54 BEHAVIOR OF THE LOWER ORGANISMS 



of hot or cold water on the cover-glass of a slide of Paramecia, or by 

 touching the cover-glass with a hot wire, or a piece of ice. The animals 

 respond by the avoiding reaction, just as when stimulated by a chemical, 

 so that the hot or cold region remains vacant. The intensity of the re- 

 action depends on the temperature, and very hot water causes a much 

 more decided reaction than very cold water. 



The avoiding reaction is seen under mechanical stimulation when a 

 specimen in swimming comes against an obstacle. It may also be shown 

 by touching the anterior end of the animal with a fine glass point. A 

 slight disturbance in the water may be induced by injecting a fine 

 stream of water against the animal with a pipette drawn to a capillary 

 point ; the animal then responds by the avoiding reaction, thus swimming 

 elsewhere. 



Special features in the reactions to various different classes of stimuli 

 will be dealt with in the next chapter. 



5. " POSITIVE REACTIONS " 



The reactions thus far described have the effect of removing the 

 animal from the source of stimulation ; they might therefore be charac- 

 terized as negative. But Paramecia are known also to collect in certain 

 regions, giving rise to what are commonly known as positive reactions. 

 How are these brought about? 



A simple experiment throws much light on the cause of such col- 

 lections. Under usual conditions the animals avoid a ^ per cent solu- 

 tion of NaCl, so that when a drop of this is introduced into a slide of 

 Paramecia, they leave it empty. But if we mix the animals with ^ per 

 cent NaCl, then introduce into a slide of this mixture a drop of 

 ^j- per cent NaCl, in the way shown in Fig. 40, we find that the Para- 

 mecia quickly collect in this drop, though under ordinary circumstances 

 they avoid it. Very soon the drop of T V per cent NaCl is swarming with 

 the infusoria, as in Fig. 43, while very few remain in other parts 

 of the preparation. The phenomena are identical with what has often 

 been called positive chemotaxis. 



Careful observation of the movements of the individuals shows, 

 as might be expected, that the Paramecia collect in the ^ per cent NaCl 

 merely because they avoid the stronger solution more decidedly. Pas- 

 sage from the ^ per cent solution to the ^ per cent solution causes the 

 avoiding reaction, while passage in the reverse direction does not. The 

 details of the behavior are as follows : The Paramecia in the ^ per cent 

 NaCl are swimming rapidly in all directions, so that many of them are 

 carried toward the drop. On reaching its boundary they do not react 



