METHODS OF STUDYING BACTERIA 33 



river or drainage-water, or where microbes adherent to dust- 

 particles or the like are carried by the wind or in some other 

 way. In a suitable stationary fluid, the actively motile germ 

 of typhoid fever can travel about four millimetres (a little 

 less than a sixth of an inch) in an hour ; whilst the extremely 

 active cholera organism may cover something like eighteen 

 millimetres, or about two-thirds of an inch, in a similar time. 

 From this it will be seen that the practical question of the 

 passage of bacteria from place to place, e.g. in the spread of 

 epidemics, is one of passive carriage in water or milk or dust, 

 in contaminated clothing or excretions or the like, rather 

 than of active locomotion by the bacteria themselves. 



Motility is usually, though not always, due to the bacterium 

 possessing one or more, and sometimes numerous, long, deli- 

 cate, vibratile, hair-like organs called flagella (" little whips ") 



Fia. 9. Examples i Flagellated Organisms, (a) Cholera 

 Vibrio with a single terminal flagellum. (&) Vibrio 

 of another species with a terminal flagellum at 

 each end. (c) and (d). B. Coli and B. typhosus, with 

 many flagella all round, those of the latter being 

 most numerous. 



or cilia ("hairs"), by means of which it swims in the fluid; 

 and there is often also an active wriggling eel- or snake-like 

 movement of the body of the organism as well. 



Some bacteria, for example the Pneumococcus (which causes, 

 amongst other diseases, acute lobar pneumonia), possess what 

 is called a capsule, sometimes of considerable thickness. This 

 investment is probably formed by a modification of the outer 

 part of the cell-wall, and is doubtless protective in its function. 

 Many bacteria also secrete or produce a viscid or jelly-like 

 material resembling mucus, amongst which they are found 

 embedded, and which often serves to glue them into masses ; 

 and this intercellular substance, as it is called, may partly 

 account for the arrangement of bacteria into the characteristic 

 grouping illustrated in the Frontispiece, figs. 1 to 6. 



The method and rate of reproduction of bacteria are matters 

 of great interest ; and we have already seen that, as their 

 scientific name, Schizomycetes or fission-fungi, implies, it is 

 by direct division, fission or splitting septation, as it is some- 

 times called that multiplication occurs. Under favourable 



