18 BACTERIOLOGY. 



times be almost spherical, at other times rod-shaped. 

 Again, the term vibrio is applied to certain bacteria which 

 may form spirals, but which commonly grow in segments 

 of a spiral. They may therefore be considered as bent, 

 twisted rods which appear under the microscope as comma- 

 like forms. The union of two of these "comma bacilli" 

 gives an elongated S form, and when more of these elements 

 unite thus they give rise to a spirillum. Long, slender, 

 flexible spirals are frequently designated as spirochcetes. 

 Additional terms will be met with when describing the 

 various characteristics of growth of bacteria. 



FIG. 2 a Bacterium; b Vibrio; c Spirochaete. 



The above classification is based upon the external 

 forms of bacteria, and is therefore artificial in character. 

 A similar classification applied to the higher forms of 

 life would lead to gross error. Thus, the worm, eel and 

 snake, viewed at a distance, show the same general appear- 

 ance, and yet they are wholly different, unrelated types of 

 life. A close inspection at once reveals striking differences. 

 In the case of bacteria, owing to the extremely small size 

 and the simplicity of the bacterial cell, it is not possible to 

 establish structural differences, and for that reason the 

 form classification, imperfect as it may be, is of necessity 

 adopted. 



The various species of micrococci will show differences 

 in their size, that is in their diameter, but otherwise they 

 closely resemble one another. Occasionally, when micro- 

 cocci grow in pairs they may show flattened, apposed sur- 

 faces, as if two biscuits were brought together. In other in- 

 stances they may be elongated or lance-shaped (see fig. 10 a). 

 In the rod-shaped bacteria or bacilli greater differences will 



