VERMES, FLY LARVAE, AND ECHINODERMS 183 



Loeb says (p. 58), " When the animals crossed the bound- 

 ary from diffuse Hght into direct sunlight, the reaction 

 caused by the increase in the intensity of the light did 

 not take place until a half or a third of the body was in the 

 sunlight (because in all phenomena of stimulation some time 

 elapses between the application of the stimulus and the re- 

 action to it). The animal checked its movement and turned 

 its head through an angle of 90°- 130° from side to side. If 

 in so doing the head again came into the shade the animal 

 returned into the shade; but if this did not happen, as was 

 more usually the case, the animal continued its movement 

 into the sunlight." Under the conditions of the experiment 

 quoted above there was therefore no cause for turning until 

 one-half or one-third of the body was in direct sunlight. 

 It is evident that after the anterior end is in the sunlight, 

 it is in lowest light intensity when it is directed from the 

 source of light. Consequently if the larvae did start to 

 turn around so as to get back into the shaded region, the 

 effective intensity would be increased and this would at 

 once cause them to turn the head back again to the position 

 in which it is shaded. Moreover Loeb says, as quoted 

 above, that if the head came into the shadow in turning, 

 the animal returned into the shade. It is therefore evident 

 that there is nothing in the observations of Loeb inconsist- 

 ent with the idea that the orienting reactions are due to 

 difference or change of intensity on the surface of the organ- 

 ism. Nor do these observations show that these reactions 

 are not accompanied by anthropomorphic sensations as 

 Loeb intimates, since every reaction shows that the larvae 

 assume positions such that there is a minimum exposure of 

 the sensitive anterior end. 



/. Sensitive region. — Both Loeb and Holmes assume 

 the anterior end to be the most sensitive part of the fly 

 larvae with reference to stimulation by light. The follow- 

 ing experiments on the effect of intensity on the rate of 

 locomotion indicate that this is not only the most sensitive 

 region, but that it is the only region sensitive to light. At 



