132 AMEBOID MOVEMENT 



metrical during the phylogenetic history and as a result became 

 unable to continue to swim in a straight path, the pertinent 

 question to ask is : Was it easier for these organisms to learn 

 to revolve on their long axis than to learn to beat their cilia 

 a little harder on the side toward which they swerved ? Observa- 

 tion of the forms before us does not afford any evidence that 

 rotation was the easiest solution. Moreover, if it was an acquired 

 habit, is it not strange that it should have been easier to acquire 

 the rotating habit for every single species of the six or seven 

 thousand unicellulars which now obey the spiral urge, as well 

 as the swarm spores and zoospores, than to change the beat of 

 the cilia in some other way, in at least a few species? This ex- 

 planation also makes inevitable the assumption that the ancestors 

 of our present unsymmetrical protozoans were symmetrical and 

 swam in straight courses without revolving, a condition of affairs 

 which contrasts strongly with present conditions, for none of the 

 most nearly symmetrical unicellulars and swarm spores now 

 swims without revolving on the long axis. It is therefore ex- 

 ceedingly improbable that spiral swimming is the result of an 

 acquired habit. 



Now what evidence is there in support of the hypothesis that 

 the spiral path is a necessary accompaniment of locomotion, ex- 

 cept as it may be broken by the effect of stimulation? 



As a problem in engineering, it is clear that the shape of the 

 body is not responsible for the spiral course, for almost every 

 conceivable shape is met with in organisms swimming in spiral 

 paths. The frequent spiral turns in the path of stylonychia can- 

 not be the result of the shape of the body, which is almost, 

 if not quite, as well adapted for swimming through the water as 

 is that of a euglena or a fish, but for revolution on its long 

 axis it is not nearly so well adapted. Moreover, some of the 

 euglenas turn the ventral or smaller lip out in the spiral turns, 

 while others turn the dorsal or larger lip out (Mast, '10). Since 

 there is no other assymmetry of shape in these euglenas, it is 

 clear that the shape of the body has nothing to do with causing 

 the spiral path. The immediate cause of spirality must therefore 

 be the work of the motile organ, and not the shape of the body. 



Similar observations on paramecium have shown that it is 



