THE BEHAVIOR OF INFUSORIA; PARAMECIUM 105 



No other stimulus acting, it begins to respond to the changes in distri- 

 bution of its internal contents due to the fact that it is not in line with 

 gravity. It tries various new positions until its anterior end is directed 

 upward, and continues in that direction. It thus reaches the surface 

 film. To this it responds by the avoiding reaction, finding a new posi- 

 tion and swimming along near the surface of the water. Now there 

 is a strong mechanical jar, some one throws a stone into the water, 

 perhaps. The Paramecium starts back, tries certain new directions, 

 and finishes by reacting to gravity in the reverse way from its former 

 reaction; it now swims downward. But this soon brings it into water 

 that is notably lacking in oxygen. To this change it responds as be- 

 fore, trying new directions till it has come near the surface again. Swim- 

 ming forward here, it approaches a region where the sun has been 

 shining strongly into the pool, heating the water. The Paramecium 

 receives some of this heated water in the current passing from the 

 anterior end down the oral groove. Thereupon it pauses, swings its an- 

 terior end about in a circle, and finding that the water coming from one 

 of the directions thus tried is not heated, it proceeds forward in that 

 direction. This course leads it perhaps into the region of a fresh plant 

 stem which has lately been crushed and has fallen into the water. The 

 plant juice, oozing out, alters markedly the chemical constitution of 

 the water. The Paramecium soon receives some of this altered water 

 in its ciliary current. Again it pauses, or if the chemical was strong, 

 swims backward a distance. Then it again swings the anterior end 

 around in a circle (Fig. 38) till it finds a direction from which it receives 

 no more of this chemical; in this direction it swims forward. 



Thus the animal swims about, continually hesitating as it reaches 

 regions where the conditions differ, trying new directions, and changing 

 its course frequently. Every faint influence in the water affects it, 

 for the animal is very sensitive. Other Paramecia swim about in the 

 same way. They do not avoid each other, but often strike together; 

 then one or both draw back and turn in another direction. The animal 

 may strike in the same way against stones or the sides of a glass vessel. 

 In such cases it may be compelled to try successively many different 

 directions before it succeeds in avoiding the obstacle, acting like a 

 blind man who finds a stone wall in his course. 



After a time our animal comes against a decayed, softened leaf. 

 At first it draws back slightly, then starts forward again, and places 

 itself against the leaf. The body cilia cease their action, while the oral 

 cilia carry a strong stream of water to the mouth. It so happens that 

 this leaf has lately fallen into the water and has no bacteria upon it, so 

 that the Paramecium receives no food. Nevertheless the animal "tries" 



