96 BEHAVIOR OF THE LOWER ORGANISMS 



of the attached animal move in the usual manner under the influence of 

 stimuli, though these movements would not have the usual effect. As a 

 matter of fact, in most cases we see nothing of the kind. The cilia 

 either do not move at all, or move in a manner different from that occur- 

 ring in free specimens. The essential factor in the interference is a 

 physiological one. When reacting to the contact stimulus, the animal 

 is less easily affected by other stimuli, and when reacting to the other 

 stimuli, it is less easily affected by the contact stimulus. Since the two 

 stimuli in question require behavior of opposite character, it is indeed 

 inevitable that one should give away to the other, or at least modify the 

 behavior toward it ; both cannot receive the usual reaction. 



Combinations of other stimuli have been less investigated than those 

 just considered. In any combination the reaction to gravity gives way, 

 as we have seen, to the reaction due to other factors. Paramecia swim- 

 ming upward react to other stimuli without hindrance, and Paramecia at 

 rest against a surface often show no orientation with reference to gravity. 

 The reactions to chemical and electrical stimuli completely supplant 

 the reactions to gravity. In a vertical tube Paramecia may form col- 

 lections in any region that becomes impregnated with carbon dioxide or 

 may avoid any region which contains a repellent chemical. If an elec- 

 tric current is passed through a vertical tube, the Paramecia react to it 

 in exactly the same manner as under other conditions, swimming toward 

 the cathode whether this is above or below. Sosnowski (1899) and 

 Moore (1903) have shown that many different stimuli modify the reac- 

 tion to gravity, changing the direction in which the animals swim. If 

 Paramecia in the culture fluid swim upward, mixture with tap water, or 

 with chemicals of various sorts, often causes them to swim downward. 

 This effect soon disappears, however, and the animals return to the top. 

 Increase of temperature to 30 degrees (Sosnowski), or decrease to 2 de- 

 grees (Moore), often has the same temporary effect. The same result 

 is at times produced by shaking or jarring the tube containing the ani- 

 mals ; they go to the bottom, returning in a short time to the top. The 

 effect of all these agents varies with different cultures of Paramecia ; in 

 some cultures the reaction to gravity is easily changed, in others with 

 difficulty or not at all. 



Reactions to chemicals often interfere with the reaction to the 

 electric current. If through a preparation of Paramecia that are 

 gathered in an area containing carbon dioxide, as in Fig. 68, A, an 

 electric current is passed, the animals swim to the cathode side of the 

 area, then stop. All gather in this region, seeming to make vain efforts 

 to cross the invisible boundary (Fig. 68, B). Observation of individuals 



