136 ISABELLA M. L>RUMMOND. 



twist of the oesophagus and visceral conuecfcives lies alto- 

 gether in the anterior region of the mantle cavity, and in 

 front of the anus. All the region behind this, we must 

 believe, has equally undergone a torsion of 180°, and yet hei-e 

 we have clearly marked that peculiar disposition of the 

 mantle cavity which Amaudrut seeks to explain. It seems 

 to me that this is not to be regarded as due to torsion at all, 

 but to unequal growth ; already in Stage E, as we have seen, 

 the main features were present, the mantle cavity reaching 

 over the mid-dorsal line behind and being much less ad- 

 vanced in front, while the rectum showed no tendency to a 

 corresponding disposition. It is not, in fact, till Stage H 

 that the position of the anus ceases to be where theoretically 

 it should be according to the degree of torsion, and here the 

 displacement is due to a sudden bend of the rectum to the 

 right quite close to the anus. The final disposition of the 

 rectum is due to a forward growth in the direction indicated 

 by this bend after torsion is. complete, and may perhaps be 

 due, pliylogeuetically, to the advantage gained by the animal 

 in having the anus in a position as far removed from the gill 

 as possible. 



Boutan's explanation of the cause of torsion has already 

 been mentioned, and it has been shown to be hai-dly 

 applicable in the case of Paludiua. His conception of this 

 antagonism of growth between visceral hump and foot is, 

 much more than Pelseneer's, that of an ontogenetic cause, 

 fur he expresses the opinion most definitely that if this 

 antagonism could be suppressed torsion would not take place. 

 Now amongst the monstrosities already referred to it is 

 certainly true that I did not find one with an abnormally 

 small foot and yet a visceral hump which had undergone 

 torsion, but, on the other hand, fig. M ii shows how both 

 foot and hump njay be very highly developed and yet no 

 torsion take place. Surely the antagonism of growth, if such 

 exist, must be much greater in this case than, for instance, 

 in Stage (J, where torsion has already begun. This leads 

 on to IJoutan's view of torsion as the cause of asymmetry. 



