86 BACTERIA LESS. 



its numbers increase those of Bacterium termo diminish, 

 until Bacillus becomes the dominant form. Its cells (Fig. 

 1 6) are rod-shaped and about 6/x ( T y<) mm.) in length in the 

 commonest species. Both motionless and active forms are 

 found, the latter having a flagellum at each end. The 

 zooglaea condition ?.s often assumed, and the rods are fre- 

 quently found united end to end so as to form filaments. 



Vibrio resembles Bacillus, but the rod-like cells (Fig. 1 7, A) 

 are wavy instead of straight. They are actively motile and 

 when highly magnified are found to be provided with a 



"**: 



FIG. 15. Micrococcus. I, single and double (dumb-bell shaped) 

 forms : 2 and 3, chain-forms : 4, a zooglaea. 



flagellum at each end. Vibriones vary from 8/x to 25^, in 

 length. 



Spirillum is at once distinguished by its spiral form, the 

 cells resembling minute corkscrews (Fig. 17, B &: c) and 

 being provided with a flagellum at each end (c). The 

 smaller species, such as S. tenue (B) are from 2 to 5 /x in 

 length, but the larger forms, such as S. volutans (c) attain a 

 length of from 25 to 3o/x. In swimming Spirillum appears 

 on a superficial examination to undulate like a worm or a 

 serpent, but this is an optical illusion : the spiral is really a 

 permanent one, but during progression it rotates upon its 



