24 THE NATURE OF BACTERIA. 



and Bacterium, based upon the presence of flagella, is a con- 

 venient one, but its adoption would produce considerable 

 confusion in the terms which have been commonly accepted 

 in the last few years. At all events in most bacteriological 

 literature the term Bacillus does not have the significance 

 above mentioned and simply refers to any rod-shaped bac- 

 terium. It should be noted also that whereas the word 

 Bacteria refers to the whole group of fission fungi which 

 we are to study, the genus Bacterium has reference only to 

 a small division of rod-shaped bacteria. 



3. Spiral Bacteria. Spirillum. In this group the rods 

 are spirally coiled to form either long or short spirals (Fig. 

 4, c). They are not so abundant as the cocci and rod forms, 

 although some of them are of importance, inasmuch as they 

 play an active part in the decay of organic tissues. They 

 are sometimes motile and sometimes stationary. The only 

 two divisions of the group that. we need to notice are as 

 follows : 



Spirillum. Ordinary spiral rods, stiff and inflexible (Fig. 

 4, c,i). 



Spirochata. Spiral rods which are flexible like a spiral 

 spring (Fig. 4, c, 2). 



4. Higher Bacteria. Cladothrix, Leptothrix, Streptothrix, 

 Actinomyces. (Fig. 7.) Under this head are included a 

 few forms of fungi which resemble other bacteria in some 

 respects, but differ in others. They are composed of 

 threads which are commonly larger than the threads of bac- 

 teria and which may show frequent branching, a character- 

 istic not usual in bacteria. They also have a peculiar 

 method of forming reproducing bodies. The group is not 

 one of very great importance. 



This classification gives only what are usually recognized 

 as the genera of bacteria. A further classification of the 

 group into species is at the present time in a condition of 

 the greatest confusion. Many hundreds of species have 



