CHAP. XII.] BACTERIA. 409 



In other cases again, however, there is some reason to 

 believe that the so-called cilia are simply prolongations of 

 the cell-membrane, and destitute of any power of inde- 

 pendent movement. In the latter case the movement of 

 the organism must be due to some other cause, such as the 

 contractility of the protoplasmic body as a whole. Many 

 forms, such as the Vibrio7ies^ so common in putrefying 

 matters, appear obviously to have a wriggling or serpenti- 

 form motion, but this is an optical illusion. In this Bac- 

 terium the body does not rapidly change its form ; but its 

 joints are bent zigzag-wise, and the rotation of the zig-zag 

 upon its axis, as it moves, gives rise to the appearance of 

 undulatory contraction. A cork-screw turned round, while 

 its point rests against the finger, gives rise to just the same 

 appearance. 



In other forms, however, such as Spirillum, true contrac- 

 tile curvatures have been observed. 



A nucleus has not yet been detected in any of these 

 plants. The delicate membrane of the Bacteria is in all 

 cases surrounded on the outside by a gelatinous layer, often 

 difficult of detection. In the still state, however, this gela- 

 tinous substance is often developed to a very large extent, 

 forming a continuous mass of considerable size, in which 

 the individual cells appear embedded. This is termed the 

 Zooglcea form of the Bacteria. The membrane of the Bac- 

 teria, with its gelatinous layers, corresponds to the cell-wall 

 of Protococcus or yeast. Its reactions, however, vary in 

 different cases, sometimes agreeing with those of cellulose, 

 and sometimes indicating an albuminous composition. 



All forms of Bacteria, of which the life-history has been 

 fully investigated, are found to produce resting spores. 

 These bodies make their appearance when the supply of 

 food has become exhausted or the conditions have otherwise 



