66 BEHAVIOR OF THE LOWER ORGANISMS 



Strong acid solutions cause the avoiding reaction as do other chemi- 

 cals. If a drop of strong acid solution is introduced into a preparation 

 of Paramecia, the animals collect about its periphery, where the acid 

 is diluted by the surrounding water, just as in Fig. 49. Individuals 

 which swim against the inner strong acid respond by giving the avoid- 

 ing reaction in a very pronounced way, — swim- 

 ming far backward and turning toward the aboral 

 side, for perhaps two or three or more complete 

 turns. They react also at the outer boundary of the 

 acid ring, so that within the ring the individual 

 Paramecium follows such a path as is shown in 



Fig. 5°- 



Often the reaction is not produced at the inner 

 • , Fl ?" 5 1 ° - 7 Patho . an boundary of the ring, by the strong acid, until the 



individual Paramecium J °' J . 



in such a ring as is shown Paramecium has entered far enough to be injured, 

 in Fig. 49. or even killed. A drop of strong acid introduced 



into a preparation is usually soon surrounded by a zone of dead 

 animals. Acids, as we have seen (p. 64), belong with those substances 

 which do not produce the avoiding reaction till they have become 

 directly injurious. 



Paramecia do not, under usual conditions, collect in oxygen. If 

 we introduce an air bubble or a bubble of oxygen into a slide prepara- 

 tion of Paramecia, they do not as a rule collect about it. But if the 

 outer air is excluded from this preparation by covering its edges with 

 vaseline, and it is allowed to stand for a long time, the behavior changes. 

 The oxygen has of course become nearly exhausted and now the Para- 

 mecia gather about the air or the oxygen. The collections are formed 

 in exactly the same way as are those in acids. 



Thus the experiments show that all reactions to chemicals take place 

 through the avoiding reaction, and this reaction is produced by a change 

 in the intensity of action of the chemical in question. With some chem- 

 icals, or under certain conditions, it is a change to a greater intensity 

 that produces the avoiding reaction ; in other cases it is a change to a 

 less intensity that produces the reaction. With acids both an increase 

 and a decrease beyond a certain intensity produce reaction. We may 

 express the facts for all chemicals in the following general way. For 

 each chemical there is a certain optimum concentration in winch the 

 Paramecia are not caused to react. Passage from this optimum to 

 regions of either greater or less concentration causes the avoiding reac- 

 tion, so that the animals tend to remain in the region of the optimum, 

 and if this region is small, to form here a dense collection. For acids 

 and for oxygen the optimum is a certain very low concentration. For 



