40 



PROTOZOA 



[CH. 



state of swimming forward is resumed. If into a film of water full 

 of active Paramecia a drop of dilute solution of sugar is introduced 

 the animals pay no attention to it, but soon by chance one traverses 

 it. When it tries to pass out of the drop on the other side it is 

 stimulated and its one reaction called forth. As a consequence it 

 is forced back into the centre of the drop, and the reaction is 



FIG. 13. Paramecium caudatum x about 250. After Butechli. 

 1. Mouth at bottom of groove. 2. Oesophagus. 3. Food vacuole just 

 being formed. 4. Contractile vacuoles. 5. Trichocysts which have 

 exploded : the unexploded ones line the cuticle. 6. Cilia. 7. Mega- 

 nucleus. 8. Micronucleus. 9. Contractile fibrils. 



repeated every time the animal approaches the edge, so that the 

 Paramecium remains a prisoner. Soon another is caught and after 

 a time the drop is crowded. They can only be released when the 

 sugar is all finally assimilated by them. If on the other hand the 

 drop consists of moderately strong acid then the reaction is called 



