MORPHOLOGY AND PHYSIOLOGY OF BACTERIA. 109 



The word "bacterium" was formerly used to designate short 

 bacilli which generally formed no spores, while the word bacillus 

 was restricted to the longer forms in which spore formation 

 occurred. This use is no longer common, although not rarely 

 the name bacterium is still given to a species- — for instance, 

 bacterium coli commune. 



Spirilla present a very great variety of form. The short 

 "comma-shaped bacilli" are only parts, at most, of spirals, 

 although the microbes of cholera do sometimes form long 

 spirals. On the other hand, there are among spirilla large 

 and long sinuous figures which present most remarkable 



Fig. 42. — Involution Fobms of the Spirillum of Cholera. — {Van 

 Ermengem.) 



pictures under the microscope; for example, the spirillum 

 of relapsing fever. Spirilla without very marked windings are 

 sometimes called " vibrios " ; and long, wavy forms with cork- 

 screw-like windings " spirochcRtce "; and only the rigidly spiral 

 forms "spirilla." 



Besides the purely morphological classification already 

 mentioned, bacteria are sometimes grouped according to certain 

 physiological qualities. In general botany, saprophytes are 

 plants that grow on decaying vegetable matter. In a bacterio- 

 logical sense, saprophytes are bacteria which grow in external 

 nature on dead organic matter, and parasites are bacteria 



