82 Studies in Animal Behavior 



plates. Although orientation in the usual way was 

 impossible, the amphipods moved toward the light 

 by pushing against the glass with their legs and by 

 flexing and extending the body. The long axis was 

 kept pointed approximately toward the light, but 

 this position was maintained by movements quite 

 different from those involved in orientation under 

 normal conditions. Since the phototaxis of this 

 species is readily modified by experience it is not im- 

 probable that something more than direct reflex ac- 

 tion is involved. Perhaps the explanation of why 

 animals do the things that they like to do is involved 

 in any complete account of the orientation of higher 

 forms. 



For more primitive types in which behavior is lit- 

 tle if at all modified through the agency of asso- 

 ciative memory there is no reason for doubting that 

 orientation is effected by means of reflex action. But 

 granting this to be true there is room for a variety 

 of ways in which orientation may be brought about. 

 Many animals respond very readily to a sudden 

 change in the intensity of light, while they show 

 little or no response when exposed to constant illumi- 

 nation. In some cases a sudden increase of light 

 intensity produces a reaction, but more frequently 

 the response is evoked when the intensity of the light 

 is diminished. Hungry leeches (Glossiphonia) 

 raise up and extend the anterior end of the body 

 when a shadow passes over them, and mosquito 

 larvs wriggle downward under the same circum- 



