PARAMECIUM 



conditions, between 24 and 28 C. A number of animals placed 

 on a slide, which is heated at one end, will swim about in all direc- 

 tions, giving the avoiding reaction where stimulated, until they 

 become oriented so as to move toward the cooler end. This is the 

 method of trial and error, that is, the animal tries all directions 

 until the one is discovered which allows it to 

 escape from the region of injurious stimula- 

 tion. 



Gravity in some unknown way causes Parame- 

 cia to orient themselves with their anterior ends 

 pointed upward. This brings them near the 

 surface of the water. If a number are equally 

 distributed in a test tube of water, they will 

 gradually find their way to the top (Fig. 37). 

 The most probable theory to account ' for this 

 is that the substances within the body, being 

 of different specific gravities, move about when 

 the animal changes its position and act as stim- 

 uli, relief being obtained only when the animal 

 has placed its body with its long diameter 

 perpendicular to the earth's surface and its 

 anterior end up (68). Under certain condi- 

 tions Paramecium exhibits positive geotropism. 



If Paramecia are placed in running water, 

 they orient themselves with the anterior ends 

 upstream and swim against the current. This 

 is probably caused by the interference of the 

 current with the beating of the cilia, for as soon as an animal 

 reaches a position with anterior end upstream, the water no 

 longer tends to reverse the cilia. 



Paramecia may be subjected to an electric current in the labora- 

 tory, but they are never influenced by it under normal conditions. 

 A weak current causes a movement toward the cathode; a strong 

 current reverses the direction of the beating of the cilia and causes 

 the animals to swim backward toward the anode. Many other 



FIG. 37. Paramecia 

 collected at 

 the top of a 

 vertical tube. 

 (From Jen- 

 nings after 

 Jensen.) 



