702 RAYMOND PEARL. 



the posterior portions of the body induces the crawling move- 

 ment. This is to be regarded as the specific reaction of this 

 portion of the body. Weak stininhition of the same region 

 causes local contraction at the point stimulated in the case of 

 mechanical stimuli, while weak chemical stimuli applied to 

 this region are ineffective. 



7. The ventral surface of the body of Planaria is strongly 

 positively thigmotactic, and the dorsal surface is negatively 

 thigmotactic. 



8. When the organism is placed in an inverted position it 

 performs the righting reaction. This reaction consists in a 

 turning of successive parts of the body about the longitudinal 

 axis through 180°. During the process the animal takes the 

 form of a spiral. The anterior end is brought into the up- 

 right position first. On analysis the righting reaction is 

 found to be a special case of the reaction to strong stimuli 

 (the negative reaction). It is brought about by an extension 

 of one side of the body, while the other side maintains its 

 original length (pp. 676 — 679). The reaction is given when- 

 ever the ventral surface is removed from a solid or the surface 

 film of the water. 



9. To the constant electric current Planaria reacts by 

 turning the anterior end towards the kathode. Complete 

 orientation and moveuient towards the kathode may occur. 

 The turning towards the kathode is brought about by an 

 extension of the anode side of the body. The current causes 

 a contraction of nuiscular elements whose long axes are 

 parallel to the direction of the current (pp. 690 — 693). The 

 current very quickly paralyses planarians on which it acts. 



The rhabdocoele Stenostoma leucops orients to the 

 current with the anterior end towai'ds the kathode, and 

 moves towards this pole. This orientation is brought about 

 by changes in the positions and conse(|uent effective beat of 

 the cilia, exactly like those which occur in the case of the 

 ciliato infusoria. Ciliii, on the portit)ns of the l)ody directed 

 towards the kathode pole, take on reversed positions. 



10. All the normal reactions to stinmli are of the luiture of 



