138 ISAP.ELLA M. DRUMMOND. 



grovvtl), independent oC torsion^ wliieli should cause tlie apex 

 of the visceral liump to remain upon the left side of the foot, 

 should alter this growth of the liver, and cause the coil to 

 become sinistral. 



With regard to the cause of asymmetry, another view 

 remains to be mentioned here, namely, that of Grobben (8), 

 who, while accepting Pelseneer's main conclusions, finds 

 himself unable to regard the antagonism between foot and 

 visceral hump as a true ontogenetic cause of torsion. He 

 grants Pelseneer's view that the growth of the foot necessi- 

 tates a vertical displacement if the anus is to continue to 

 approach the mouth, and in order to explain how this 

 is produced he has recourse to Plate's suggestion of 

 an unequal gi-owth of the two originally symmetrical 

 liver lobes. For reasons already stated when Plate's 

 theory was under discussion this is not altogether satis- 

 factory, for the growth of the liver does not keep pace 

 with torsion, and the chief development of it takes place 

 after torsion is complete. As a phylogenetic cause it may 

 have phiyed its part, but probably not quite in the manner 

 that is described by Plate. 



Thiele (18) also takes up a position somewhat intermediate 

 between the two extreme points of view on torsion, foi-, 

 while agreeing with Pelseneer that before torsion the shell 

 must have been bent with its apex pointing forwards, as 

 though forming the beginning of an exogastric coil, he 

 approaches Lang more nearly than anyone else in his view 

 of how torsion lias been effected. Lie dismisses Plate's view 

 of the liver as the disturber of the original symmetry of the 

 body, and believes this part to be played by the gonad, in 

 which he also sees the cause of the coiling of the visceral 

 hump. Thus, as, in the Gasteropods, the gonad is formed 

 only on the left, the coil also lies with its apex in the left. The 

 condition of affairs which is tlius reached is shown in his 

 lig. 3, p. 15, and this be believes to be a position of unstable 

 e([uilibi-ium, conse(]uently a sudden rotation is effected till the 

 visceral huuq) comes to rest in the normal adult position. 



