THE MANIFESTATIONS OF LIFE 53 



ena are sometimes observed; as, for example, intimate 

 association with diatomes in order that they may profit 

 by the oxygen thrown off by the little plants. Verworn 

 observed a group of bacteria (Spirochaete plicatilis) 

 surrounding a Pinnularia in great numbers, though 

 elsewhere in the preparation they were absent. The 

 bacteria were all at rest and were present in greatest 



Fia. 9. Mutualism of diatome and bacteria. The bacteria by which the 

 diatome is surrounded are profiting by the oxygen it gives off in its metabolism. 



numbers near the centre of the organism. Suddenly 

 the diatome moved off a short distance, when the bacteria 

 left behind and remaining quiet a short time, swam after 

 it, surrounding it again in a similar cluster. It was no 

 doubt the oxygen given off by the plant that attracted 

 the bacteria. In microscopic specimens, prepared by 

 covering a drop of water with a cover-glass, Hertwig 

 points out that necessity for oxygen eventually deter- 



