160 



THE STUDY OF ANIMAL LIFE 



CHAP. 



to changes in temperature, illumination, chemical com- 

 position of the medium, and so on. As there is no nervous 

 system, but simply a specific inborn protoplasmic organi- 

 sation, which registers experiences, we may use the 

 phrase " organic reaction " for this kind of answer-back to 

 stimulus. It is in a far-off way comparable to a "reflex 

 action ' in a higher animal. Its rudimentariness is 

 obvious in those cases where the same answer the only 

 one the creature has learnt is given to every question 

 or stimulation. To most stimuli the slipper-animalcule 

 (Paramecium) responds by reversing its cilia, turning a 



FIG. 47. BEHAVIOUR OF PARAMECIUM. 

 (After Jennings.) 



1. The Infusorian approaches an obnoxious influence, A. 



2. It reverses its ciliary action and retreats. 



3. 4, 5. It turns on its own axis. 



6. It advances again, and if it re-encounter A it will repeat its reaction. 

 Paramecium swims habitually with a spiral movement of its body, for the 

 aboral cilia strike more strongly than the others. 



little on its axis, and then going ahead again in a slightly 

 altered direction. This often enables it to evade an 

 obnoxious influence and it is an illustration of the rudi- 

 ments of behaviour. Prof. Jennings has shown that the 

 Amoeba in its mysterious gliding selects lines of flow 

 which relieve it from hurtful stimulation. 



4. Trial and Error among Unicellular Animals. The 

 observations of Jennings in particular have shown that 

 many Protozoa exhibit what may be called a trial and 



