THE NATl'KK <>F I'.ACTERIA. 25 



being by far the smallest living organisms known, and de- 

 manding the highest powers of the microscope for their study. 

 The spheres, for example, have a diameter varying from .25 /*. 

 to 1.5 IJL (0.000012 to 0.00006 inch). The rods have a diam- 

 eter of about the same dimensions, but their length may be 

 considerably greater, especially when they grow into long 

 slender threads. All are, however, far below the limits of 

 human vision unaided by the microscope. 



Many bacteria have the power of motion which is produced 

 by slender motile hairs arising from their body and which by 

 lashing to and fro produce a locomotion (Fig. 5). These hairs 



FIG. 5. 



Bacilli showing flagella. (Migula.) 



are called flagella. They are sometimes numerous and scat- 

 tered all over the body (Fig. 5, a), sometimes few and grouped 

 atone or both ends (Fig. 5, b\ and in other cases only a single 

 flagellum is found (Fig. 5, c). In many bacteria they are en- 

 tirely wanting. 



By the use of these characters bacteria are divided into 

 three easily recognizable divisions : 



i. Spherical Bacteria. Coccus. This group includes all 

 bacteria which are spherical. They are further subdivided in 

 accordance with their method of division. In some of them 



