120 ISABELLA M. DT^UMMONP. 



asymmetry of the Gasteropods, which upholds essentially the 

 same view of torsion for the Prosobranchs as Pelseneer and 

 Aniaudrut had already enunciated^ and which derives its 

 chief value from the author's claim to have actually observed 

 the vertical torsion take place in the case of Acmoea. 



It is at once evident that the processes which, broadly 

 speaking, characterise respectively the two classes into which 

 we divided the theories under discussion, will not have 

 entirely similar results. Both, indeed, alike have, as their 

 chief results, the forward doi^sal position of the anus and the 

 crossing of the visceral connectives, for it was to account for 

 those facts that the theories were originally framed ; but, on 

 the other hand, the twisting of the oesophagus, if true, could 

 never have arisen from the processes which Biitschli 

 describes, while the growth of the mantle must be conceived 

 quite differently, according to which hypothesis is accepted. 

 If Biitschli is correct, what was originally right remains on 

 the right side throughout ; while, according to the view of 

 vertical torsion, the mantle, and therefore also the shell, share 

 in the displacement of the palleal organs. The same holds 

 good for most of the viscera, and is especially clearly illus- 

 trated in those organs which lie dorsally or ventrally ; for 

 while on the theory of unequal growth a lateral shifting 

 might easily take place in the same manner as is the case for 

 the palleal organs, a dorso-ventral displacement is only 

 readily understood on such a theory as Pelseneer's. Thus 

 Plate has to account for the gonad, which is dorsal in 

 the Chitons, having a ventral position in the Gasteropoda, by 

 supposing that a lobe of the great liver of the left side grew 

 dorsally to it and pressed it against the foot. 



Taking these considerations separately, and beginning with 

 the last, the facts of embryology seem to me to show in a quite 

 unequivocal manner that actual rotation of the organs has 

 taken place round an axis coinciding with the oesophagus in 

 its direction. It is for this purpose that the drawings of 

 figs, n to 17 were made, and a comparison of tlieso with one 

 another, bearing in mind that they are all orientated on the 



