174 LIGHT AND THE BEHAVIOR OF ORGANISMS 



Summary 



(i) Arenicola larvae are positive to light in their free- 

 swimming state. They rotate on the long axis and swim 

 on an irregular spiral course owing to frequent sharp lateral 

 movements of the head. 



(2) If held under a cover-glass so that they cannot 

 rotate, it is found that the head is suddenly turned directly 

 toward the source of light when either of the two sides 

 is illuminated, frequently to such an extent that the 

 anterior end of the body is at right angles to the posterior 

 end. 



(3) The larvae have two prominent eye-spots, which con- 

 sist of a hyaline portion partly surrounded by an opaque 

 caplike structure. The hyaline portion, which is probably 

 sensitive to light, is directed dorso-anterio-laterally. In 

 the free-swimming state orientation takes place by a greater 

 swerving toward the source of light on the spiral course 

 every time an eye-spot faces the light just as in Euglena. 

 In Arenicola, since it has two eye-spots, the increase in 

 swerving takes place in two different positions on the spiral, 

 i.e., twice during a complete rotation on the long axis. In 

 Euglena, since there is but one eye-spot, it takes place only 

 in one position on the spiral, or once during a complete 

 rotation. Euglena however responds when the eye-spot 

 faces the source of light because when it is in this position 

 the eye-spot shades the tissue. It responds only to a 

 decrease of intensity while it is positive. Arenicola larvae, 

 on the contrary, respond to either an increase or a de- 

 crease of intensity on the sensitive tissue on either side, 

 but they always turn toward the more highly illuminated 

 side. 



(4) The stimulus causing this reaction, a reaction by 

 means of which the organism orients, is probably due either 

 to a decrease or to an increase of intensity on either side. 

 The orienting reaction is probably a differential response 

 to a localized stimulus. Our evidence however does not 



