4 MANUAL OF BACTERIOLOGY. 



which divide by fission. They are sometimes called schiz- 

 omycetes. In every-day language they are known as microbes 

 and also as germs. They are generally classed with the fungi. 

 In some respects they seem quite closely related to the algae or 

 simplest green plants, and, on the other hand, they have strong 

 points of likeness with some of the unicellular animals belong- 

 ing to the infusoria. 



Bacteria are divided into two great groups: 



I. The lower bacteria include those forms which are of most 



^^ importance at present, from a medical 



.!"jy.. ^^*v \ ^ pj and economic point of view, and con- 



> sist of : 



Micrococci. Bacilli. Spirilla. Cocco-baciUi—mostlj short, thick, 



oval forms which also occur in rods. 



Micrococci, or cocci (singular, coccus) — spherical forms. 



Bacilli (sing., bacillus) — long and straight, or rod-shaped 

 bacteria. 



Spirilla (sing., spirillum) — consisting of spiral filaments like 

 the turns of a corkscrew, or parts of spirals shaped like commas. 



II. The higher bacteria, which consist of long filaments 

 made up of segments more or less united. In some of these the 

 filaments show dichotomous branching. This group is more 

 fully discussed under the non-pathogenic bacteria. Part III. 

 A few of them are pathogenic. 



The extreme smallness of the bacteria is hard of compre- 

 hension. We may say, of most of them, that from S,ooo to 

 25,000 placed end to end would make a line about an inch in 

 length. The tiny speck which adheres to the end of a fine 

 platinum wire when this is used to obtain preparations from 

 cultures is found upon examination with the microscope to con- 

 sist of many thousands of bacteria. 



It is well known that bacteria are present on most of the 

 objects about us. They occur on the skins of men and other 

 animals as well as in the mouth, stomach and intestines, and 



