576 Introduction to the Study of Science 



the group is called streptococcus (Fig. 171 b). If reproduction 

 occurs in two planes with such a mass as (c), the group is micro- 

 coccus. Other forms of groups as in (d) are produced, de- 

 pending especially upon the nutritive conditions. 



Bacteria of still different forms may be discovered in the 

 microscopic field. Many types assume a cylindrical, or rod- 

 like form (e), which may be either straight or bent. The bacteria 

 found in sour milk are rod-shaped of different lengths. But 



o 



00 



FIG. 171. Typical forms of bacteria: a, 6, c, and d, typical groups of cocci ; 

 e, straight rod-shaped bacteria and ways of grouping ; / and g, spiral forms, /, 

 the vibrio of cholera ; h, motile forms, often called bacilli, equipped with flagella 

 as means of locomotion. Compare these forms of bacteria with the protozoa, 

 Fig. 172. 



the extremely short ones are single bacteria, and the longer ones 

 are short rods joined together end to end. One may find a 

 short rod dividing itself into two sections. This is the usual 

 manner of reproduction. Continued observation will reveal 

 the full growth and reproduction of the minute plants, for this 

 may easily occur within an hour. 



The rod-shaped bacteria are divided into two classes, the 

 motionless and the motile. The motile has organs of locomotion 

 called flagella, exceedingly fine, hair-like extensions which are 

 curved in various directions. It is impossible to detect the 

 flagella without staining and a very high-power microscope. 

 It is customary to call all motile bacteria bacilli (bacillus, 

 singular). An example is the typhoid bacillus (Fig. 171 h). 



