MORPHOLOGY AND PHYSIOLOGY OF BACTERH.. I07 



of the classification of bacteria, it must not be supposed that 

 the species of bacteria are not permanent. For instance, it 

 would be incorrect to imagine that the micrococci and 

 spirilla become converted into species of bacilli, or for 

 the bacilli of one species to be transmuted into those of 

 another. This does not contradict the statement that we may 

 frequently, through erroneous and imperfect information, be in 

 the habit of including unlike species under one name, or of 

 classifying mere varieties of one species as entirely different 

 species. At present it is sufficient for practical purposes to divide 

 bacteria into two great groups, the lower bacteria and the higher 

 bacteria; and to subdivide the lower bacteria into: micrococci, 

 spherical forms; bacilli, rod-shaped forms, one diameter being 

 in excess of the others; spirilla, twisted like a corkscrew, 



Sarcina;. 



making long spirals or simply parts of spirals (comma-shaped 

 forms).* 



Recent investigations indicate that several species of bacteria 

 often are closely related to one another, so as to form a well- 

 marked group. Such a group is constituted by the bacillus 

 of typhoid fever, bacillus coli communis and similar forms. 

 The spirillum of cholera and other comma-shaped spirilla 

 resembling it may be held to constitute another group. Still 

 another is that containing the tubercle bacillus, smegma bacillus 

 and other .acid-proof bacilli. 



The micrococci are subdivided into staphylococci, where 



* Migula's system of classifying bacteria has found favor with many writers. 



