4 BACTERIA 



(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 unbranched 

 threads by means of delicate sheaths, branching threads 

 are then formed. Division takes place in three directions 

 of space, producing sarcina-like groups of gonidia, 

 which, when free, are spherical. 



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

 flagella, divided only in one direction^ and contain sulphur 

 granules. 



VI. BEGGIATOACE^:. Cells united to form threads that are not 

 sheathed: have scarcely visible septa; divide in one direction, 

 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 bio- 

 logical behavior in culture media and in the tissues of animals un- 

 der 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 observation of the development of bacteria in culture 

 media, and the reactions produced in it, and in the bodies of ex- 

 periment animals, that we can identify them positively from others 

 of a foreign species. No bacteriologist is able by a simple micro- 

 scopical examination of a given bacterium, to identify it absolutely 

 at all times. 



The higher groups of fungi may be classified conveniently as 

 follows : 



