IMOYM.MKNTS, KTC, OF FRKSII-WATER PLANAKIANS. 687 



much less ]')rononiicod. 'I'lie aiiiinal in the transverse position, 

 at the moment of makino- tlie current, will sim])lv tnrn the 

 anterior ]iart of the body somewhat toward the kathcxh' niid 

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

 a short distance towards the new kathode, and then stop 

 again. The oinentation becomes less and less precise the 

 longer the current acts. The position most fre([uently tnken 

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

 current for a short time is shown in Fig. 4■^, 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 fairly Aveak intensities. With verj' weak 

 currents no striking effect is produced. With a current 



+ 



Fig. 43. — Diagram showing partial orientation of Plaiiaria to the 

 constant, current. 



which is just stong enough to cause a general movement of 

 Paramoccium towards the kathode, the only eifect on a 

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

 some cases a very slight turn of the head towards the kathode 

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

 gliding 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 verv strono- contraction of the ventral lonsjitudinal 



