126 



BEHAVIOR OF THE LOWER ORGANISMS 



result is that all the specimens which swim in any direction but that 

 toward the cooler water are quickly stopped and turned, while all that 

 pass toward the cooler water continue in that direction. Since all the 

 specimens in the heated region are moving very rapidly and turning at 

 very brief intervals, in a short time all will have become directed 

 toward the cool water. Hence soon after the water has been heated at 

 one end of the trough, a stream of Oxytrichas will be seen passing 



toward the cool water. The 

 animals are all "oriented" in 

 a common direction, but the 

 orientation has taken place by 

 exclusion — through the fact 

 that movement in any other 

 direction is at once stopped. 



If one end is cooled to lo 

 degrees C. or below, while 

 the other is left at the usual 

 temperature, the Oxytrichas 

 react in this same way in the 

 cold region ; hence they leave 

 it, as they before left the heated 

 region. The reaction in the 

 case of cold is much less strik- 

 ing and less complete than that 

 produced by heat. This is because the cold has the effect not only 

 of producing the avoiding reaction, but also that of making the move- 

 ments slower, and of finally benumbing the animals, so that they cease 

 to move. Thus it takes much longer for the animals to pass out of a 

 cold region than out of a warm region, and many of them do not suc- 

 ceed in escaping before the cold has stopped their movements. 



The reaction of Oxytricha is essentially similar to that of Parame- 

 cium. But in Oxytricha the method of reaction is much more evident, 

 because the movements are slower, and there is usually no revolution on 

 the long axis. 



In many other infusoria the reaction to heat and cold has been 

 shown to take place in the same manner as in Oxytricha. In some 

 species the individuals show this type of.behavior, yet with slight modifi- 

 cations that are such as to make the reaction quite ineffective, so that 

 the animals do not escape from the heated region, and are finally killed. 

 This may be observed in Bursaria truncatella. If one end of a trough 

 containing specimens of Bursaria is heated, the animals respond with 

 the avoiding reaction, as Oxytricha does. They begin to swim back- 



FiG. 85. — Bursaria swimming backward in a 

 circle when heated. Ventral view. 



