REACTIONS TO HEAT AND COLD IN THE 

 CILIATE INFUSORIA. 



To explain the movements of organisms toward or from a source of 

 stimulus, we find given almost universally in one shape or another a 

 certain general formula. This is the schema set forth, with unessen- 

 tial variations, by Verworn (1899, pp. 500-502) for the orientation of 

 a ciliate or flagellate infusorian to a one-sided stimulus, and by Loeb 

 (1897, pp. 439-442) for the tropisms of organisms in general. Essen- 

 tially, the schema is as follows : An agent acting upon the organism 

 from one side causes the locomotor organs of that side to contract 

 either more strongly or less strongly than those of the opposite side. 



a 



FIG. i.* 



In the former case (Fig. i) the animal is turned away from the source 

 of stimulus, till it comes into a position in which the motor organs of 

 the two sides are similarly affected. Then progressing straight for- 

 ward, it of course moves away from the source of stimulus (negative 

 taxis or tropism). If the motor organs on the side most affected are 

 caused to contract less strongly than those on the opposite side (Fig. 2) 



*Fio. i. Diagram of a negative reaction of an organism, according 10 the 

 tropism schema. The motor organs which act more effectively are shown more 

 heavily drawn. The more pointed end is the anterior. A stimulus is supposed 

 to impinge upon the organism a from the direction indicated by arrows; this 

 causes the motor organs directly affected by the stimulus to beat more strongly, 

 as indicated by the darker shade. The result is to turn the anterior end in the 

 direction indicated by curved arrows. The organism thus occupies successively 

 the positions a, b, c, finally coming into the position d. Here the motor organs 

 of the two sides are equally affected by the stimulus, hence there is no further 

 cause for a change of position. The usual forward motion of the organism now 

 takes it away from the source of stimulus, as indicated by the straight arrow at d. 



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