72 BEHAVIOR OF THE LOWER ORGANISMS 



the avoiding reaction ceases, and they continue to swim in that direc- 

 tion. At that end of the trough which is cooled below the optimum, 

 similar effects are produced, save that the reaction is less rapid, and the 

 Paramecia therefore leave this region much more slowly than they do 

 the heated end. 



Thus after a time the direction of movement of all the individuals in 

 the hot or cold end of the trough has become changed, and all are moving, 

 often in a well-defined group, toward the optimum region. Thus we 

 may observe in these temperature reactions a well-defined common 

 orientation of a large number of organisms ; all are headed toward the 

 optimum. This orientation is brought about, as we have seen, by ex- 

 clusion. That is, movement in any other direction is stopped, through 

 the production of the avoiding reaction, so that all finally travel in this 

 one direction. Or, to put it more accurately, the Paramecia try every 

 possible direction, through the avoiding reaction (Figs. 37-39), till 

 finally they all find the only one which does not cause stimulation; in 

 this direction they continue to move. The method of reaction, by 

 systematic trial of all directions, is such as to find any existing avenue 

 of escape, no matter how narrow it may be. 



4. REACTION TO LIGHT 



To ordinary visible light Paramecium is not known to react in any 

 way. If light is allowed to fall on the animals from one side only, or 

 if one portion of the vessel containing them is strongly righted while the 

 rest is shaded, this has no observable effect on their movements or 

 distribution. 



But Hertel (1904) has recently shown that to powerful ultra violet 

 light Paramecium does react. The ultra violet rays employed by Hertel 

 came from a magnesium spectrum ; they were of a wave length of 

 280 fifx. When part of a drop of water containing Paramecia 

 was subjected to this light, the animals in the lighted region at once 

 began to move about rapidly. They therefore passed quickly into 

 the region not lighted. Specimens moving about in this shaded region 

 stopped at once on reaching the boundary of the lighted area, and turned 

 away. It is evident that the reaction to light is by the usual avoiding 

 reaction, though the details of the movement were not observed by 

 Hertel. 



When the animals were unable to escape from the light, their move- 

 ment became uncoordinated, and in ten to fifty seconds it ceased. The 

 animals were dead. 



