REACTIONS OF INFUSORIA TO ELECTRIC CURRENT 159 



than the opposing ones, and because the opposed action of the cilia on 

 the anodic and cathodic halves of the body soon bends the slender body. 

 It thus squirms about from one side of the transverse position to the 

 other, taking many shapes besides those figured. It remains quiet only 

 for certain periods in the transverse position with peristome to the cath- 

 ode, when it is in contact with a surface: this is a result of the inter- 

 ference of the contact reaction with the reaction to the electric current. 

 Under the action of the current alone, the reaction of Spirostomum does 

 not tend to bring it to a position where it is not effectively stimulated, 

 for no such position exists. In this respect the electric stimulus shows 

 again a marked contrast with other stimuli. 



In Opalina ranariim the first marked effect of the electric current is 

 to cause the animals to swim to the anode instead of to the cathode. 

 Its reaction seems thus in striking contrast with that of other ciliate 

 infusoria. We must examine the reaction in Opalina, following Wallen- 

 gren (1902), to see how this result is brought about. 



Opalina is a large, flat, disk-shaped, parasitic infusorian, living in 

 the large intestine of the frog. For experimental work it is examined 

 in physiological salt solution, as it soon dies in water. There is no 

 mouth, since food is obtained by absorption over the entire body sur- 

 face. The body is closely set with fine cilia. The anterior end of the 

 body is more pointed than the posterior. From the anterior portion 

 there extends backward at one edge a convex region, ending at a sort 

 of notch in the middle of the body (Fig. 107, x). This convex region 

 is set with cilia having, as we shall see, a somewhat different function 

 from those of the remainder of the body. The side bearing this con- 

 vexity is usually known as the right side. 



Opalina swims with anterior end in front, at the same time usually 

 revolving on its long axis. When stimulated by contact with a solid, 

 or in other ways, it turns toward the side bearing the convexity — the 

 right side. Observation shows that this movement is due to the fact 

 that the cilia on the convexity of the right side now strike forward in- 

 stead of backward, thus necessarily turning the animal toward the side 

 bearing them. In this way the typical avoiding reaction of Opalina is 

 produced. 



If a preparation of Opalina in physiological salt solution is sub- 

 jected to the action of a weak electric current, the animals swim to the 

 anode. Examining the individuals, it is found that the cilia on the 

 anode half of the body strike backward, those on the cathode half for- 

 ward, exactly as in Paramecium. Why then does Opalina swim to the 

 anode instead of to the cathode? 



The secret of this difference lies in the following facts. The cilia 



