2 26 LIGHT AND THE BEHAVIOR OF ORGANISMS 



nately to the right and left in the process of locomotion, 

 supports the conclusion reached in the study of Caprella, 

 as do also the orienting reactions of zoeae and lobster larvae. 

 Lobster larvae in the earlier stages always orient with the 

 posterior end toward the source of light. When they are 

 positive they swim toward the light with this end ahead. 

 When they are negative they swim in the opposite direction 

 with the anterior end ahead. Hadley (1908, pp. 264-276) 

 showed that if the direction of the rays is changed in any 

 way after the larvae are oriented, they immediately and 

 directly turn until the anterior end is shaded again. If the 

 position of the source of light is so changed that the dorsal 

 or the ventral surface is exposed, it is evident that the sides 

 may still be equally illuminated. If the dorsal surface, for 

 example, becomes illuminated when the ray direction is 

 changed they turn toward the ventral surface. This shows 

 very clearly that orientation in these forms is not regulated 

 solely by relative intensity of light on symmetrically located 

 structures, for the illumination on opposite sides throughout 

 this whole reaction may have been equal. I observed simi- 

 lar reactions in the zoeae of several different species of the 

 Brachyura and Caridea. In some of these animals the eyes 

 extend laterally beyond the surface of the body so far that 

 both are illuminated no matter whether the anterior or 

 posterior end is directed toward its source. 



In all these forms it is evident that the turning toward the 

 dorsal or the ventral surface, when the source of light is 

 lowered or raised, must be due to the fact that different 

 portions of the eyes become illuminated, unless it is regu- 

 lated by vision. If it is, the former orientation appears to 

 be a differential response to a localized stimulation. The ob- 

 servations of Holmes and others indicate that orientation in 

 a number of other arthropods also depends upon the surface 

 of the eyes exposed. Holmes (1905) covered different parts 

 of the eyes of Ranatra with an opaque substance and found 

 that it responds just as though the environment in the 

 direction of the blackened portion of the eyes were dark. 



