MOLLUSKS, ARTHROPODS AND VERTEBRATES 217 



although a tendency to deviate toward the side of the nor- 

 mal eye regularly occurred." Radl (1903, p. 62) says, 

 " Die Calliphora vomitoria bewegt sich fast ebenso gerade 

 mit einem geschwarzten Auge, wie wenn sie auf beiden 

 sieht, und es ist mir nicht leicht, diese Erscheinung zu 

 erklaren." Holmes (1905) discovered that Ranatra with 

 one eye blackened at first deviates strongly toward the 

 functional eye in going toward a source of light, but that 

 this deviation decreases and that the path becomes much 

 more nearly direct after repeated trials, indicating that the 

 animal learns to adjust itself to the new conditions and that 

 its reaction mechanism is not so simple as Loeb's theory 

 demands. This is still more clearly demonstrated by the 

 interesting observations of Holmes on the fiddler crab, Uca 

 pugnax. I cannot do better than to quote his conclusions 

 based on these observations (1908, p. 496): " The point of 

 principal interest in the phototaxis of the fiddler crabs is the 

 relation of their lateral orientation to the theories of tro- 

 pisms. Can we regard orientation as a direct response in 

 which the animal is involuntarily forced into line, or is it 

 rather to be considered as coming under the pleasure-pain 

 type of behavior, and as therefore related to the voluntary 

 seeking of a certain end which is exhibited in the behavior 

 of higher forms ? In order to explain the orientation of a 

 highly organized form like an insect or crustacean in which, 

 in most cases, response to light takes place through the 

 eyes, we may assume that light falling more strongly on one 

 eye sets up impulses which are transmitted more or less 

 directly to the leg musculature. We may assume that the 

 extensors of the opposite side are stimulated, or the flexors 

 on the same side, or both, and that in consequence of this 

 distribution of impulses the animal moves until its body 

 is in line with the rays. In such a case the movements 

 involved in orientation are the same as those employed in 

 ordinary locomotion, only the activity of the legs on one or 

 the other side is accentuated according to the position of 

 the body in relation to the direction of the rays. 



