VERMES, FLY LARVAE, AND ECHINODERMS 201 



the exposure, then they very slowly extended and turned 

 the anterior end from the source of light every time. The 

 movements were so slow that they could be readily fol- 

 lowed in detail under a hand lens. There was no evidence 

 of even the slightest preliminary turning toward the source 

 of light. It must therefore be concluded that these animals 

 have the power of differential response to localized stimula- 

 tion by light. This conclusion is in harmony with the 

 results of Parker and Arkin, Miss Smith, Adams, and 

 Harper. 



Parker and Arkin also found that if only the middle or 

 the posterior third of the body is exposed there are more 

 negative head movements than positive. I was unable to 

 confirm these results. Specimens were repeatedly allowed 

 to orient in a horizontal beam of light of 15 ca. m. intensity 

 on smooth moist black paper supported by a glass plate as 

 described above; and after they had started to crawl away 

 from this source of light a portion of the body was exposed 

 in an intensity of 200 ca. m. to lateral illumination from a 

 Nernst glower. The glower was mounted vertically so that 

 it cast a well-defined sharp shadow. By means of an opaque 

 screen containing a rectangular opening a beam of light 

 could be thrown upon any portion of the body and held 

 there by moving the screen in harmony with the movement 

 of the animal. The orientation of specimens was thus fre- 

 quently studied and the direction of movement carefully 

 noted while theycrawled across the field, a distanceof 20 cm., 

 alternately with and without some portion of the body 

 exposed to unilateral illumination. The exposure of any 

 portion back of the sixth segment had no appreciable effect 

 on the direction of motion. If however the screen was at 

 any time brought forward so that the relatively intense 

 lateral rays fell on the anterior end there was always an 

 immediate response if the specimens were active. If the 

 anterior end chanced to be directed from the source of light 

 in its swinging movements when the exposure was made, it 

 was at once thrown sharply toward the light; if it chanced 



