674 RAYMOND PEARL. 



ahead, the spiral is nnwouiul by the raised edge of each 

 twist dropping down and attaching to the bottom as soon as 

 it is in a position where this is possible. Thus, of course, 

 when the animal has traversed a distance equal to its own 

 length it will have come entirely into the normal position 

 again. The reaction is really a rotation of the body on its 

 long axis through 180°. The mechanism of the turning is 

 such that only a part of the body rotates at a time, — first 

 the anterior end, then the portion next behind that, and so 

 on, till the whole animal has turned over. This rotation by 

 sections, as it were, causes the spiral form which the animal 

 takes on in the reaction. 



The number of turns into which the body is thrown in 

 forming the spiral varies Avith the length of the individual, 

 and appai'ently to some extent witli its physiological con- 

 dition. There may be only a half-turn in the whole body, or 

 there may be one complete turn ; or, again, one and a half 

 turns; or, finally, as many as two complete turns in the body. 

 One complete or nearly complete turn, as shown in Fig. 36, 

 is the usual form of the reaction. In large individuals 

 more twisting is frequently seen. Evidently all the twisting 

 that is absolutely essential for the righting of the specimen 

 is the half-turn given by the turning of the anterior end 

 ventral side down. 



The determination of the direction in which the spiral is 

 thrown, or, in other words, the side of the body towards 

 which the anterior end turns in order to get right side up, 

 was for some time a very puzzling problem. A collection of 

 statistics on the matter showed that the anterior end twisted 

 towards the right and towards the left ^ in an approximately 

 number of cases. This is precisely the result which would 

 be expected if the matter were due to chance only, but the 

 reaction did not give the appearance of being a chance 

 matter. Finally, the determining factor was found to be the 

 relation of the dorsal surface to the bottom. A cross-section 



' Tn tlie figure (Fig. oH) tlie worm is repiTseiifed willi (lie spiral tlu-owii 

 towards tlie left.. 



