AND ITS BIOLOGICAL SIGNIFICANCE. 335 



This similar relation to the environment is usually assigned as a reason for the simi- 

 larity of paired organs, and the lack of such similar relation to the environment 

 may become an equally good ground for loss of similarity in structure. The lower, 

 originally left, toe is now next to the bottom when the animals are creeping (as they 

 sometimes still do), and will more often come in contact with the bottom than will 

 the right one. When a thread of mucus drips from the toes and catches on some 

 object on the bottom, suspending the animal, it will more often be that from the 

 lower (left) toe. The conditions for a change in structure are therefore present. 



It is clear, however, that the reasons for the change in the relative length of the 

 toes is not so strikingly evident as those for the other changes above described. The 

 same may be said a fortiori in the case of the asymmetry of the lateral antennae. 

 I am unable to discover any reason for the change from bilateral symmetry to the 

 excessively unsymmetrical condition found in Rattulus cylindricus Imhof and Diu- 

 rella stylata Eyferth (Fig. E, 2). 



On the whole, however, it is evident that the unsymmetrical form and structure 

 of the Rattulidse is an adaptation to their method of life and movement. The ques- 

 tion may be asked why this unsymmetrical form has not been developed in other 

 free-swimming rotifers, which revolve on their long axes and swim in a spiral course, 

 just as do the Rattulidse. But it must be remembered that the same question may 

 be asked in regard to almost any adaptation. Why are there certain creeping animals, 

 such as the starfish, that are not bilaterally symmetrical? The general answer to 

 such a question must be, that we find some animals adapted to then- conditions in 

 one way, some in another; that all are not adapted in the same manner does not alter 

 the fact that certain adaptations exist in given cases. On the specific question as 

 to why many of the other rotifers have not become unsymmetrical, there is perhaps 

 one consideration which furnishes a partial answer. All the rotifers which have 

 become unsymmetrical belong to the loricate groups; the soft-bodied Rotifera have 

 remained bilateral. The latter are flexible, and in the process of swimming in a spiral 

 the body can by its flexibility adapt itself directly to the spiral course. There is 

 therefore no necessity for the spiral form to become fixed. With the loricate rotifers 

 the form is fixed, hence it must become permanently adapted to the method of 

 movement, if it becomes adapted at all. 



