THE DEVELOPMENT OF PALUIUNA VIVIPARA. 137 



The normal Gasteropod larva^ he says, is perfectly bilaterally 

 symmetrical, and remains so till torsion takes place ; but as 

 soon as this begins the asymmetrical growth of the internal 

 organs begins, and, if torsion could be averted, symmetry 

 would be maintained. Certainly iu this connection the sym- 

 metry of Monstrosity II is striking, but that asymmetry and 

 torsion are closely connected no one doubts, and whichever 

 were the cause of the other, or if both were the outcome of 

 some common cause, the result would be the same. In the 

 normal course of development it has been repeatedly pointed 

 out by other writers that asymmetry in some form or other is 

 found before torsion begins. In Paludina torsion begins so 

 early that it is difficult to be quite sure of this, but the liver, 

 at least, is never wholly symmetrical, and the unequal 

 development of the original rudiments of the mantle cavity 

 takes place as nearly as possible simultaneously with the 

 beginning of torsion. 



Once more, Boutan turns to antagonism between foot and 

 visceral hump in order to explain the coiling of the latter. 

 If, when the creeping sole of the foot is developed, this can 

 stretch out without lateral displacement of the visceral hump, 

 then the shell, he believes, will remain symmetrical ; but if 

 not, then the hump will be pushed to one side or other, and 

 the sense of the future coil will depend upon which side it is 

 pushed towards. Such a view, it would seem, would be 

 quite impossible to accept after even a cursory view of the 

 facts of development. For if, as would seem to be the case, 

 the coiling of the visceral hump is primarily the result of a 

 definite process of growth in the liver (cf. figs. IG and 17), 

 this is altogether independent of the exact relation in which 

 this oi'gan finds itself to the foot. As a matter of fact, the 

 dextral coil of Paludina begins before torsion is quite 

 complete, and therefore, while the apex of the visceral hump 

 is still to the left of the foot, by a strong growth of the liver 

 towards the right, which, when torsion is complete and the 

 visceral hump nearly symmetrical, points to the right and 

 upwards. It is altogether inconceivable that any accident of 



