MOVEMENTS, ETC., OF FRESH-WATEK. PLANARIANS. 607 



evitable because, as has been shown above, unihitoral 

 sfcimulatiou of the posterior region of the body in a normal 

 individual does not cause the negative reaction, but instead 

 merely causes the animal to move ahead faster by crawling. 

 If these paths for the crossing of impulses which are so 

 immediately eifective after the operation are present in the 

 uninjured specimen, one would, expect the reaction to be of 

 quite a different character from what actually occurs. A 

 stimulus applied near the posterior end would naturally cross 

 over at once and produce a bending on the opposite side at 

 the same level. Or the stimulus might diffuse, so that the 

 entire opposite side Avould be affected and the worm would 

 become uniformly curved on that side. But as a matter of 

 fact we find that the turning affects only the anterior portion 

 of the body. If it is urged that after operation the crossing 

 of impulses takes place through the general protoplasm the 

 difficulties encountered are no less, for it must be shown how 

 passage of an impulse through the protoplasm to cause a 

 perfectly well co-ordinated reaction can appear so quickly and 

 produce such perfect results at once. If tested immediately 

 after the operation, before the general lowering of tonus is 

 felt, the reaction time for the negative response of a posterior 

 piece of the body will not differ appreciably from that of a 

 normal worm. Now, according to the views of the advocates 

 of the theory that after operations involving loss of nervous 

 tissue, impulses may be conducted through the general 

 protoplasm, it is held that such conduction is always at first 

 appreciably slower than in nervous tissue. It would also 

 seem on purely a priori grounds that this must be true. 



Thus it is seen that there are serious objections to the view 

 that the negative reaction is the result of a contraction on the 

 side of the body opposite to that stimulated — that is, that it is 

 a crossed reflex. The question now arises, if the reaction is 

 not produced in this way, in what other way can it be 

 produced ? Evidently it is (juite possible that the anterior 

 part of the body can be turned away from the stimulus by a 

 lengthening of the side stimulated, quite as well as by a 



