4 BACTERIA 



whole branches, and by swarm spores or motile gonidia 

 having flagella. 



(b) Crenothrix. Filaments are fixed to a nutrient base. 

 Are usually thinner at the base than at the apex, formed 

 of unbranched threads that divide in three directions of 

 space, and produce in the end two kinds of gonidia, 

 probably of bisexual nature. 



(c) Phragmidiothrix. Cells are first united into un- 

 branched threads by means of delicate sheaths, branching 

 threads are then formed. Division takes place in three 

 directions of space, producing sarcina-like groups of goni- 

 dia, which, when free, are spherical. 



(d) Thiothrix. Are unbranched cells, sheathed, without 

 flagella, divided only in one direction, and contain sulphur 

 granules. 



VI. BEGGIATOACE^E. Cells united to form threads that are 

 not sheathed: have scarcely visible septa; divide in one direc- 

 tion, and motile only by an undulating membrane, not by 

 flagella. 

 (a) Beggiatoa. Cells containing sulphur granules. 



Bacteria may furthermore be classified according to their biolog- 

 ical characteristics, which may be wonderfully different. The ulti- 

 mate differentiation of one species from another depends not only 

 on the morphology, which may be precisely similar, but on its biolog- 

 ical behavior in culture media and in the tissues of animals under 

 identical conditions. Again, different individuals of a given species 

 may vary extraordinarily one from another in form and size, yet the 

 chemical behavior is invariably the same. Hence it is only by obser- 

 vation of the development of bacteria in culture media, and the re- 

 actions produced in it, and in the bodies of experiment animals, that 

 we can identify them positively from others of a foreign species. No 

 bacteriologist is able by a simple microscopical examination of a 

 given bacterium, to identify it absolutely at all times, -y^ 



