12 BACTERIA ! FORMS AND REPRODUCTION 



vibrio ; a rigid spiral form in shape like a corkscrew is 

 a spirillum. 



The terms bacterium and its plural bacteria are fre- 

 quently used in a general sense when speaking of one or 

 more or any of these minute organisms without reference 

 to their form. The word bacterium is, however, used 

 also in a limited sense to denote a 

 particular non-motile rod-like cell which 

 is shorter than a bacillus. Under certain 

 circumstances bacterial cells may become 

 distorted and irregular in shape. Such 

 FIG. 6. involution are known as involution-forms (Fig. 6), 



forms of a bacterium , . . , . . ,, , . , 



and are met with chiefly where the 

 medium in which they are living is becoming exhausted 

 of its nutritive substances, or where there is an accumu- 

 lation of compounds which are detrimental to the 

 vitality of the organisms : they are diseased, degenerate 

 forms. 



The capsule or skin surrounding the central body 

 of a bacterium as a rule is very thin, 

 and as it does not readily absorb dyes 

 it is rarely visible. In some species, 

 however, the capsule is of a mucil- 

 aginous or slimy nature, and swells up 

 with water to a thickness of several 

 times that of the rest of the cell (Fig. 7). FIG 7 ._ 

 It may then be faintly stained in several weii-deveioped capsules, 

 ways, and can be seen as a pale cell-wall surrounding 

 the darker central body of the bacterium. Very 

 frequently the swollen envelopes of large numbers of 

 bacteria adhere to each other, and form a gelatinous 

 mass or zooglcea colony of variable size, which is 

 often met with as a slimy skin-like " scum " on 



