BACTERIA 5 



to think of them as a group of single-celled organisms probably 

 representing primitive forms that existed before differentiation 

 into animal and vegetable kingdoms occurred. 



Classification. Bacteria may be divided into two subgroups, 

 a lower and a simpler form and a higher and more developed one. 

 The members of the lower form are minute masses of protoplasm 

 surrounded by an envelope^ each cell a living unit containing 

 all the vital capacities of an independent organism. 



Although there are hundreds of different species there are only 

 three general forms, spheres (cocci), rods (bacilli), and spirals 

 (spirilla). The spheres may be large or small, and may group 

 themselves differently; the rods may be long or short, thick or 

 slender, the ends may be rounded or sharply rectangular ; the spirals 

 may be flexible or stiff, they may have one, two, or many coils, 

 but still spheres, rods, and spirals comprise all types. So far as 

 is known, it is never possible by any means to permanently change 

 the form of the members of one group to that of another ; that 

 is, under suitable conditions cocci always produce cocci, bacilli 

 always produce bacilli, and spirilla always produce spirilla. 



The higher bacteria (trichobacteria) show an advance on the 

 lower forms in that they consist of united segments, branched or 

 unbranched, which are surrounded by a sheath and which are 

 more or less interdependent. 



Transition forms no doubt exist between the lower and higher 

 forms, and for this reason certain organisms are difficult to classify. 

 The tubercle bacillus, for example, under ordinary conditions is a 

 typical rod but sometimes it produces branching filaments, and 

 for that reason it is classed by some writers with the higher bac- 

 teria. 



THE LOWER BACTERIA 



Terminology. The terms microbe, microorganism, and germ 

 are frequently used to designate bacteria ; they may, however, be 

 applied to any form of microscopic life. 



The name bacterium is given to any single member of the group 

 of bacteria, regardless of its own form. 



