602 RAYMOND PEARL. 



possible by varying the strength of the stimulus to produce 

 only certain parts of the whole reaction without the 

 remainder, and, furthermore, that a part of one reaction may 

 in rare instance be combined with a part of the other 

 (v. sup., p. 587). 



d. Mechanism of the Reactions. — A question which 

 is of the greatest importance in all work on the reactions of 

 organisms is, what is the mechanism of the reaction ? In 

 the case of the flat-worm this becomes, what is the neuro- 

 muscular mechanism of the reactions ? Very little direct 

 evidence bearing on this question can be obtained from the 

 reactions themselves. Taking the positive and negative 

 reactions as they occur, there are several different sets of 

 muscles and of nerve connections by means of which they 

 might conceivably be brought about. The best evidence on 

 the question is the indirect evidence from operation experi- 

 ments, in which parts of the mechanism are injured or 

 removed. 



1. Relation of the Brain to the Reactions. — The 

 first specific problem which may be taken up may be stated 

 thus : is the brain necessary for the performance of the 

 normal reactions to mechanical stimuli ? Or, in other words, 

 will a planarian from which the brain has been removed 

 react normally to stimuli ? This question can be answered 

 from the study of specimens which have been cut in two 

 transversely, and consequently we may proceed at once to a 

 description of the reactions of the pieces resulting from such 

 an operation. A typical specimen is cut in two transversely 

 at the level of a point about halfway between the head and 

 the origin of the pharynx, as shown in Fig. 10. As has been 

 mentioned above, the cut itself acts as a strong mechanical 

 stimulus, and the immediate effect of the operation is to set 

 both pieces crawling, the anterior one ahead and the 

 posterior one usually backward. 



If now the pieces are allowed some hours to recover from 

 the immediate effect of the operation, and then stimulation is 

 triedj the following results ai'e obtained : — With the anterior 



