VERMES, FLY LARVAE, AND ECHINODERMS 1 71 



toward the source of light in two positions on the spiral 

 course just as Arenicola larvae do. If in place of a union 

 of two individuals we should have a union of three, it would 

 result in an organism that could turn toward the source of 

 light in three positions on the spiral. And if a sufficient 

 number were united it is clear that the organism could turn 

 toward the source of light in all positions on its course. 

 Such organisms we have in the colonial forms Volvox, 

 Eudorina, and Pandorina. All of these consist of numerous 

 individuals united, and all can turn toward the source of 

 light directly no matter which side is illuminated. It is 

 however probable that this analogy is, with reference to 

 Arenicola larvae, merely superficial. 



/. Orienting stimulation. — In positive Euglenae it was 

 found that orientation is due to a reaction caused by a 

 reduction in effective light intensity due either to a change 

 in the intensity of the field or to rotation of the organism 

 owing to the fact that it is more sensitive when the ventral 

 surface is illuminated than when the dorsal surface is. In 

 Volvox the orienting stimulus is likewise due to a reduction 

 of effective intensity. To what is it due in Arenicola lar- 

 vae? Is it due to a decrease of intensity caused by the 

 shadow of the pigment on the hyaline portion of the eye- 

 spot on the side turned from the light, or to an increase of 

 intensity on this structure in the eye-spot turned toward 

 the light, or to an absolute difference of intensity on the 

 two sides in accord with the theories of Verworn and Loeb? 



Two methods were used in attempting to answer these 

 questions. In both the larvae were mounted under a large 

 cover-glass supported by means of a ring of vaseline. The 

 cover was then pressed down until the space became so 

 narrow that the larvae could not rotate, (i) A piece of 

 sheet metal containing an opening i cm. square was hung 

 about 3 mm. from a Welsbach mantle. From the middle 

 of one side of the opening there projected nearly to the 

 center a spinelike process. The incandescent mantle was 

 focused on the slide by means of the plane mirror and Abbe 



