CHAPTER IX 



REGULATION IN THE DIRECTION OF MOVEMENT WITH 



REFERENCE TO THE SOURCE OF LIGHT IN VERMES, 



FLY LARVAE, AND ECHINODERMS 



Many of the organisms belonging to these groups respond 

 very definitely to stimulation by light. In some, the 

 response results in orientation, in others it consists merely 

 of an increase or decrease in rate of movement, and in 

 still others it consists chiefly of a sudden contraction. We 

 shall concern ourselves here primarily with forms which 

 orient, emphasizing particularly the orienting reactions. 

 The reactions of the blowfly larvae will be discussed in 

 this section owing to their worm-like structure and 

 method of locomotion. 



I . A renicola cristata — Larvae 



a. Description. — Arenicola deposits its eggs in great 

 numbers in masses of jelly-like substance. In the course 

 of a few days the eggs develop into finger-shaped free-swim- 

 ming larvae about 0.3 mm. long. These organisms are 

 strongly positive in their reactions to light and negative in 

 their reactions to gravity. They contain two ciliary rings, 

 one near the anterior and the other near the posterior end ; 

 these are connected by a median ventral band of cilia. On 

 either side near the anterior end is an eye-spot. Lillie 

 (1903, p. 345) says, " Each [eye-spot] consists of a compact 

 clump of pigment or excretory granules on the surface of 

 the brain." I studied the eye-spots in living specimens 

 slightly flattened with the cover-glass, under an oil immer- 

 sion lens and found that they consist of a brownish granular 

 cup-shaped portion which partially surrounds an ellipsoidal 



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