AMEBOID MOVEMENT 131 



But in the explanation of body form in one of the rotifers he 

 (1. c., p. 376) says: "In some of these primatively bilateral ani- 

 mals this spiral method of swimming has resulted in the pro- 

 duction of an unsymmetrical form analogous to that of the in- 

 fusoria." 



It is of course quite possible theoretically, that some of the 

 unsymmetrical structures on an organism that habitually swims 

 in spirals, are the result of its spiral swimming, and that other 

 structures which go to make the organism unsymmetrical, are 

 the cause of the spiral swimming. This hypothesis is not an at- 

 tractive one, however, for, because of the endless variety of 

 asymmetrical differentiation in spiral swimming organisms, it 

 would be impossible to tell for the large majority of organs or 

 organelles whether they were the cause or the effect of spiral 

 swimming. 



Before taking up the hypothesis that all 'moving organisms are 

 subject to the tendency to move in spiral paths, a hypothesis which 

 accords with all the known pertinent facts, it may be well to exam- 

 ine the thesis that rotation on the long axis is an adaptation which 

 has been developed to compensate for the effect of an unsym- 

 metrical shape of the body. 



It will be noted first that this question cannot be decided by 

 direct observation or experiment. The entire body of real evi- 

 dence is written in phylogeny, and that is for this purpose a 

 closed book. It is only the interpretations of observations that 

 bear on this problem, and it is these interpretations that it is of 

 interest to examine. 



Referring now only to the ciliates, all of which have numerous 

 motile organs, it has been observed by numerous writers that 

 cilia are not confined to one or two methods of contraction, but 

 that there is great latitude in the extent and direction of their 

 activity. This is very well illustrated by a paramecium or a stentor 

 whose ciliary systems enable these animals to execute a great 

 variety of maneuvers depending upon the character of stimula- 

 tion, the amount of food in the body, etc. (Jennings, '06, Schaef- 

 fer, '10). The cilia are under the control of the animal in the 

 same way as the legs and arms of a man are under his control. 

 Now supposing that the bodies of these organisms became unsym- 



