VI] GENERAL CHARACTERS OF BACTERIA 119 



exception, since this species is motile {Centralbl. fur Bad. 

 VI. 2). 



In aerobic bacilli and spirilla which are possessed of 

 motility this is intimately connected with a supply of 

 oxygen. Though some species seem to obtain this readily 

 even when in deep fluids (e.g. bacillus of hay, certain species 

 of proteus), many others cease to move when the supply of 

 oxygen becomes insufficient. Engelmann has made some 

 very interesting experiments with certain motile bacilli, 

 showing the direct influence of oxygen on their motility. 

 When motile bacilli, owing to insufficient oxygen or after 

 the consumption of the oxygen previously present, come to 

 rest, by adding to them new oxygen in a drop of fresh fluid 

 containing air, the motility is resumed. On removing the 

 oxygen and adding carbon dioxide or hydrogen gas, am- 

 monia, chloroform, or ether, the movement ceases, but on 

 removing these gases and replacing them again by oxygen 

 (or air) the movement is again resumed. 



Motile baciUi and spirilla when growing in a fluid medium 

 have a great tendency to seek the surface of the fluid — i.e. 

 move towards the part where they can obtain oxygen, and 

 here form more or less coherent pellicles, in which they are 

 in a resting state, and in which a rapid multiplication goes 

 on ; but it is quite incorrect to assume that an organism 

 which in a fluid medium forms a pellicle is a motile 

 organism, since some species which form a pellicle are not 

 motile, and some species of motile organisms do not form a 

 pellicle. 



On making a comparative study of the presence of 

 flagella best by v. Ermengem's method, two things will be 

 found of interest : (1) that there are flagella present even 

 in bacilli which in the fresh state show no locomotion 

 or only a very feeble one ; and (2) that the length and 



