MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 615 



stimulation at this point the specimen contracted longitudi- 

 nally, and then started moving ahead again in exactly the 

 same direction in which it was going before stimulation. It 

 was impossible to induce any turning awny following stimula- 

 tion of the side x, although this was tried many times. 



Now it is evident that this specimen comes very near 

 to being an isolated longitudinal half-plauarian. All the 

 ^ti'uctures of the original one half ai-e present, and there 

 is only a very little of the other side of the body produced in 

 the line of new tissue, down the originally cut edge. In this 

 new tissue there is probably very little differentiation, and the 

 muscle layers are not well formed. It was brought out above 

 (p. 610) that an isolated half of the body ought to be able to 



Fig. 22.— 0. Operation diagram, b. Piece which regenerated from A 

 in Diagram r/. The new tissue is indicated by stippling. 



give only one reaction, or, in other words, ought to be able to 

 turn the body in only one direction in response to stimulation, 

 provided this turning is due to an extension of "the stimulated 

 side. We find precisely this result in the regenerating speci- 

 men just discussed. It turns away from stimuli applied at y 

 because on that side are present all the muscles necessary for 

 extension just as in a normal animal. It does not turn away 

 from stimulation of the side x because it has not the necessary 

 muscles for extension on that side. On the view that the 

 turning away is due to contraction on the side opposite that 

 stimulated, there is no reason why stimulation at x should not 

 cause the animal to turn away from the stimulus, because the 

 opposite side {y) has all its muscular mechanisms intact. 



