THE WORLD OF MICROSCOPIC LIFE 51 



type is the rod-shaped form called bacilli (Latin: 

 bacillus — rod) ; then there are the spherical cocci, 

 and the spiral bacteria (the spirilli and spirochetes) 

 and sometimes forms of peculiar shape. Bacteria 

 have no clearly-defined nucleus and they present 

 but little differentiation of internal structure, al- 

 though their very small size would probably pre- 



■ l D 6°8 



Fig. 5. Forms of Bacteria. A, staphylococci; B, streptococci; 

 C, bacilli of anthrax ; D, bacilli of the plague ; E, Spiroch&ta 

 pallida; F, tetanus bacilli showing spores in one end; G, the 

 typhoid bacillus. 



elude us from observing it even were it present. 

 Sometimes there are whip-like appendages or 

 flagella, which are employed in locomotion, al- 

 though many forms move about actively without 

 possessing any external appendages at all. 



All the bacteria are invisible to the naked eye. 

 It is impossible to specify their minimum size; 

 even with the most powerful microscopes there are 

 forms which can barely be seen, and it is practically 

 certain there are others which are too minute to be 



