IV. 



BACTERIA. 



4 



UNDER the general title of Bacterium a considerable variety 

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

 included. 



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

 spirally coiled masses of protoplasmic matter enclosed in a 

 more or less distinct structureless substance, devoid of 

 chlorophyll and multiplying by transverse division. The 

 smallest are not more than 80 ^ 00 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 thick- 

 ness of little more than I0 ooo tn 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 condition, 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 

 spirally coiled body. No such structure, however, can be 

 made out in the straight Bacteria, and it remains doubtful 

 whether they possess cilia which are too fine to be rendered 



