28 



BEHAVIOR OF THE LOWER ORGANISMS 



accidental conditions, or, as is more probable, is determined in some way 

 by the structure of the organisms, has not been discovered. In the 

 infusoria, as we shall see, the latter is the case. 



The reversal of movement of course carries the organism away from 

 the agent causing it. We find that the same reaction is produced when 

 the bacterium comes to a region where some repellent chemical is diffus- 

 ing in the water. This is well shown when a drop of ^ per cent NaCl is 

 introduced with a capillary pipette beneath the cover-glass of a prepara- 

 tion swarming with actively moving Spirilla. The bacteria at first keep 

 up their movement in all directions, but on coming to the edge of the 

 drop of salt solution the movement is reversed. Hence none of the bacteria 

 enter the drop, and it remains empty, hke the chemicals in Fig. 24. 





B 



Fig. 24. — Repulsion of bacteria by chemicals. A. Repulsion of Chromatium weissii by 

 malic acid diffusing from a capillary tube. After Miyoshi (1897). B, Repulsion of Spirilla by 

 crystals of NaCl. a. Condition immediately after adding the crystals; b and c, later stages 

 in the reaction. After Massart (1891). 



They react in this way toward solutions of most acids and alkalies, as 

 well as toward many salts and other chemicals. A drop of these chem- 

 icals remains entirely empty when introduced into a preparation of 

 Spirilla. 



This simple reversal of movement is the method by which avoidance 

 of any agent takes place ; in other words, it is the method of the nega- 

 tive reactions in bacteria. Bacteria also collect in certain regions, as we 

 have seen, — about air bubbles, green plants, food, etc. ; they have 

 thus what are called "positive reactions" as well as negative ones. 

 What is the behavior in the formation of such collections ? 



One finds, rather unexpectedly, that the positive reaction is produced 

 in essentially the same way as the negative one, — by a simple reversal 



