CHAPTER I. 



§ 2. FORMS OF BACTERIA. 



The different forms which bactena are known to assume may all be 

 reduced to three types : 



(1) Coccus, or round bacteria (Fig. 2a). All included under Coccaceae. 



(2) Bacillus, or rod bacteria (Fig. 2b). „ „ Bacteriaceae. 



(3) Spikillum, or spiral bacteria (Fig. 2c). „ „ Spirillaceae. 



These shapes are constant; that is, a coccus-species always retains 

 its spherical form, and the same can be said of the bacillus and 

 spirillum. In the case of the bacillus group, however, it often happens 

 that the rods are so small that they appear almost like cocci. This 

 follows from the fact that any particular species has not always the 

 same length, or even the same breadth, but if a species of this kind 



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Fig. 2. 



be constantly kept under observation, and cultivated in different ways, 

 it will be found that the rod shape is constant, however much it may 

 vary in length and breadth. 



Looking more closely into the first group we find that the cocci 

 arrange themselves in different ways. When division takes place, the 



