72 MILK [vi 



jelly-like material ; and in such species as thrive in 

 moist or fluid surroundings, the outer layer of the 

 cell wall is of a jelly-like nature, and is provided in 

 some cases with minute whip -like filaments, the 

 cilia, by means of which they are enabled to swim 

 about, and in this respect resemble animal life. The 

 way in which such bacteria move differs considerably, 

 some having a circular spinning motion, while others 

 have a spiral motion, and others, again, glide along. 

 When the organism is exposed and dried, this outer 

 jelly-like layer becomes a hard crust. Those of a 

 rod -like shape are known as bacilli or bacteria, the 

 former being longer than the latter. Those possess- 

 ing a spiral or corkscrew shape are known as spirilla, 

 or when in a shorter form as -comma bacilli. The 

 term bacteria, however, is generally used in a wider 

 sense to denote the micro-organisms above enumerated. 

 In addition to these there are two other classes of 

 micro-organisms : viz. yeasts, which are comparatively 

 large oval bodies ; and the moulds so familiar to all 

 as giving rise to hairy patches on various kinds of 

 food, especially jam (see Fig. 10, p. 73). 



Method of Development of Bacteria. A point of 

 much interest, and one also of great importance, is 

 the method in which bacteria develop. This is done 

 in a variety of ways. Some grow in length, some 

 increase by splitting up into two parts, which is known 

 as fission (Fig. 11, p. 74), and some by throwing off 



