Size and Motility 17 



the spiral forms as spirilla. Besides these, there are 

 also certain groups of bacteria that occur as chains, 

 or aggregations of individuals, encased in sheaths, 

 from which they may escape and go through the 

 process of multiplication. These organisms are well 

 represented by certain classes of the so-called iron- 

 bacteria. 



Size. — There are great differences in the size of bac- 

 teria of the same species. Still greater differences in 

 size, however, exist between those of different species. 

 The spherical bacteria have an average diameter of 

 about g 5 i of an inch, but those with a diameter of 

 60000 of an inch are not uncommon. The rod-shaped 

 bacteria and spirilla attain, in some instances, a much 

 greater size, — up to toVo of ^^ i^.^^ in length — although 

 such dimensions are very exceptional. The anthrax 

 bacillus, a fairly large organism, is about -grroo" of an 

 inch wide, and 25000 to 25000 of an inch long. 



There undoubtedly exist bacteria so small as to 

 be practically invisible with the highest magnification. 

 We know, at any rate, that some of them pass through 

 unglazed porcelain filters whose pores are so small as 

 to prevent the passage of ordinary bacteria. 



Motility and organs of motion. — The bacterial body 

 consists of a cell-wall surrounding the protoplasm. 

 Protoplasm is the living substance of the organism. 

 It may present a homogeneous appearance under the 

 microscope, or it may be granular in structure. The 

 cell-wall and the protoplasm within it are not of the 

 same composition. Attached to the cell-wall are the 

 bacterial organs of motion known as flagella (Fig. 7). 



