CLASSIFICATION. 13 



those species which only form rod-shaped cells, and filaments com- 

 posed of rod-like segments ; or straight filaments not distinctly seg- 

 mented, which may be rigid or flexible. 



The SPIRILLA are also included in a single genus, embracing all 

 of those species in which the filaments are spiral in form and the 

 segments more or less spiral or "comma-shaped" filaments either 

 rigid or flexible. 



This simple morphological classification of the monomorphous 

 group of Baumgarten corresponds with the nomenclature now gene- 

 rally in use among bacteriologists. We speak of the spherical bac- 

 teria as cocci or as micrococci, of the rod-shaped bacteria as bacilli, 

 and of the spiral bacteria as spirilla. 



It is true, however, that we are sometimes embarrassed to decide 

 whether a particular microorganism belongs to one or the other of 

 these morphological groups or so-called genera.^ Aihong the bacilli, 

 for exainple, we may have, in the same pure culture, rods of very 

 different lengths, some being so short that if alone they might be 

 taken for cocci, while others may have grown out into long fila- 

 ments. But if we are assured that the culture is pure the presence 

 of rod forms establishes the diagnosis, and usually the cocci-like 

 elements, when carefully observed, will be seen to be somewhat 

 longer in one diameter than in the other. The German bacterio- 

 logists generally insist upon placing among the bacilli all straight bac- 

 teria in which, as a rule, one diameter is perceptibly greater than 

 that transverse to it ; and several species of well-known bacteria 

 which were formerly classed as micrococci are now called bacilli 

 e.g., Friedlander's bacillus ("pneumococcus"), Bacillus prodigiosus. 



The distinction made by Cohn and others between the genus 

 Bacterium (Duj.) and the genus Bacillus (Cohn) cannot be main- 

 tained, inasmuch as we may have short rods and quite long fila- 

 ments in the same pure culture of a single species ; and, again, the 

 character upon which the genus Vibrio (Ehr.) was established 

 viz., the fact that the filaments are flexible and the movements 

 sinuous is not a sufficient generic or even specific character, for in 

 a pure culture there may be short rods which are rigid, and long 

 filaments which are flexible and have a sinuous movement. We 

 therefore to-day speak of all the elongated forms as bacilli, unless 

 they are spiral and have a corkscrew-like motion, in which case they 

 are known as spirilla. 



The bacteria are also classified according to their biological char- 

 acters, and it will be necessary to consider the various modes of 

 grouping them from different points of view other than that which 

 relates to their form. This is the more important inasmuch as we 

 are not able to differentiate species by morphological characters 



