THE BEHAVIOR OF INFUSORIA; PARAMECIUM 73 



5. ORIENTING REACTIONS, TO WATER CURRENTS, TO GRAVITY, AND 



TO CENTRIFUGAL FORCE 



In the reactions which we have thus far considered, the infusoria do 

 not become oriented in any precise way with relation to the direction of 

 action of the stimulating agent. But to water currents, to gravity, and 

 to centrifugal force the animals at times react in such a way as to bring 

 about a definite orientation, with the body axis of all the reacting 

 individuals in line with the external force. In a water current the 

 anterior end is directed up stream ; under the influence of gravity the 

 anterior end is directed upward, while when subjected to a centrifugal 

 force the anterior end is directed against the action of the force. 



How are these results produced, and why do the organisms take a 

 definite axial orientation under the action of these stimuli, while they do 

 not under most other stimuli ? 



In the reactions to water currents and to gravity, direct observation 

 has shown that the orientation is produced through the movements 

 which we have called the avoiding reaction. Under the action of a 

 centrifugal force, observation of individuals is impossible, but beyond 

 doubt the reaction is the same as that due to gravity. 



A. Reactions to Water Currents 



The reactions to water currents can best be studied in a tube like 

 that shown in Fig. 55. By covering the two open ends with rubber caps 

 filled with air, and 

 pressing on these, the 

 water containing the 



animals in the tube Fig. 55. — Tube used in studying the reactions to water cur- 



can be driven through 



the narrow part of the tube with any desired velocity. With a certain 

 velocity of current most of the individuals, both those that are free 

 swimming and those that are resting against the glass, are seen to 

 place themselves in line with the current, with anterior end up stream. 

 Some of the individuals usually do not react. In those that do, the 

 reaction is brought as follows : As soon as the current begins to act, 

 producing a disturbance in the water, the animals give the avoiding 

 reaction in a not very pronounced form. That is, a given individual 

 swims more slowly or stops, and swerves more strongly toward the 

 aboral side, thus swinging the anterior end about in a circle, as in 

 Figs. 37 and 38, "trying" various directions. It then starts forward 

 again in one of these directions. This reaction may be repeated sev- 

 eral times, till the infusorian finally comes into a position with anterior 



