REACTIONS OF INFUSORIA TO LIGHT AND TO GRAVITY 133 



Why does the animal react in this way, even when the vessel is not 

 divided into regions of light and darkness, but is lighted from one side ? 

 The essential problem is, Why does a specimen swimming transversely 

 or obliquely to the direction of the light rays give the avoiding reaction 

 and continue this until the anterior end is directed away from the 

 source of light ? 



To understand this, certain facts need to be recalled. We know 

 that the anterior end is much more sensitive than the remainder of the 

 body. We know that an increase in illumination causes the avoiding 

 reaction. We know that this is true even when the anterior end alone is 

 subjected to such a change. Now, Stentor swims in a spiral of some 

 width, so that its anterior end swings always in a circle, and is pointed 

 successively in many different directions. If the animal is swimming 

 transversely or obliquely to the direction of the light rays, the anterior 

 end in one phase of the spiral path is directed more nearly toward the 

 source of light, in another phase more nearly away from it, so as to be 

 partly shaded, as is illustrated for Euglena in Fig. 94. The result is, 

 of course, that the sensitive anterior end is subjected to repeated changes 

 in intensity of illumination ; at one instant it is shaded, at the next the 

 light shines directly upon it. As we know from other experiments, the 

 change from light to darkness produces no reaction, while the changes 

 from darkness to light produce the avoiding reaction. Every time, 

 therefore, that the anterior end swings into the light, the avoiding reac- 

 tion is caused; the animal therefore swings its anterior end in a large 

 circle, trying many directions. Every time it swings its anterior end 

 away from the source of light into the shadow of its body, on the other 

 hand, no reaction is produced; the position thus reached is therefore 

 retained. This process continues, the animal trying new directions 

 every time its anterior end swings toward the light, until in a short time 

 the anterior end must inevitably become directed away from the light. 

 In this position the anterior end is no longer subjected to changes in 

 illumination, for the axis of the course coincides with the axis of the light 

 rays, and the body maintains a constant angle with the axis of the course. 

 The amount of light received by the anterior end therefore remains con- 

 stant. Hence there is no further cause for reaction, and the organism 

 retains the position with anterior end directed away from the source of 

 light. 



Attached specimens of Stentor do not become oriented with refer- 

 ence to the light. They may occupy any position with reference to the 

 direction from which the light comes, even though the light shines di- 

 rectly on the anterior end. We have seen previously that contact inter- 

 feres with many of the reactions of organisms. But if the animals are 



