86 BACTERIA less. 



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

 1 6) are rod-shaped and about 6/x (^1^ nun.) in length in the 

 commonest species. Both motionless and active forms are 

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

 zooglcea condition is often assumed, and the rods are fre- 

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



r/,(^/'/((7 resembles Bacillus, but the rod-like cells (Fig. 17, a) 

 are wavy instead of straight. They are actively motile and 

 when highly magnified are found to be provided with a 



^••"i 



P"lG. 15. — Micrococcus. I, single and double (dumb-bell shaped) 

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



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

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

 length of from 25 to 30/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 



