232 LIGHT AND THE BEHAVIOR OF ORGANISMS 



All the other coelenterates studied orient directly. We 

 are however unable to say how light produces the orient- 

 ing stimulus. Orientation probably is due to a differential 

 response to a localized stimulation. 



(6) In Arenicola larvae orientation takes place much as 

 it does in the flagellates and ciliates; but the larvae can turn 

 directly toward the right or the left side. They appear to 

 have the power of differential response to localized stimula- 

 tion; the stimulation is probably due either to a reduction 

 of light intensity on the shaded side, or to an increase of 

 intensity on the illuminated side. 



In the blowfly larvae only the very tip of the anterior 

 end is sensitive to light. They turn this tip to the right 

 and left alternately during the process of locomotion. If it 

 becomes more highly illuminated as it turns toward either 

 side it is turned farther in the opposite direction. The 

 larvae apparently continually test their position with refer- 

 ence to the light. The lateral head movements make it 

 possible for these creatures to orient much more accurately 

 than they otherwise could. The orienting stimulus is un- 

 doubtedly due to an increase of intensity. 



The earthworms can turn directly from the light. They 

 probably have the power of differential response to localized 

 stimulation by light. The random movements of the ante- 

 rior end may serve to localize the stimulation. The direc- 

 tion of movement is, however, not entirely determined by 

 localized stimulation. It is largely influenced by internal 

 factors. The worms frequently turn toward the source of 

 light when stimulated. The " selection of random move- 

 ments," essentially as described by Holmes, is an important 

 factor in orientation. 



The anterior end of the earthworm is much more sensitive 

 to light than the rest of the body. Lateral illumination of 

 any portion of Allolobophora back of the sixth segment does 

 not appreciably affect orientation, although this part of the 

 animal is undoubtedly sensitive to light. 



The planarians orient directly. A sudden change of 



