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BULLETIN OF THE UNITED STATES FISH COMMISSION. 
movement, while those in the oral groove and the gullet continue active. As a con- 
sequence locomotion ceases; the animal comes to rest against the solid body, while 
the cilia of the oral groove continue to drive a stream of water to the mouth. This 
reaction to a solid body may be called tliigmotaxis. If the loose solid body is a mass 
of bacterial zoogloea, the stream of water carries numbers of bacteria to the mouth, 
where they pass into the internal protoplasm of the animal and are digested; thus 
Paramecium gets its food. But the animal conducts itself in exactly the same way 
toward other loose fibrous bodies, such as bits of cloth, paper, sponge, or the like, the 
presence or absence of material that will serve as food having nothing to do with the 
production of the reaction. On the other hand, the substances held in solution in 
the water have a marked effect on the tendency of the Paramecia to react in the 
manner above described. If the water is faintly acid in reaction, the Paramecia are 
much more inclined to come to rest as just described. This is especially noticeable in 
water containing carbon dioxide. The presence in the water of an alkali in solution 
has, on the other hand, precisely the opposite effect, tending to inhibit the thigmotactic 
reaction. 
2. Any other change in the conditions, of such a nature as to act as a stimulus, 
causes a definite change in the movements, which is of a stereotyped character, being 
of the same nature for almost every stimulus. When stimulated, Paramecium swims 
backward, turns toward its own aboral side, then swims forward again. The same reac- 
tion is produced by stimuli of the most varied kinds— by acids, alkalis, and neutral salt's, 
by heat, by cold, by mechanical shock. The reaction is the same whether the stimulus 
first affects the anterior end, the posterior end, one side, or the entire surface of the 
auimal at once. The direction in which the animal swims has thus no relation to the 
localization of the stimulus. If the stimulus comes from the anterior end, swimming 
backward of course carries the animal away from it; if the same stimulus comes from 
the posterior end, swimming backward carries the animal toward it. If an injurious 
chemical substance diffuses in such a way as to first come in contact with the posterior 
end of a resting Paramecium, the latter therefore swims backward directly into the 
substance and is killed. The turning is likewise without relation to the position of 
the stimulus. The animal always turns toward its own aboral side, so that the 
absolute direction in which it turns depends upon the chance position of the aboral 
side when the turning begins. As the animal continually revolves, both when swim- 
ming forward and when swimming backward, it is impossible to predict in which 
direction the aboral side will lie after the animal has swum backward a distance from 
its position when stimulation occurs ; and observation shows that when Paramecium 
strikes on one side against an obstruction, it is fully as likely to turn toward the 
obstruction, after swimming backward, as to turn away. In the former case it of 
course strikes the obstruction again; the whole reaction is then repeated. Owing to 
the continual rotation on the long axis, the aboral side will probably be in a new 
position next time, so that the animal will turn in a new direction. If this repetition 
is continued, the Paramecium is certain finally, by the laws of chance, to avoid the 
obstacle. 
The factors determining the direction of motion in Paramecium are thus internal; 
the direction of its movements has no relation to the position of external objects. This 
result is of fundamental significance for interpretation of the movements of these 
creatures, and throws a flood of light on many of the phenomena of their life. Study 
of some other Infusoria in the light of the result on Paramecium has shown that the 
