332 ASYMMETRY IN CERTAIN LOWER ORGANISMS, 



formed would operate against this method of moving. The reason why the right 

 side is convex, the left concave, is likewise evident; this is a necessary result of 

 making the body a segment of a right spiral. 



The asymmetry of the internal organs may be recognized as a consequence 

 of the external form. Since the right side has become concave, there is less space 

 here than in the convex left side; hence we find a tendency for some of the organs 

 on the right side to become reduced. This is especially noticeable in the trophi or 

 laws, in which, as we have seen, the right side is usually small or rudimentary. 



There are some points which still need elucidation, however. Why are the teeth 

 at the anterior dorsal margin of the lorica confined to the right side? Why has the 

 foot become twisted into such a position that the toes can be bent only to the right? 

 And why does the right toe degenerate? 



These points will be better understood if we examine the behavior a little further. 

 As we have seen, the animals continually swerve toward the dorsodextral part of 

 the body — that which bears the ridge. This result is due to two components : first, a 

 tendency to swerve toward the dorsal side, as when lifting the body from the bottom 

 (a tendency which is present in almost all free-swimming rotifers) ; and secondly, the 

 revolution toward the right. The resultant of these two components is a swerving 

 toward the dorsodextral region. 



Now the delicate head bearing the cilia and sense-organs, is the only portion 

 of the body which is not protected from injury by the lorica as the animal swims. 

 Owing to the continual swerving toward the dorsodextral side, if the delicate head 

 ever strikes against any object, it must be on the dorsodextral side. Hence the 

 tooth or teeth are placed in this region, where they will receive any blows which would 

 otherwise fall upon the unprotected head. This striking against objects is by no 

 means rare in the ordinary swimming of the animal. It often swims along with its 

 spiral path tangent to a surface, almost every turn bringing the animal against the 

 surface. But it is of course always the dorsodextral angle which comes in contact 

 with the surface, the tooth or teeth here protecting the soft head from injury. This 

 will be appreciated by an examination of Figure I, if the line x-y is conceived to be 

 the surface tangent to which the animal is swimming. 



Moreover, the usual reaction to a stimulus in the Rattulidse is, as in the Infusoria, 

 closely related to the method of locomotion and to the unsymmetrical form. When 

 a Diurella or Rattulus while swimming freely through the water meets an obstacle, 

 it alters its course simply by turning still farther than usual toward the dorsodextral 

 side. If the obstacle is small, it is thus at once avoided. If the obstacle, on the other 

 hand, is large — for example, a flat surface which prevents farther movement in the 



