VERMES, FLY LARVAE, AND ECHINODERMS 167 



hyaline portion and that this is directed dorso-anterio-later- 

 ally. The hyaHne structure in the eye-spot is probably highly 

 sensitive to light, while the pigmented portion appears to 

 serve in shading the inner surface so as to admit light only 

 from in front and from the side on which the eye-spot is 

 located. 



b. Locomotion. — The larvae have two methods of loco- 

 motion. They swim by means of the cilia for a few days 

 then settle to the bottom and crawl. The crawling move- 

 ment is brought about by means of muscular contraction 

 much as in many other annelids. During this method of 

 progression they are slightly negative and there is scarcely 

 any indication of orientation. In the free-swimming posi- 

 tive state however they orient quite accurately. Both 

 methods of reaction are adaptive. The positive reaction 

 in the free-swimming state serves to keep the larvae at the 

 surface of the water and causes them to scatter widely. 

 The negative reaction serves to keep them at the bottom 

 and to guide them into the mud where most of their future 

 days are to be spent. 



In swimming they proceed much like the dilates. They 

 rotate counter-clockwise on the long axis and travel on a 

 spiral course. The ventral surface, the surface containing 

 the median band of cilia, constantly faces out in the spiral, 

 contrary to what might be expected if these cilia are 

 functional. The organism Is slightly curved, the ventral 

 surface being concave. It may be that this causes the 

 constant swerving toward this surface, which together with 

 rotation on the long axis results in the spiral course. 



c. Orientation. — Orientation is not so accurate as is 

 generally assumed. If casually observed It is true that the 

 larvae do appear to move directly toward the source of 

 light, but if the course of a given individual exposed to light 

 from a single compact source is carefully followed it is found 

 that there are frequent deviations. As the organisms pro- 

 ceed they frequently turn the anterior end slightly but 

 suddenly toward either side by means of muscular contrac- 



