lyo 



LIGHT AND THE BEHAVIOR OF ORGANISMS 



^ .■y ^, ^/ 

 f f f f 



717 



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I n 



Fig, 26. I. Arenicola larva in the free-swimming state, proceeding on a spiral 

 course, m, n, directions of light; a-h, different positions on the spiral; b, dorsal 

 surface up, right eye-spot toward n; d, ventral surface up, left eye-spot toward n. 

 If the ray direction is changed by simultaneously exposing n and shading w when 

 the larva is in position a or c, no reaction takes place until it reaches b or d, then 

 it bends the head sharply toward the source of light and turns in its course. In 

 the former position it turns toward the right side of the body, in the latter toward 

 the left. This indicates that the larvae have the power of differential response 

 to locaUzed stimulation, and that the orienting stimulus may be due to a change 

 of light intensity. 



II. Much enlarged sketch showing the general structure and position of the 

 eye-spots as seen under an oil immersion objective. The eye-spots are com- 

 posed of a dark brownish caplike portion, y, which partially surrounds a colorless 

 portion, x, directed slightly dorso-laterally; z, band of cilia. 



