BEHAVIOR OF UNICELLULAR ORGANISMS. lOI 



It thus appears that the direction in which a Paramecium 

 turns after stimulation by a chemical substance is not deter- 

 mined by the position of the stimulating agent, nor indeed by 

 any external factor, but by an internal factor, — by structural 

 differentiations of the animal's body. This is demonstrated in 

 a striking manner by immersing the Paramecia directly into a 

 chemical solution of such a nature as to act as a stimulus. 

 The entire surface of the animal is then bathed by the chemi- 

 cal, so that there is nothing in the external conditions to deter- 

 mine in which direction the animal shall move. Nevertheless, 

 under these circumstances, it swims backward, turns toward 

 the aboral side, and swims forward, usually repeating the 

 operation indefinitely. Very striking is also the experiment of 

 causing the chemical to act first upon the posterior end of the 

 animals. This may be done as follows : A large number of 

 Paramecia are frequently observed with anterior ends pressed 

 against the surface of a bit of bacterial zoogloea (thigmotactic 

 reaction), so that the posterior ends are all pointed in the same 

 direction. Now, a capillary glass rod, coated with some chemi- 

 cal, is introduced into the water behind the Paramecia. The 

 chemical gradually diffuses through the water, of course first 

 reaching the posterior ends of the Paramecia. But these, when 

 they respond, react exactly as in the other cases ; they swim 

 backward some distance and turn toward the aboral side. It 

 often occurs that in thus swimming backward they enter the 

 densest part of the chemical and are killed by it. 



These experiments indicate that not only the direction of 

 turning after swimming backward, but also the swimming back- 

 ward itself is determined by internal factors, and is independent 

 of the position of the source of stimulus. This conclusion 

 seems strictly true for chemical stimuli both in Paramecium 

 and in other Infusoria experimented with. As will be shown 

 later, other experiments throw a light upon the cause of this 

 uniform backward motion when stimulated by a chemical. 



Summing up, then, we may say that when Paramecium is 

 chemically stimulated it swims backward, turns toward its own 

 aboral side, then swims forward. As a rule, the anterior end, 

 moving forward, comes first in contact with the chemical, so 



