MOTOR RESPONSES 327 



Since reversal in the stroke of the ciHa begins on the cathodal side, 

 it would seem that it must be associated either with an increase in the 

 concentration of the negative ions or with a decrease in the concentration 

 of the positive ions, on the cathodal side of the semipermeable structures 

 on this side of the organisms. Either a decrease of polarization or an 

 increase in the water content of the cytoplasm may be involved. If this 

 is true, then transfer from culture fluid to distilled water and from dis- 

 tilled water to solutions of monovalent cation salts should, since this re- 

 sults in reversal of the stroke of all the cilia, produce similar changes 

 in the concentration of ions or water. In other words, the transfer from 

 culture fluid to distilled water should produce a decrease in the con- 

 centration of positive ions on the outside of the semipermeable struc- 

 tures, or an increase in the water content at the surface. A transfer from 

 distilled water to solutions of monovalent cation salts should produce 

 like changes. Whether or not this obtains is at present unknown, but one 

 would expect it to obtain, if, as is frequently asserted, permeability is 

 increased by monovalent cation salts. 



D. COLONIAL ORGANISMS 



No detailed observations have been made on the response to electricity 

 of any of the colonial organisms except Volvox. Volvox orients very 

 precisely in direct current. It swims toward the cathode under some condi- 

 tions and toward the anode under others. Carlgren (1899) maintains 

 that reversal in the direction of orientation is correlated with the dura- 

 tion of exposure to the current. Terry (1906) and Bancroft (1907) 

 contend that it is correlated with the intensity of the light received and 

 the duration of exposure to it. Mast (1927c) found that Volvox swims 

 toward the cathode when it is photopositive, and toward the anode when 

 it is photonegative, i.e., that the response to electricity is specifically cor- 

 related with the response to light. 



In photopositive colonies in which rotation on the longitudinal axis 

 is inhibited by means of pressure, the flagella on the cathodal side stop 

 beating immediately after the circuit is closed. They remain inactive 4.5 

 to 6 seconds and then begin to beat again (Fig. 117). If the circuit 

 is now opened, those on the anodal side stop for a few moments and 

 then beat again. If the colonies are swimming and rotating on the longi- 

 tudinal axis in the normal way, but are not proceeding directly toward 



