• REACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 177 



leave the edge of the drop hke a flash, make a small circle and 

 return again somewhat farther back. Then they proceed along 

 the edge again and when the retina again becomes shaded they 

 turn again. This may be repeated several times in succession, 

 the reaction occurring each time approximately in the same 

 location. Similar reactions occur when the retina becomes 

 illuminated, but the turning is not nearly so sharp. 



These results together with those presented above prove 

 conclusively that the eye functions in all of the photic reactions 

 observed. 



The fact that changes in the position of the organism in relation 

 to the source of light resulting in changes in the illumination in 

 the eye may induce changes in the direction of locomotion is of 

 great importance, for it gives an insight into the process of 

 orientation, as will be shown presently. 



It is renlarkable that, while the eye in the tadpoles of Amarou- 

 cium is unquestionably useful, it functions, in some cases, for 

 only a few minutes during the life-time of the individual, and 

 its eyeless successors (zooids), which may live for several years 

 producing, vegetatively, thousands of generations none of which 

 have eyes but all of which may produce tadpoles with eyes fully 

 formed and apparently perfect in function at birth. This seems 

 to show that the origin of function in the eye of the individual is 

 independent of experience, and that it must be in some way 

 specifically associated with a substance (an eye determiner) 

 which, without any apparent relation to function, is capable of 

 passing from tadpole to tadpole through thousands of genera- 

 tions and millions of individual zooids. ** 



PHOTIC ORIENTATION 



If Amaroucium tadpoles are exposed in a horizontal beam of 

 light, they swim fairly directly toward or from the light, i.e., 

 they orient fairly precisely. If the direction of the beam is 

 changed, they immediately turn sharply toward or from the 

 light until they are again oriented. What, now, are the factors 



"Grave deals extensively with this problem. 



