IV. 



BACTERIA. 



Under the general title of Bacterium a considerable variety 

 of organisms, for the most part of extreme minuteness, are 

 included. 



They may be defined as globular, oblong, rod-like or 

 spirally coiled masses of protoplasmic matter enclosed in a 

 more or less distinct structureless substance, devoid of chloro- 

 phyll and multiplying by transverse division. The smallest 



are not more than th of an inch in diameter, so that 



under the best microscopes they appear- as little more than 

 mere specks, and even the largest have a thickness of little 



more than th of an inch, though they may be very long 



in proportion. Many of them have, like Protococcus, two 

 conditions — a still and an active state. In their still con- 

 dition, however, they very generally exhibit that Brownian 

 movement which is common to almost all very finely divided 

 solids suspended in a fluid. But this motion is merely 

 oscillatory, and is readily distinguishable from the rapid 

 translation from place to place which is effected by the really 

 active Bacteria. 



In one of the largest forms, Spirillum volutans, it has been 

 possible to observe the cilia by which the movement is 

 effected. In this there is a cilium at each end of the 



