GIO EAYMOND PEAEL. 



along its outer margin rather than by actively contracting on 

 its inner cut margin ? This question may be answered by 

 operative experiments of a different character. If the side 

 stimulated, acting independently, produces the reaction by 

 lengthening on its own outer side, then an isolated longitu- 

 dinal half of the body ought to be able to give only one 

 reaction wherever stimulated, or, in other words, it ought 

 always to turn towards the same side. Furthermore, such a 

 piece ought always to turn towards the cut edge, since onl}' 

 on the side opposite to this has it a margin possessing 

 the necessary circular muscles for extension (vide sup., 

 pp. 556, 557). On the other hand, if the contrai'y view is 

 correct, that the turning away is due to contraction of the 

 longitudinal muscles on the side opposite that stimulated, 



Fig. 18.— Showing the appearance of a longitudinal half of a 

 phinarian wlien at rest. 



then such an isolated longitudinal half of the body ought to 

 be able to turn either way, according to the localisation of 

 the stimulus, since there are longitudinal muscle-fibres along 

 the cut edge as well as along the other. We may determine 

 from experiments which of these two views is correct. 



Unfortunately, it is impossible to get any clear evidence 

 on this point from entirely separated longitudinal halves of 

 tiie Avorm. When a planarian is split in two lengthwise 

 each of the pieces immediately becomes strongly contracted 

 longitudinally on the cut side, the apparent purpose of this 

 reaction being to reduce the exposed surface at once to a 

 minimum. After this strong contraction has taken place, 

 giving the piece the form shown in Fig. 18, no further 

 pi'ogressive movement can take place, and the general tonus 



