RELATION BETWEEN INTENSITY OF LIGHT AND 



RATE OF LOCOMOTION IN PHACUS PLEURO- 



XECTES AND EUGLENA GRACILIS AND ITS 



BEARING ON ORIENTATION. 



S. O. MAST AND MARY COVER, 

 THE ZOOLOGICAL LABORATORY OF THE JOHNS HOPKINS UNIVERSITY. 



INTRODUCTION. 



According to the De Candolle-Verworn theory of orientation 

 which has been widely accepted, the action of the locomotor ap- 

 pendages in organisms is dependent upon the intensity of the light 

 received by the receptors connected with them ; so that if one side 

 of a bilaterally symmetrical organism, Volvo.\~, e.g., is more highly 

 illuminated than the other the locomotor appendages on one side 

 beat more effectively than those on the opposite side, resulting 

 in turning until the two sides are equally illuminated and the 

 organism is oriented. Torrey ('13), Bancroft ('13), Loeb ('18). 

 and others maintain that orientation in asymmetrical organisms 

 like Phacus and Euglcna is in principle precisely the same as 

 orientation in bilaterally symmetrical organisms. These organ- 

 isms rotate on their longitudinal axes and proceed on a spiral 

 course with a given surface continuously directed outward. The 

 locomotor appendages are, consequently, on one side when the 

 organism is in one position on the spiral and on the opposite side 

 when it is in another position. Owing to this it isTield by those 

 mentioned above, that orientation in these asymmetrical organisms 

 is essentially like that in bilaterally symmetrical organisms. If 

 this contention holds it is evident that the rate of locomotion 

 should depend upon the luminous intensity of the field. With 

 this in mind we have investigated the relation between the in- 

 tensity of light and the rate of locomotion in Phacus and Eiiglcnu. 



Pliacus pleuronectes. 



Phacus is a small green flagellate somewhat like Euglena in 

 structure and behavior. The species studied closely resembles 



203 



