594 EAYMOND PEARL. 



the middle line. There is no turning towards or away of 

 any part of the body. The lack of any special effect of 

 unilateral stimulation is not surprising, for the reason that 

 rapid movement in a forward direction will get the animal 

 away from harmful stimuli affecting this region^ in the long 

 run, more quickly than any other. Further, there Avould be 

 no advantage in the production of a positive reaction by 

 stimuli at the posterior end. If we think of these reactions 

 as having been developed by natural selection there would 

 be no possibility of such a reaction having arisen, for the 

 reason that practically any favourable stimulus would be 

 encountered by the anterior end before it possibly could be 

 by the posterior. Very weak mechanical stimulation of the 

 posterior end of the body causes only a local contraction at 

 the point stimulated. 



4. Reactions to Stimulation of the Ventral Sur- 

 face. — In the descriptions of the reactions to mechanical 

 stimuli up to this point we have been considering stimuli 

 applied to the dorsal surface and to the margins of the body. 

 It may be well to describe briefly what the reactions in 

 response to localised stimulation of the ventral surface are. 

 This matter can best be tested when the animal is moving on 

 the under side of the surface film, with its ventral side 

 uppermost. It might be supposed before the trial was made 

 that this habit of the animal would afford ideal conditions for 

 testing its reactions to ventral stimulation, but, as a matter 

 of fact, the conditions are anything but ideal. The flexibility 

 and elasticity of the surface film makes it almost impossible 

 to touch it with a stimulating point anywhere within a radius 

 of a centimetre about a planariau without causing the animal 

 to be jerked bodily to one side or the other, quite sharply 

 and for some little distance. This is, of course, a mere 

 mechanical effect, which takes ])lace with lifeless bodies also. 

 Furthermore, as has been mentioned in an earlier section, it 

 appears to be very difKcult for planarians to quickly change 

 the direction of their movement when on the surface film (as 

 is necessary in reacting to stimuli). On account of these 



