THE DEVELOl'MKNT OF I'ALUDINA VIVIl'AKA. 139 



Tliiele seeks very little support for bis view iu the facts of 

 development, except in the rapid rotation observed by 

 Boutau in Acinoea, and it is seen at a glance to be wholly 

 inapplicable to the development of Paludina where rotation is 

 gradual, and where both gonad and coil are only formed 

 when torsion is far advanced. It brings us, therefore, no 

 nearer to forming a conception of the ontogenetic course of 

 torsion, and the development of Paludina gives no evidence 

 to support it as a phylogenetic theory. 



Whatever view may be held with regard to phylogeny, in 

 ontogeny it seems to me that we are ultimately thrown back 

 upon the problems of heredity, and for tlie present we must 

 agree with Guiart (9) when he says, "Mais a ceux qui nous 

 denianderont la cause mecanique de cette torsiou, et qui nous 

 reprocheront de ne pas I'avoir trouvee chez I'embryon, nous 

 repondrons simplemeut ceci. II ne faut pas confondi-e 

 ontogenie et phylogeuie, la cause n'existe pas chez I'embryon, 

 nuiis chez le moll usque primitif." From the nature of the 

 case the evidence which ontogeny can give upon the phylo- 

 genetic cause is merely negative. 



Summary. 



To sum up, then, theories of Gasteropod torsion may be 

 divided into two classes : 



a. Those which view the present position of the palleal 

 complex as due to a forward movement along the right side 

 of the body, which resulted from greater growth of the left 

 side of the body than of the right. 



h. Those which view the present position of the palleal 

 complex as due to a ventral flexion followed by a vertical 

 rotation of the whole visceral hump upon the head. 



The evidence for the second of these views seems greater 

 than that for the tirst, in that — 



1. A vertical displacement through 18U'^ of all the organs 

 contained in the visceral hump takes place in the course of 

 ontogeny. 



