THK DEVEI,orMKNT OF PAl.UDINA VIVIPAEA. 135 



obsfcaclo to the close approacli of the luouth aud anus, and 

 tlierefore vertical torsion takes the place of the ventral 

 flexure. This is really no true explanation at all, for we are 

 left iu the dark as to how the foot brings about this new state 

 of affairs, and we do not get much nearer if we say, with 

 Boutan, that there is an antagonism of growth between the 

 foot and the visceral hump. It seems to me that in the 

 development of Paludina it is altogether out of place to speak 

 of such an antagonism, for torsion begins at a time when the 

 foot is still but a comparatively insignificant ventral pro- 

 jection, and long before the formation of the creeping sole. 



Amaudrut also is unsatisfactory in this respect, for he 

 attributes torsion ultimately to voluntary effort on the part 

 of the animal to get its gills into a better situation. With 

 the main part of Amaudrut's paper embryology has nothing 

 to do ; there are, however, one or two points in which it seems 

 to me ho is mistaken, owing to a too exclusive regard for the 

 anterior region of the body. His description of the shell and 

 visceral hump as aiding the torsion by their weight is wholly 

 inapplicable to embryology ; while his account of the 

 manner iu which the peculiar shape of the mantle cavity and 

 the disposition of the organs included in it are induced 

 receives complete contradiction. "A peu pres dans le memo 

 plan transversal qui passe pas le ganglion visceral posterieur," 

 he says, " se trouvent la partie terminal de la region tordue 

 du tube digestif, le fond de la cavite respiratoire, la partie 

 posterieure de la branchie et le cocur. Ce plan marquant en 

 arriere la limite extreme de la torsion, les organes cpii s'y 

 trouvent out du executes un mouvement de rotation d'envirou 

 180° pour se rendre dans leur position definitive." This holds 

 good for all organs behind this position, but in front of it 

 obviously torsion will be less. It is these facts of which he 

 makes use to explain the apparent slope of the mantle cavity 

 from the left towards the right anteriorly, and of the rectum 

 from mid-dorsal, where theoretically it should be, to the 

 right side of the body, the characteristic position lor the 

 Prosobrauchia. In Stage II, however, the region of the 



