328 



MOTOR RESPONSES 



either pole when the current is made, flagellar inactivity on the cathodal 

 side is continuous, owing to the continuous transfer of the zooids from 

 the anodal to the cathodal side. The colonies therefore turn toward the 

 cathode until they face it directly, and the transfer of zo5ids from side 

 to side ceases. 



Orientation. — In photonegative colonies precisely the opposite occurs. 

 The flagella on the anodal side stop beating after the circuit is closed. 



+ 



Figure 117. Sketch showing in a stationary photopositive colony of Volvox the effect 

 of a galvanic current on the currents of water produced by the flagella. A, outline of 

 colony oriented in light; B, same colony immediately after the circuit was closed; a, 

 anterior end; straight arrows, direction of illumination; curved arrows, currents pro- 

 duced by the flagella; +. positive pole; — , negative pole. (After Mast, 1927.) 



and the colonies turn toward the anode until they face it and then swim 

 toward it (Mast, 1927c). 



Galvanic orientation is consequently correlated with photic orientation, 

 but the processes involved diff^er, for, as previously stated, photic orienta- 

 tion is due to a change in the direction of the stroke of the flagella on 

 opposite sides, while galvanic orientation is due to decrease or cessation 

 in the activity of the flagella on one side. 



Electric charge on the colonies. — Galvanic orientation in Volvox also 

 differs from that in Farajuecium, for while the one is due to decrease or 

 cessation in activity on one side, the other, as previously stated, is due to 

 reversal in the direction of the eff^ective stroke of the cilia on one side. 



Since a given colony of Volvox may be either photopositive or photo- 

 negative in the same environment, the difference in response to the light 



