8 BACTERIA 



Structure and For^n, Having now located micro-organisms 

 in the economy of nature, we may proceed to describe their 

 subdivisions and form. For practical convenience rather 

 than academic accuracy, we may accept the simple division 

 of the family of bacteria into three chief forms, viz.: — 

 ( (i) Round cell form — coccus. 



Lower Bacteria \ (2) Rod form — bacillus. 



( (3) Thread form — spirillum. 



Higher Bacteria — Leptothrix, Streptothrix, Cladothrix, etc. 



A classification dependent as this is upon the form alone 

 is not by any means ideal, for it ignores all the higher and 

 complicated functions of bacteria, but it is, as we have said, 

 practically convenient. 



I. The Coccus. This is the group of round cells. They 

 vary in size as regards species, and as regards the conditions, 

 artificial or natural, under which they have been grown. 

 Some are less than -j^-J-fj-g- of an inch in diameter ; others are 

 half as large again, if the word large may be used to describe 

 such minute objects. No regular standard can be laid down 

 as reliable with regard to their size. Hence the subdivisions 

 of the cocci are dependent not upon the individual elements 

 so much as upon the relation of those elements to each other. 

 A simple round cell of approximately the size already named 

 is termed a micrococcus {^lupo^^ small). Certain species of 

 micrococci always or almost always occur in pairs, and such 

 a combination is termed a diplococcus. Some diplococci are 

 united by a thin capsule, which may be made apparent by 

 special methods of staining; of others no limiting or uniting 

 membrane can be seen with the ordinary high powers of the 

 microscope.' Again, one frequently finds a species which is 

 exactly described by saying that two micrococci are in con- 

 tact with each other, and move and act as one individual, 

 but otherwise show no alteration ; whilst others are seen 



^ A one-twelfth oil immersion lens is requisite for the study of the lower 

 bacteria. 



