330 ASYMMETRY IN CERTAIN LOWER ORGANISMS, 



This will perhaps best be appreciated by examining the figure of Diurella tigris Muller 

 (Fig. E, 1). The anterior part of the ridge is considerably to the right of the median 

 line, as is also the anterior tooth. At the posterior end, on the other hand, the indi- 

 cations are that what was primitively dorsal has passed to the left, while the right- 

 hand one of the paired structures has taken a dorsal position. The body has become 

 not only twisted on its primitive straight axis, but bent also, so as to form a segment 

 of a spiral. As a result of this the left side has become convex, the right side concave. 

 These features are of course much more marked in some species than in others. The 

 remainder of the unsymmetrical characteristics, as we shall see, may be considered 

 secondary results of this twisting and bending, so that it is with the latter that we are 

 primarily concerned. 



What is the significance of this peculiar twisted condition in the Rattulidse? 



The key to the asymmetry of this group is to be found, as in the case of the 

 Infusoria, in the movements and behavior of the animals. The unsymmetrical con- 

 dition in the Rattulidse is of course not primitive, for these animals were originally 

 bilaterally symmetrical. The fundamental plan of structure is still that of bilateral 

 symmetry; certain parts have been reduced or changed in position, so that asymmetry 

 has resulted, but the bilateral ground-plan is easily traceable. 



The nearest relatives of the Rattulidse are still bilaterally symmetrical. Prob- 

 ably no one familiar with the Rotifera will be inclined to question the view that the 

 Rattulidse are derived from the Notommatidse. Almost all investigators that have 

 expressed themselves in the matter have taken this view, and indeed it is clearly 

 evident. 



The Notommatidse are typically creeping forms. They live among the weeds, 

 on the surfaces of which they creep about by means of their cilia, keeping the mouth 

 as a rule against the surface. It is true of course that they can also swim freely through 

 the water, but their characteristic movement is along a surface. The species least 

 differentiated in special ways (Proales) are those of which the creeping habit is most 

 characteristic. 



In animals which thus creep about, keeping one side against a surface, we may 

 naturally expect bilateral symmetry. Such animals fulfil all the conditions which 

 were set forth in the beginning of this paper for bilateral symmetry. It is not sur- 

 prising, therefore, that we find the Notommatidse to be bilaterally symmetrical. 



But in the Rattulidse we find that the characteristic movement is no longer that 

 of creeping along a surface; they are more frequently found swimming about in the 

 free water. And if we examine their movements carefully, we find that, as in the 

 Infusoria, they swim in a spiral course. 



