. Behavior of the lower organisms. Animal behavior; Invertebrates. 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 turn
. Behavior of the lower organisms. Animal behavior; Invertebrates. 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?" 51° 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 que
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