SIZE AND STRUCTURE OF THE BACTERIAL CELL. 35 



edly motile bacteria it is very difficult to demonstrate their 

 presence. 



Bacteria exhibit real motion only in liquid or on moist 

 media, and then only when in the actively growing con- 

 dition. When in the seed or spore form they do not possess 

 motion; neither do they possess active motion when floating 

 about in the air as fine dust-like particles. 



Real motion is observed in most of the spiral-shaped 

 bacteria. The bacilli are, as a rule, motile. The micro- 

 cocci are, on the other hand, usually non-motile. Two or 

 three micrococci are known to possess motion. Two motile 

 sarcines have also been studied. 



The motion exhibited by bacteria will vary with the 

 different species, and probably depends upon the number 

 and arrangement of the organs of locomotion. Some rods 

 show a slow, forward, wabbling movement. Others glide 

 rapidly and steadily forward. In some the forward motion 

 is accompanied by a rotation of the cell around its long 

 axis, while in others a "somersault" movement is to be 

 seen. Frequently the actively motile cell will suddenly 

 reverse its motion and travel backward. It may come to a 

 sudden stop, and then as suddenly, again begin to move. 



The motility of a culture depends largely upon its age, 

 upon the composition of the soil, and upon the temperature. 

 In the case of anaerobic bacteria the presence of oxygen 

 soon inhibits motion. The higher the temperature, as a 

 rule, the more marked will be the movement. Thus, certain 

 bacteria scarcely show real motion at the ordinary room 

 temperature because of the presence of a slimy secretion. 

 When placed, however, at the temperature of the body the 

 motion becomes well marked. 



Active motion is carried on by means of certain organs 

 known as flagella or whips. The whips are very delicate, 

 long, thin threads of protoplasmic substance. They are 

 analogous to similar appendages on motile infusoria, on 

 plants and on ciliated epithelium. Owing to their extreme 



