively 
5 and d, which lie in the same plane). 
Thi 1 , with th 1 
axis (a and c are 
than 
This spiral course the dorsa 
side to the outside of the spiral, is 
characteristic for many Rotifera 
. 
THE AMERICAN. NATURALIST. 
[Vor. XXXV. 
(Fig. 8) becomes at once a well-ordered 
progression in a spiral path (Fig. 9). 
No one who has seen this sudden 
change from random circles to a path 
having all the essential qualities of a 
straight line can fail to appreciate thé 
biological significance of the rotation 
on the long axis in compensating the 
tendency to swerve in a given direction. 
This tendency to swerve toward the 
dorsal side seems present in the major- 
ity of the free-swimming Rotifera, and 
is compensated almost universally by 
the revolution on the long axis, causing 
the resulting path to be a spiral with 
the dorsal side directed toward the 
outside of the spiral (Fig. 9). All 
rotifers observed by the writer revolve 
to the right, and no reversal of the 
direction of revolution was ever seen. 
In some of these primitively bilat- 
eral animals this spiral method of 
swimming has resulted in the pro- 
duction of an unsymmetrical form 
analogous to that of the infusoria. 
In the small aberrant family of Rat- 
tulidæ this adaptation to a spiral 
movement is most striking. Rattulus 
tigris, for example, has a Urs 
body, forming actually a segment o 
a spiral, and there is in WE. 
high spiral ridge on one side. T is 
ridge begins behind the middle, near 
the mid-dorsal line, and passes for- 
ward, at the same time curving onee 
to the right side. The animal am 
in a spiral of which this ridge 2" 
its own twisted body form 4 part. 
