180 ISAF.KLLA M. DIMIMMOND. 



between the two general theories undei- consiflefation relates 

 to the symmetrical growth of the mantle, and this is the only 

 one the evidence upon which seems still to point in favonr of 

 Biitschli's view ; for, if the whole visceral hutnp has nnder- 

 gone a rotation, we should expect to find signs in the 

 innervation. But the definitive right side belongs to the 

 original left, and vice versa; whereas it is well known that 

 the right pleural ganglion gives rise to the mantle nerve of 

 the definitive right side of the body, and the left pleural 

 ganglion to that of the definitive left. How this can be 

 explained npon Pelseneer's view is not quite clear, unless it 

 may be that the mantle nerve is cmbryologically a late out- 

 growth of the pleural ganglion, and is altogether post- 

 torsionnl ; but if this be so we shall have to admit a 

 discrepancy between embryology and phylogony. The case, 

 however, is not altogether easy, even for the upholders of 

 Biitschli's theory of the symmetrical growth of this region 

 of the body, for the innervation is not wholly symmetrical. 

 As Bouvier (3) remarks, 'Ma branchie n'est rien autre chose 

 qu'une formation palleale, et les meraes nerfs qui I'innervent 

 se repandent en meme temps dans le manteau," and, as is 

 well known, the definitive left ctcnidium is innervated from 

 a ganglion belonging to the original right side of the body. 

 The question, then, is one of some difhculty whichever view 

 one takes, and of hardly greater difficulty in the one case 

 than in the other. 



An examination of the broad features of the two great 

 classes into which we divided the theories of Gasteropod 

 torsion leaves, then, a balance of embryological evidence in 

 favour of that class of which Pelseneer was the first exponent. 

 It will be well now to examine the individual theories more 

 in detail. Biitschli's views have already undergone criticism 

 at the hands of Pelseneer and others as not corresponding to 

 the facts of ontogeny as one sees them. Amaudrut criticises 

 hi in more particularly with regard to the position of the 

 supra- and sub-intestinal ganglia. The supra-intestinal 

 ganglion, he says, is usually situated further back than its 



