52 BEHAVIOR OF THE LOWER ORGANISMS 



ditions Paramecium soon becomes acclimatized ; it is change that 

 causes reaction. 



To this general statement there are certain exceptions. If we place 

 the infusoria in conditions of such intense action that they are quickly 

 destructive, — for example, in 2 per cent potassium bichromate, or 

 in water heated to 38 degrees C, — the animals continue to react till they 

 die. For two or three minutes they rapidly alternate swimming back- 

 ward with turning toward the aboral side and swimming forward, 

 till death puts an end to their activity. Thus very injurious conditions 

 may produce reaction independently of change. But as a general rule, 

 it is some change in the conditions that causes the animal to change its 

 behavior. The animal, having been subjected to certain conditions, 

 becomes now subjected to others, and it is the transition from one state 

 to another that is the cause of reaction. This is a fact of fundamental 

 significance for understanding the behavior of lower organisms. 



But it is not mere change, taken by itself, that causes reaction, but 

 change in a certain direction. This is shown by observation of the 

 behavior of the individuals as they pass from one set of conditions to 

 another. If we place Paramecia on a slide in ordinary water, then in- 

 troduce into the preparation, by means of a capillary pipette, a drop of 

 ^ per cent sodium chloride, as shown in Fig. 40, we find that the 

 animals react at the change from the water to the salt solution, so that 

 they do not enter the latter. If, on the other hand, the animals are first 

 mixed with ^ per cent salt solution, and a drop of water is introduced 

 into the preparation (as in Fig. 40), they do not react at passing from the 

 salt solution to the water. In the same way, Paramecia at a temperature 

 of 30 degrees react at passing to a higher temperature, but not at passing 

 to a lower temperature. Paramecia at 20 degrees, on the other hand, 

 react at passing to a lower temperature, not at passing to a higher. To 

 these relations we shall return. 



A relation which is worthy of special emphasis is the following : The 

 direction toward which the animal turns in the avoiding reaction does 

 not depend on the side of the animal that is stimulated, but is deter- 

 mined by internal relations. The animal always turns toward the aboral 

 side. It is true that with chemical stimuli the stimulation usually 

 occurs on the oral side, so that the animal turns away from the side 

 stimulated. But, as we have just seen, it turns in the same way when 

 all parts of the body are equally affected by the stimulating agent. 

 Furthermore, it is possible to apply mechanical stimuli to various parts 

 of the body, and observe the resulting reaction. If with the tip of a fine 

 glass point we touch the oral side of Paramecium, the infusorian turns 

 directly away from the point touched. But if we touch the aboral side, 



