60 THE BEHAVIOR OF LOWER ORGANISMS. 



does a change in the illumination of the entire body. Indeed, Engel- 

 mann (1882, a) showed that a change in illumination over the remainder 

 of the body is ineffective in producing the reaction, so that in every case 

 the reaction is due to the change in illumination at the anterior end. 

 Now, in the orientation reaction the conditions are present for produc- 

 ing changes of illumination at the anterior end of precisely the character 

 which would, in view of our other experimental results, bring about 

 the reactions observed. This will best be shown by again examining 

 in detail from this point of view a concrete case. 



In Fig. 23 we will suppose that the Euglena at i is at first swimming 

 toward the source of light. When it reaches the position 2 the light is 

 changed, so that it now comes from the direction indicated by the 

 arrows at the right. By this change the intensity of illumination at 

 the anterior end is decreased, since before the light came from directly 

 in front and affected the entire end, while now it falls upon but one 

 side. We know from other experiments that as a result of such a 

 change the organism reacts by swerving more toward the dorsal side, 

 at the same time continuing to revolve on the long axis. This is ex- 

 actly what happens now ; by the increased swerving the organism is 

 carried from position 2 to position 3. In this change the anterior end, 

 swinging still farther away from the source of light, is still less illumi- 

 nated than before. As a result of this farther decrease in illumination 

 the reaction is continued or increased ; combined with the revolution 

 on the long axis it carries the organism successively to positions 4, 5 

 and 6. In this part of the movement the anterior end becomes pointed 

 more directly toward the source of light, and is hence more strongly 

 illuminated ; there is therefore nothing in this movement to cause a 

 reaction. The strong swerving toward the dorsal side then ceases or 

 becomes less. But in the next phase of the spiral course (from 7 to 8), 

 there is necessarily at least the normal amount of swerving toward the 

 dorsal side, and this carries the organism to a position (8), where 

 the intensity of the light acting on the anterior end is decreased. As 

 a result of this decrease we know that the " motor reaction " must again 

 be induced ; the organism swings then farther toward the dorsal side* 

 This movement, combined with the revolution on the long axis, carries 

 the Euglena through 9 and 10 to n. Here again the swerving de- 

 creases, because the change was from a less illuminated to a more 

 illuminated region. Hence after reaching 12 the Euglena swerves only 

 a little away from the light, to 13 ; then, as a result of the decrease in 

 illumination at the anterior end caused by this movement, it swerves 

 far toward the light, through 14 and 15 to 16. This movement causing 

 greater illumination, the reaction ceases. The light is now shining full 

 on the anterior end. The organism therefore swims forward in the 



