VERMES, FLY LARVAE, AND ECHINODERMS 203 



middle is in the region of strongest illumination, than when 

 the posterior end is there. One would therefore expect a 

 greater proportion of negative head movements under the 

 former conditions than under the latter, which is just what 

 Parker and Arkin found. 



The tabulated conclusions of these authors are formu- 

 lated with the supposition that all positive head movements 

 in Allolobophora exposed to lateral illumination are " due 

 to other stimuli than light " (I.e., p. 153). My observa- 

 tions do not confirm this conclusion. As already stated, it 

 was found that if the anterior end is bent toward either 

 side when the exposure is made, it simply turns toward the 

 opposite side regardless of the direction of the rays. Orien- 

 tation in these forms is by no means entirely due to differen- 

 tial response to localized stimulation. Selection of random 

 movements or trial movements, as Holmes, Harper and 

 Jennings pointed out, undoubtedly plays a very large part 

 in the process of orientation in the earthworm under ordi- 

 nary conditions. 



The swinging movements of the anterior end are in the na- 

 ture of trial movements. They may be induced by external 

 conditions, but their character and direction are determined 

 by the structure of the organism and various physiologi- 

 cal processes. They make it possible for the organism 

 to orient much more accurately than it otherwise could. 

 When the anterior end is directed straight ahead and the 

 organisms are oriented, this end is more or less shaded and 

 not in a position to be readily stimulated by changes in the 

 direction of the greatest illumination. If it were immov- 

 ably fixed to the rest of the body in this position the entire 

 organism might turn toward either side considerably with- 

 out receiving an orienting stimulation. In place of turning 

 the entire body it raises the anterior end so as to magnify 

 the difference of intensity on opposite surfaces, extends it 

 so that it becomes more sensitive, and swings it from side 

 to side so that the different surfaces become alternately 

 shaded and illuminated, thus producing changes of inten- 



