Flagella. Zooglcea 19 



These may be short or long; at times they are much 

 longer than the body of the organism. 



Different species show very marked variations in 

 the number of flagella and their arrangement on the 

 body. There are a number of important organisms, 

 among them the nodule bacteria of legumes, that 

 possess only a single flagellum at one end. Such organ- 

 isms are, therefore, designated as pseudomonas, while 

 those with one flagellum at each end are classed as 

 microspira. The spirilla frequently possess a bundle 

 of flagella at each end, while the rod-shaped bacilli 

 may have them attached at any place on the body. 



The bacteria capable of moving about in a liquid 

 are called motile; the others, non-motile. The latter 

 include species which have, in some instances, been 

 demonstrated to possess flagella. The form of the organ- 

 ism has, also, some relation to motility, for it has been 

 found that among the spherical bacteria, motility 

 is comparatively rare; whereas, among the rod-shaped 

 species, this property is common to many. 



Zooglcea. — In some instances, the cell-wall of the 

 bacteria is surrounded by a zone of a gum-like substance, 

 which may encourage the aggregation of cells into 

 irregular^ masses, called zooglcea. The formation of 

 zooglcea may occur in solid, as well as in Hquid, media. 

 Membrane formation on the surface of liquid media 

 is characteristic of species that require the unhindered 

 access of air for their development. 



Rate of increase. — Bacteria multiply by splitting 

 into halves. This process may be completed in one- 

 half hour; at times, even more rapidly. Under less 



