CHAP. XII.] BACTERIA. 409 



the movement is effected. In other Bacteria however, 

 there is reason to believe that the so-called cilia are sim ply- 

 prolongations of the cell-membrane, and destitute of any 

 power of independent 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 Vibriones^ so common in 

 putrefying matters, appear obviously to have a wriggling or 

 serpentiform motion, but this is an optical illusion. In this 

 Bacterium 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 whicli 

 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 



