THE BEHAVIOR OF BACTERIA 27 



made. In some bacteria the two ends are similar, and movement may 

 take place in either direction. In others the two ends differ, one bearing 

 flagella, while the other does not. In these species the movement is still 

 further determined ; the end bearing the flagella is anterior in the usual 

 locomotion. In none of the bacteria can we distinguish upper and lower 

 surfaces or right and left sides. As the bacterium swims, it revolves 

 continually on its long axis; the significance of this revolution will be 

 considered in our account of behavior in the infusoria. 



2. REACTIONS TO STIMULI 



The movements of the bacteria are not unordered, but are of such a 

 character as to bring about certain general results, some of which at 

 least are conducive to the welfare of the organism. If a bacterium 

 swimming in a certain direction comes against a solid object, it does 

 not remain obstinately pressing its anterior end against the object, but 

 moves in some other direction. If some strong chemical is diffusing in 

 a certain region, the bacteria keep out of this region (Fig. 24). They 

 often collect about bubbles of air, and about masses of decaying animal 

 or plant material. Often they gather about small green plants (Fig. 25), 

 and in some cases a large number of bacteria gather to form a well- 

 defined group without evident external cause. 



How are such results brought about? To answer this question, we 

 will examine carefully the behavior of the large and favorable form, 

 Spirillum 1 (Fig. 23, c). Spirillum is a spiral rod, bearing a bunch of 

 flagella at one end. In a thriving culture a large proportion of the indi- 

 viduals bear flagella at both ends and can swim indifferently in either 

 direction. It is said by good authorities that such specimens are pre- 

 paring to divide. 



When Spirillum comes against an obstacle, it responds by the sim- 

 plest possible reaction, by a reversal of the direction of movement. 

 In specimens with flagella at each end the new direction is continued 

 till a new stimulation causes a new reversal. In bacteria with flagella 

 at only one end, the movement backward is continued only a short time, 

 then the forward movement is resumed. Usually when the forward 

 movement is renewed, the path followed is not the same as the original 

 path, but forms an angle with it ; the bacterium has thus turned to one 

 side. Whether this turning is due to currents in the water or other 



1 There are several species of Spirillum found in decaying organic matter. The species 

 have not been clearly determined in most of the work on behavior, and this is not of great 

 importance, as the behavior is essentially the same in character throughout. 



