14 BIOLOGY AND TECHNIQUE 



adhere to the loop. Exactly what the significance of the capsules 

 is cannot yet be decided. 



There is, however, definite reason to believe that there is a direct 

 relation between virulence and capsulation ; capsulated bacteria are 

 less easily taken up by phagocytes than are the iion-capsulated mem- 

 bers of the same species. Also, as Porges and others have shown, 

 capsulated organisms are not easily amenable to the agglutinating 

 action of immune sera. Many bacteria (plague, anthrax) which are 

 habitually uncapsulated on artificial media acquire capsules within 

 the infected animal body. Also in some species (pneumococci), the 

 loss of capsule formation as cultivated on the simpler media is accom- 

 panied by a diminution of virulence. 



Organs of Locomotion. When suspended in a drop of fluid many 

 bacteria are seen to be actively motile. It is important, however, in 

 all cases to distinguish between actual motility and the so-called 

 Brownian or molecular movement which takes place whenever small 

 particles are held in suspension in a fluid. 



Brownian or molecular movement is a phenomenon entirely ex- 

 plained by the physical principles of surface tension, and has abso- 

 lutely no relation to independent motility. It may be seen when 

 particles of carmine or any other insoluble substance are suspended 

 in water, and consists in a rapid to and fro vacillation during which 

 there is actually no permanent change in position of the moving par- 

 ticle except inasmuch as this is influenced by currents in the drop. 



The true motility of bacteria, on the other hand, is active motion 

 due to impulses originating in the bacteria themselves, where the 

 actual position of the bacterium in the field is permanently changed. 



The ability to move in this way is, so far as we know, limited 

 almost entirely to the bacilli and spirilla, there being but few in- 

 stances where members of the coccus group show active motility. In 

 all cases, with the exception of some of the spirochetes, where mo- 

 tility may occasionally be due to an undulating membrane margin- 

 ally placed along the body, bacterial motility is due to hair-like 

 organs known as flagella. These flagella have rarely been seen during 

 life, and their recognition and study has been made possible only by 

 special staining methods, such as those devised by Loeffler, van 

 Ermengem, Pitt, and others. 



In such stained preparations, the bacterial cell bodies often 

 appear thicker than when ordinary dyes are used, and the flagella 

 apparently are seen to arise from the thickened ectoplasmic zone. 



