Harper, Behavior of Corethra. 441 



to some other form. It may, however, serve to emphasize the 

 description of the facts to compare the mode of orientation in these 

 tw^o widely different forms, inasmuch as the explanation has been 

 given a quasi universaHty in its mode of statement. We quote it 

 in abridged form. The earthworm is attuned to hght of low inten- 

 sity. Therefore under unilateral stimulation, the muscle tonus on 

 the more illumined side is reduced, with the result that the less 

 illumined side, having its normal tonus, is caused to contract more 

 and leads to negative orientation towards the rays of light. Muta- 

 tis mutandis, the same explanation would apply to positive orien- 

 tation. 



If this explanation is applied to the Corethra larvae, it is found 

 that under unilateral stimulation they too contract the muscles 

 of the less illumined side. The similarity ends here, for the result- 

 ing movement in Corethra is toward the light and these animals 

 are photopositive, while the earthworm is negative. Corethra 

 is "attuned" to light of rather high intensity. According to the 

 theory the muscle tonus of the more illumined side ought to be 

 increased. But owing to Corethra's system of locomotion this 

 would drive it away from the light. The reason for this differ- 

 ence in behavior is in brief that the motion of translation is in the 

 earthworm caused immediately by the action of the muscles, while 

 in Corethra it is caused by the recoil from the muscular movements 

 (Fig. 5). On the other hand the positive reaction of food taking 

 would conform to the conventional explanation as in this case it is 

 the stimulated side which contracts. But this fact weakens the 

 significance of the classification of movements in the same cate- 

 gory as positive which differ in a quite opposite way in their form. 

 It is clear that the classification of movements as 4- and — has 

 involved the tacit assumption that since movements are forced by 

 the environment anyway, their direction is the only thing worth 

 considering. It is manifest from these examples that the mode of 

 the reaction is peculiar to the system of behavior of the larva. 

 We are compelled to look to the structure and behavior of the ani- 

 mal as a whole for explanation of its conduct. As a matter of 

 fact Corethra shows a highly adaptive behavior, in which all the 

 modes of reaction fit together and are explicable only in relation 

 to one another. 



Again, the theoretical mode of orientation of the tropism scheme 

 may be applied to a negative response of Corethra, called forth, 



