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XII. 

 BACTERIA (Schizomycetes). 



UNDER these names 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 membrane, in most cases 

 devoid of chlorophyll and multiplying by transverse division. 

 The smallest are not more than ^j^nrth f an mcn m dia- 

 meter, 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 l0 Q 00 th of an inch, though they 

 may be very long in proportion. Many of them have two 

 conditions a still and an active state. In their still condi- 

 tion, 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 oscil- 

 latory, and is readily distinguishable from the rapid transla- 

 tion from place to place which is effected by the really active 

 Bacteria. 



In a large number of forms the presence of cilia has been 

 detected. In Spirillum volutans, for example, one of the 

 largest species, there is a cilium at each end of the spirally 

 coiled body. In some cases it is probable that these cilia 

 consist of protoplasm, and are really the organs by which 



