258 BACTERIOLOGY. 



placed in one class and under others in another. Thus 

 the power to produce spores may be totally lost or held 

 in abeyance for a time. 



The relations to oxygen may be gradually altered, 

 so that an anaerobic species grows in the presence of 

 oxygen. Parasitic bacteria may be so cultivated as to 

 become saprophytic varieties, and those which have 

 no power to grow in the living body given pathogenic 

 properties. 



The possibility of making any thoroughly satisfac- 

 tory classification is rendered still more difficult by the 

 fact that many necessarily imperfect attempts have 

 already been made, so that there is a great deal of con- 

 fusion, which is steadily increased as new varieties are 

 found or old ones reinvestigated and classified differ- 

 ently in the different systems. 



As one of the more successful attempts to classify 

 bacteria, the system devised by Migula is here given, 

 simply as an example. The morphology of bacteria is 

 used as the basis of the divisions : 



FAMILIES. 



I. Cells globose in a free state, not elongat- 

 ing in any direction before division 

 into 1, 2, or 3 planes . . . 1. Coccacese. 

 II. Cells cylindrical, longer or shorter, 

 and only dividing in one plane, and 

 elongating to twice the normal length 

 before the division. 



(1) Cells straight, rod-shaped, without 

 sheath, non-motile, or motile by 



means of flagella . . . .2. Bacteriaceae. 



( 2 ) Cells crooked, without sheath . 3. Spirillacese. 



(3) Cells enclosed in a sheath . . 4. Chlamydobacteriace.'e. 



(4) Cells destitute of a sheath, united 

 into threads, motile by means of an 



undulating membrane . . .5. Beggiatoaceae. 



