VERMES, FLY LARVAE, AND ECHINODERMS 191 



end forward. When the anterior end is thus extended the 

 two cone-shaped elevations at the very tip (Fig. 30) become 

 fully exposed; and these, owing to the lateral movements 

 of the anterior end, face alternately to the right and the 

 left. When the animal fastens the anterior end to the sub- 

 stratum and pulls up the posterior end, the cone-shaped 

 structures cannot be seen. They appear to be drawn in, 

 and the whole anterior end is turned under somewhat as 

 the arch is formed in the looping process. This causes the 

 tip to be thoroughly concealed and shaded. 



When the larvae are first exposed to sudden lateral illu- 

 mination in direct sunlight, they respond immediately by 

 throwing the anterior end toward one side violently, no 

 matter in what position this end chances to be. If it hap- 

 pens to be directed from the source of light when the sun- 

 light is flashed upon the organism, it turns toward the source 

 of light, and if the sunlight is immediately intercepted after 

 the larva turns, it will continue in the direction toward 

 which the anterior end points; if it is not intercepted, the 

 anterior end is thrown in the opposite direction, and then 

 the larva may follow this turn and become oriented imme- 

 diately, or it may swing the end back and forth a few times 

 before becoming oriented. If the anterior end faces the 

 light when it is exposed to the sun it is first thrown in the 

 opposite direction and orientation takes place just as de- 

 scribed above. The anterior end is thus turned in the direc- 

 tion opposite to that in which it is when the exposure is 

 made. It is therefore evident that under these conditions 

 the larvae will turn toward a strong unilateral illumination 

 about as often as from it. 



According to the tables, however, turning toward the 

 source of light becomes less frequent after the organism is 

 exposed for a time and much less frequent if the intensity is 

 decreased. What is the cause of this? If the larvae are 

 carefully observed when they are suddenly exposed to lat- 

 eral illumination by diffuse light, it is found that they 

 respond immediately only if the anterior end is turned 



