AND ITS BIOLOGICAL SIGNIFICANCE. 



321 



Flagellata, when swimming through the free water, follow a spiral course such as is 

 favored by the form of the body. Thus in Chilomonas the animal follows a spiral 

 path, with the smaller lip at the anterior end directed always toward the outside 

 of the spiral. 



The unsymmetrical structure of the ciliate and flagellate Infusoria is thus closely 

 correlated with the usual locomotion of these creatures. But this is not all. The 

 unsymmetrical structure is equally closely bound up with the method of reacting to 

 stimuli in these animals. They may be said to have an unsymmetrical method of 

 reaction, as well as an unsymmetrical form. When one of these organisms is stim- 



FIG. D. Figures of Flagellata, showing the spiral form. 



1, Phacus longicaudus Ehrbg., after Stein, from Kent; 2, Phacus pyrum Ehrbg., after Stein, from Kent; 

 3, Spironema multiciliatum Klebs, after Klebs; 4, Heteronema spirale Klebs, after Klebs; 5, Trepomonas steinii 

 Klebs, after Klebs. 



ulated in almost any manner, the way in which it reacts is conditioned by its unsym- 

 metrical structure. 



Most higher organisms govern their movements with close relation to the posi- 

 tion of external objects; in other words, they react with reference to the localization 

 of the stimulus. Thus, if we touch sharply a leech or flatworm on the left side, it 

 will turn away, to the right; if we touch it on the right, it will turn to the left. This 

 necessarily involves a somewhat complicated mechanism; a stimulus on one part 

 of the body causes a reaction in a different region from that caused by a stimulus in 

 another part of the body. A much simpler (though of course less effective) plan would 

 be to have the organism so constructed that it would react in the same manner where- 

 ever the stimulus, that it would turn in the same direction whichever side is stimu- 

 lated. This extraordinarily simple plan is what we have realized in the Infusoria. 

 They are so constructed as to turn always when stimulated toward the same side, 

 and the external manifestation of this is the unsymmetrical form. Paramecium, 



