EEACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 181 



in precisely the opposite direction from what it does in negative 

 specimens. The anterior end is consequently turned toward the 

 light in place of from it when, owing to rotation on the longitudi- 

 nal axis, the optic nerve-endings are subjected to changes in 

 illumination, and this turning continues until the organism 

 proceeds directly toward the light and rotation on the longi- 

 tudinal axis no longer results in changes in the illumination of the 

 retina large enough to stimulate. It remains oriented for pre- 

 cisely the same reason that negative specimens remain oriented, 

 i.e., because in the absence of directive stimulation it tends to 

 continue in the direction in which it is going. 



Photic orientation in the tadpoles of Amaroucium is con- 

 sequently in principle essentially the same as it is in Euglena, 

 Stentor, and numerous other organisms. It is dependent upon 

 shock reactions of the ' all-or-none ' or 'trigger' type. And the 

 eye functions in the process of orientation much like the eye-spot 

 in Euglena (Mast, '11) or the eye in Planaria (Taliaferro, '20). 



Crozier ('18, p. 491) maintains that in Chiton orientation in 

 light cannot be dependent upon reactions to changes of lumi- 

 nous intensity, because it responds to decrease in illumination 

 when it is negative as well as when it is positive. He says: ''It 

 may be emphasized here, as in the case of certain pedate holo- 

 thurians. . . . that the simultaneous presence of photo- 

 negative orientation and a precise negative response to shading, 

 without any response (of the part concerned in orienting reac- 

 tions) to increased illumination, is thoroughly inconsistent with 

 the idea that photonegative orientation is brought about by a 

 stimulation induced through any change in light intensity, as 

 such." This conclusion may be correct, but the evidence 

 presented in support of it (stated above) is not very strong : for, 

 while we have in the tadpoles of Amaroucium precisely what 

 Crozier maintains for Chiton, namely, reactions to shadows 

 in negative as well as in positive specimens, orientation in 

 these creatures is clearly dependent upon changes of luminous 

 intensity. 



