VERMES, FLY LARVAE, AND ECHINODERMS 169 



tion may be due to an Increase in illumination on one side 

 or a decrease on the other. The stimulation may be local 

 and the reaction a differential response. We shall refer to 

 this problem again later. 



The larvae are so small and move so rapidly in the free- 

 swimming state that it is exceedingly difficult to follow 

 their movements in detail during the process of orientation. 

 By carefully observing this process, however, in a low tem- 

 perature by means of which the rate of movement is much 

 reduced, it was found that it takes place somewhat as fol- 

 lows: If the source of light is changed in its position after 

 a free-swimming specimen is oriented, reactions occur im- 

 mediately only if either eye-spot is fully exposed after the 

 change is made. If the ventral or the dorsal surface is 

 directed toward the source of light after the ray direction 

 is changed, there is no reaction until the organism has 

 rotated through 90° as it proceeds on its spiral course, and 

 one of the sides comes to be illuminated. Then the anterior 

 end is turned sharply toward the source of light, frequently 

 to such an extent as to form a right angle with the posterior 

 end. This causes rapid swerving on the spiral toward the 

 light and speedy orientation (see Fig. 26). I was unable 

 to detect any change in the course due to ciliary action. 



e. Discussion. — The method of orientation in Arenicola 

 larvae has some features in common with that of Euglena 

 in the free-swimming state. In both forms there is a defi- 

 nite reaction whenever an eye-spot comes to face the source 

 of light as they proceed on their spiral courses. This reac- 

 tion consists of a turning toward the side containing the 

 eye-spot and a swerving on the spiral course in the same 

 direction, and this results in orientation. The larvae how- 

 ever, having two eye-spots, can turn toward the source of 

 light in two positions in the spiral, whereas the Euglenae 

 can turn toward It in only one. If one were to imagine two 

 Euglenae united so as to form an organism with two eye- 

 spots facing in opposite directions, it would not be difhcult 

 to conceive the organism thus formed capable of turning 



