134 ISABELLA M. DRUM MONO. 



liver is companitively iiisignificiint. Moreover, in folluwiiig- 

 the uormal development from stage to stage, it is obvious 

 .that the great original right-hand extension of the mantle 

 cavity much more nearly keeps pace with the torsion than 

 is the case with the liver; and, finally, it may be noticed that 

 this view would harmonise with Amaudrut's reasoning con- 

 cerning the supra- and sub-intestinal ganglia. It may well 

 be, however, that the growth of these organs is to a large 

 extent dependent the one upon the other, and that equilibrium 

 is maintained by the asymmetrical growth of the liver on one 

 side of the body being compensated by asymmetrical growth 

 of the mantle cavity on the other. 



We turn now to the other side of the question, but Pel- 

 seneer puts forward his theory in such a broad and general 

 form that it is difficult to enter into any detailed discussion 

 beyond the general considerations which have already been 

 adduced in his favour. The chief objection that might be 

 raised in this case is, perhaps, one which arises from the 

 difficulty, in a course of development like that of Paludina, of 

 distinguishing Pelseiieer's two processes of ventral and 

 lateral torsion. And, indeed, they do go on so closely hand 

 in hand that as a matter of fact the anus travels in an oblique 

 and never in a vertical direction. Nevertheless there is 

 clear evidence of a vertical rotation of the organs contained 

 in the visceral hum}), as has been already pointed out, and 

 in each stage it is possible to separate from the results of this 

 process a certain clearer development of the visceral hump 

 and sharper bend of the alimentary canal from that which 

 obtained in the previous stage, which must be the result of a 

 })rocess akin to Pelseneer's ventral torsion, or, as Amautlrut 

 has better styled it, ventral flexion. In the early stages the 

 distinction between the two processes is very clear — as, for 

 instance, in Stage C, where the ventral flexion is already 

 strongly marked, while the lateral torsion is but just begun. 



With regard to the cause which Pelseneer seeks for these 

 processes, however, the development of I'aludina offers no 

 conllrinalion. The growth of the fool, he says, forms an 



