i 3 4 AMEBOID MOVEMENT 



or a ciliate. Most of the Oscillatoriaceae that are capable of move- 

 ment, consist of straight filaments ; but two of the genera, Arthro- 

 spira and Spirulina, are spirally twisted in such a way that the 

 spiral axis of the filament corresponds approximately to the 

 spiral path of a particle attached to the surface film of an Oscilla- 

 toria filament, except, of course, in size. (The movement of the 

 surface film of neither Arthrospira nor Spirulina has been 

 studied). 



That the spiral shape of a rotifer, for example, may be caused 

 by swimming in a spiral path might perhaps be regarded as a 

 plausible explanation, but it seems to me that it would be more 

 satisfactory to explain the spiral shape of rotifers and Arthro- 

 spira, the direction of the oral groove of paramecium and similar 

 structures in other organisms, as due to the same fundamental 

 process that causes the spiral path in locomotion. This explana- 

 tion is purely mechanistic and avoids the teleological element on 

 which the other explanation ultimately depends. 



Most of the asymmetrical shapes of the flagellates, ciliates, 

 rotifers, etc., have originated in phylogeny without regard to 

 swimming ia spiral paths, and indeed in spite of it. In spindle- 

 shaped organisms like euglena or paramecium the amount of 

 energy required to revolve on the long axis, as compared with 

 that required for forward movement, is small, But in stylo- 

 nychia, a dorso-ventrally flattened ciliate, much more energy is 

 required to revolve the animal, proportionally, than is needed for 

 forward movement. It is of course perfectly evident that as a 

 problem in engineering it requires much more energy to revolve 

 a flate plate on its long axis than a spindle-shaped solid, in a dense 

 medium like water. But in spite of all the obstacles to revolution 

 which asymmetry of body form presents, none of them are serious 

 enough to prevent revolution from occurring, unless the keeled 

 rotifer Euchlanis (Jennings, '01) presents such a case. Observa- 

 tion would lead one to believe, however, that the compressed body 

 forms of some of the hypotrichans and some of the flagellates 

 such as phacus, have made revolution on the long axis very dif- 

 ficult ; but not difficult enough to destroy the tendency to revolve 

 and describe spirals. In short, these organisms spiralize in spite of 

 asymmetry, not because of it. 



