770 BACTERIA 



Besides these normal forms, all bacteria when subjected for 

 some time to conditions unfavourable to their nutrition, assume 

 irregular shapes, such degenerate or malformed cells being 

 designated involution-forms. 



Although variable in size, the bacteria are among the smallest 

 of living things known, the largest coccus being less than a ten- 

 thousandth of an inch in diameter. 



The cell-wall is a firm thin membrane, most frequently com- 



y^' 



°°i" 



c 



/V I 



Fig. 261. — Diagrammatic representation of the methods of vegetative 

 reproduction among common bacteria. 



a A bacillus successively dividing at a^ a , and a"'- 



l> A coccus giving rise to chains, ^*" {^Streptococci)^ pairs, h {Diplo- 



cocci)y and irregular groups, ^ {Staphylococci). 

 c A coccus giving rise by division in two directions to c"' {Micrococci) 

 and by division in three directions to c^^ {Sarcime). 



posed of an albuminoid substance — not cellulose — and the 

 protoplast within appears to be devoid of a definite nucleus. 



In some instances the outer layer of the cell-wall absorbs 

 water and swells like mucilage ; in such cases large numbers of 

 individual bacteria often become joined together and form 

 irregular gelatinous masses, known as zooglcea. 



Many bacteria have hair-like flagella or cilia (6, Fig. 262) at- 

 tached to the exterior of the cell-wall, by means of which they 

 are able to swim freely in water and other liquids. 



2. Vegetative Reproduction. — The method of vegetative 

 reproduction, which is so characteristic of the whole group of 



