MOVEMENTS, l<]TC., OF PEESH-WATER PLANA RIANS. 677 



is in contact with the bottom extends (by the mechanism 

 previously described, pp. 556, 557) in response to the stimuhis 

 given by the contact of the dorsal surface of that side of the 

 body with the bottom. At the same time the opposite half of 

 the body, by active muscular contraction, keeps its length 

 the same. Thus any bending of the body away from the 

 side stimvilated as in the ordinai'y negative reaction is pre- 

 vented, or, in other words, the long axis is kept straight by 

 the opposite side maintaining actively its normal length. 

 Now the necessary mechanical result of keeping one side of 

 a flexible system at a constant length while the other side 

 lengthens must be that the lengthening side will be thrown 

 into a series of waves. In other words, it is mechanically 

 impossible for the lengthening side to keep its whole edge in 

 the same plane. Furthermore, if in such a system it is 

 possible for rotation about a longitudinal axis to occur, the 

 system will be thrown into a spiral of the form which the 

 planarian takes in the righting reaction. Again, as soon as 

 one side of such a system under elongating stress changes its 

 level with reference to the remainder of the system, and thus 

 starts the formation of the spiral, the long axis of the 

 system (i. e. the centre of the spiral) will keep itself straight. 

 Any further force elongating one side will merely throw the 

 spiral into tighter coils without having any tendency to bend 

 its long axis. This fact is of importance in the case of the 

 planarian where the maintenance of the initial straightness 

 of the long axis is done by the opposite side of the body. Of 

 course, a symmetrical spiral cannot be formed unless the two 

 edges are of equal length, but the moment the spiral of the 

 planarian is started all necessity for one side keeping a con- 

 stant length ceases. It must be kept in mind, however, as 

 has been indicated above, that the force which produces the 

 spiral must act on one side only, and hence the side of the 

 planarian opposite that initiating the movement nuist be 

 moved passively by the other in tlie spiral formation after 

 this has once begun. The direction in which the spiral shall 

 turn will evidently not be determined by the mere lengthen- 



