MOLLUSKS, ARTHROPODS AND VERTEBRATES 233 



intensity causes definite lateral head movements. In posi- 

 tive specimens, where one side is shaded, they turn toward 

 the side which is not shaded, indicating that it is a decrease 

 of intensity on the shaded side that causes orientation. 



There is no evidence indicating that the direction of the 

 rays or the angle between the rays and the sensitive surface 

 functions in orientation in any of the forms referred to 

 under vermes, except in so far as it may cause change of 

 intensity; nor is there any evidence that light acts con- 

 stantly as an orienting stimulus. 



The echinoderms do not orient; they can move with any 

 part of the body ahead. When the direction of the light 

 is changed they change the direction of motion by moving 

 with another part of the body ahead. Precisely how the 

 direction of motion is regulated is unknown. 



(7) The moUusks, arthropods and vertebrates all orient 

 directly. There is little evidence of preliminary trial move- 

 ments in this process in these forms. Orientation is prob- 

 ably due to differential response to localized stimulation. 

 In some of the positive mollusks the orienting stimulus 

 appears to be due to a decrease of intensity on the shaded 

 side. With reference to the way in which light produces 

 the stimulation in the other forms in these groups our 

 evidence does not warrant a conclusion. 



The animals with image-forming eyes no doubt orient 

 and go toward a source of light much as they go toward 

 any other object of interest to them. 



(8) Orientation may then be due (a) to local response 

 to local stimulation, as in the rhizopods and myxomycetes; 

 {h) to shock movements, avoiding reactions, as in the dil- 

 ates, flagellates, colonial forms, vermes and larvae of various 

 kinds; {c) to differential response to localized stimulation, 

 as in some of the coelenterates, the vermes and all the 

 higher forms; {d) to sight, as in many animals with image- 

 forming eyes. 



(9) No evidence was found indicating that the direction 

 of light in the field or through the tissue (Sachs) functions 



