MICEOBES, OE BACTEEIA. 89 



be seen swimming witli it in the field of the microscope, 

 we are struck by the diiference. Infusoria come and 

 go, swiftly or slowly — they go back or move to the 

 right or left ; in a word, their movements seem to be 

 actuated in some sense by will. Nothing like this 

 is observed in the bacterium. The undulatory move- 

 ment by which it is animated is always the same, and 

 impels it straightforward, like a stone sent from a 

 sling ; it never voluntarily goes back nor out of its 

 course, but only under the influence of a foreign im- 

 pulse, such as contact with another bacterium, when it 

 rebounds, just as a projectile may rebound from a wall. 

 On encountering an obstacle, the bacterium remains 

 indefinitely undulating before it, without ever pausing 

 or showing signs of fatigue, until some external cause 

 comes to release and send it to the right or left. We 

 may often see a tangled mass of bacteria, perhaps 

 adhering by their cilia or by some other substance, in 

 which aU the. individuals continue to undulate until 

 the rupture of the mass permits them to depart in aU 

 directions. These organisms are therefore plants in the 

 character of their movements, as well as in the rest of 

 their organization. 



In bacteria each cell consists of a cellulose wall, 

 containing protoplasm, as we saw was the case in fer- 

 ments. The multiplication by fission is efiected in 

 precisely the same way in bacteria and ferments, and 

 so also is the formation of spores. Under certain 

 circumstances, when the licLuid on which they subsist 



