The Journal of 



Comparative Neurology and Psychology 



Volume XVII MARCH, 1907 Number 2 



LIGHT REACTIONS IN LOWER ORGANISMS. 



II. VOLVOX GLOBATOR. 



S. O. MAST. 



Professor of Biologia:! Science at Hope College, Holland. Michigan. 

 With Fifteen Figures. 



CONTENTS. 



1. Introduction 99 



2. Natural History loi 



3. Structure 102 



4. Functions of Eye-spot 108 



5. Locomotion 112 



6. Orientation — General Discussion 115 



a. E^ect of Internal Factors on Orientation 119 



b. E^ect of Change in Light Intensity on Orientation 121 



c. E^ect of Gravitation on Orientation 122 



d. Efect of Contact Stimuli and Rotation on Orientation 128 



7. Orientation to Light from two Sources 13' 



8. Orientation in Light Graded in Intensity 13^ 



9. Orientation of Segments 144 



10. Mechanics of Orientation '45 



1 1 . Reaction of Negative Colonies '54 



12. Cause of Change in Sense of Reaction IS7 



13. Effect of Temperature on Change in Sense of Reaction 161 



14. Effect of Mechanical Stimuli on Change in Sense of Reaction 162 



15. Threshold 163 



16. Optimum 164 



17. Reactions on Reaching the Optimum in a Field of Light Graded in Intensity 166 



18. Weber's Law 17' 



19. Summary 176 



20. Bibliography i79 



I. INTRODUCTION. 



Since the discovery of Volvox by Leeuwenhoek, over two 

 hundred years ago, it has been studied in detail by many investi- 

 gators. Nearly all noted the effect of light on the direction of 



