114 MICRO-ORGANISMS AND DISEASE [chap. 



an active locomotion, by which the individual bacteria are 

 enabled to move actively and to change their place ; this move- 

 ment shows itself either by the bacteria darting with great 

 rapidity across the field of the microscope in one or another 

 direction, or spinning round with greater or lesser velocity, 

 or briskly moving like a screw in one direction and then 

 back again. Observing a single straight bacillus in its move- 

 ment, either a darting or spinning movement in one direc- 

 tion is noticed ; when two such bacilli are connected end- 

 wise, but bent one to another under an angle, then often, 

 with a forward or backward movement of the one, a spinning 

 movement of the other is noticed, the former not really 

 actively moving but being simply propelled by the spinning 

 movement of the latter bent under an angle. When comma- 

 bacilli or spirilla move, the motion is always more or less 

 spiral. 



When longer chains or leptothrix of bacilli move, the 

 movement is always more or less serpentine. The loco- 

 motion of bacilli is either rapid or slow ; the latter may be a 

 character of the species, that is to say, the individuals as a 

 rule show only a relatively slow movement — e.g., typhoid 

 bacilli generally move comparatively slowly, and the longer 

 bacilli move in a serpentine manner. The mobile indivi- 

 duals do not continue to move indefinitely, since often an 

 individual which has been spinning round or darting about 

 gradually comes to rest and remains so for some time ; 

 besides this, all motile bacilli during the phase of division 

 are at rest, and when they form groups — i.e., when they are 

 in an active state of division — they do not move. But of 

 such groups here and there an individual may be seen to 

 separate itself from the margin and to move briskly away ; 

 on breaking up a group, crowds of motile bacilli sally forth. 

 The writer has watched single bacilli of the human Middles- 



