BACTERIOLOGICAL INTRODUCTION 19 



comma form is most usual, the spirilla form appearing 

 only when the medium is so moist that the con- 

 ditions resemble those of a fluid culture. In these 

 latter (Fig. 10) the spirillum form is most common, 

 and organisms of great length are found. In the 

 right half of Fig. 10 the breaking down of the long 

 spirillum with the formation of the derivative forms 

 (comma bacilli and vibrios) is seen, and, in the lower 

 portion, the remarkable flattening out of the spirals, 

 which makes it difficult, not unfrequently, to distin- 

 guish these flattened spirals from bacilli. 



Some of the cocci and a very large number of the 

 bacilli and spirilla are motile. The activity of the 

 movement varies greatly in different species, and in 

 the same species under different circumstances. The 

 bacilli appear to progress by rapid side-to-side or 

 lashing movements ; the spirilla, on the other hand, 

 by a rotary movement round the long axis. These 

 movements are due to the presence of flagella. The 

 flagella are threadlike, wavy prolongations of the 

 protoplasm of the cell, and are many times the 

 length of the organisms to which they are attached. 

 Their number and arrangement are very variable. 

 Some organisms have a single flagellum situated at 

 one pole of the cell (monotricha), as shown in 

 Figs. 78 and 87 ; others a single thread at each 



