ilOVEMKNTS, KTC, OF FRESH-WATEIJ PLANAKIANS. 687 



mucli less pronounced. The animal in the transverse })osition, 

 at the moment of making- the current, will simply turn the 

 anterior part of the body somewhat toward the kathode and 

 then stop. Reversal of the current causes the head to swing 

 a short distance towards the new kathode, and then stop 

 again. The orientation becomes less and less precise the 

 longer the current acts. The position most frequently taken 

 by a specimen after it has been submitted to the action of the 

 current for a short time is shown in Fig. 4o, where it is seen 

 that the orientation of even the anterior end is not very 

 precise. In all such cases the specimen remains perfectly 

 quiet after the first turn towards the kathode until the 

 current is reversed or broken. 



The behaviour described is that which is typical for currents 

 of medium to fairl}- weak intensities. With very weak 

 currents no striking effect is produced. With a current 



+ 



Fig. 43. — Diagram showing partial orientation of Planaria to tlie 

 constant, current. 



which is just stong enough to cause a general movement of 



Paramoecium towards the kathode, the only eifect on a 



planarian gliding at right angles to the current is to cause in 



some cases a vei-y slight turn of the head towards the kathode 



at the moment of making. The specimen does not stop the 



g-liding movement, and is not forced into any orientation, but 



may give a slight turning reaction, which changes its course 



from one squarely at right angles to the current to one 



turned a little diagonally towards the kathode. In many 



cases such a current produces no effect whatever. With very 



strong currents the planarian stops at the moment of making, 



jerks the anterior end around towards the kathode more or 



less, and then curls up into the form shown in Fig. 44, as a 



result of very strong contraction of the ventral longitudinal 



