242 THE BEHAVIOR OF LOWER ORGANISMS. 



in some directions it is pointed more nearly toward the light, in others 

 more away from the light, so as to be partly shaded by the rest of the 

 body. These changes in illumination cause the reaction ; the organism 

 tries pointing in various directions. When it comes into such a posi- 

 tion that the anterior end is no longer subjected to changes in intensity 

 of illumination, it continues to swim forward in that direction. Such 

 a position is found only when the axis of the spiral path is in the direc- 

 tion of the light rays. In an organism which reacts when the intensity of 

 illumination is decreased, such a position is stable only when the anterior 

 end is directed toward the source of light ; in an organism which reacts 

 when the intensity of illumination is increased, only when the anterior 

 end is directed away from the source of light (details in second of these 

 contributions). Thus the organism tries various directions till one is 

 found which does not subject it to changes in intensity of illumination 

 at the anterior end ; in this direction it swims forward. 



The reaction which produces orientation to light can be stated more 

 simply, but less completely and accurately, as follows : When light 

 coming from a certain direction falls upon the sensitive anterior end of 

 a negative organism, such as Stentor, this causes the reaction above 

 described. The animal, after backing, tries a new set of directions, 

 by whirling its anterior end about in a circle. It continues or repeats 

 this until a direction is found in which the light no longer falls on the sen- 

 sitive anterior end. It is then oriented with anterior end away from the 

 source of light. In the positive organisms, such as Euglena, the method 

 of reaction is the same, save that it is the shading of the anterior end 

 that causes the reaction. When the anterior end is shaded the organ- 

 ism reacts in the usual way. It tries successively many different direc- 

 tions, by whirling its anterior end about in a wide circle ; when the 

 anterior end becomes pointed toward the source of light, the organism 

 continues forward in that direction. 



In those infusoria which creep along the bottom, as Stylonychia or 

 Oxytricha, the reaction method is of a slightly simpler character, 

 though identical in principle. These animals when creeping do not 

 rotate on the long axis. When stimulated in any of the ways described, 

 they dart back, then turn to their right. They thus keep in contact 

 with the bottom, and may turn through any number of degrees up to 

 360 or more (Fig. Si). The reaction places them thus successively 

 in every position with reference to the source of stimulus that is possi- 

 ble so long as they remain on the bottom, and in each position the 

 adoral cilia are bringing samples of water to the anterior end and 

 mouth, as in Fig. 81. When a position is reached where the stimulus 

 no longer acts, the reaction ceases, and the animal moves forward in 

 that direction. The reaction is sometimes repeated several times before 

 the definitive position is attained. 



