EELATIONS OF KIDNEYS IN HALIOTFS TUBEHCULATA. 91 



withstanding the views he expresses^ Pelseueer's results in this 

 matter tend to strengthen the theory supported in this paper. 



If this theory be adopted it is possible to say, now 

 Pelseneer has settled the " Cemoria " difficulty, that in all 

 Anisopleura the (post- torsional) right is par excellence 

 the excretory side, even in those forms which still have a 

 median anus. This opens up a possibility of very great and 

 far-reaching interest as regards gastropod morphology. 



Pelseneer's theory of the gastropod twist is now generally 

 accepted as amended in the matter of terms by Amaudrut 

 and others. 



According to this the far-off untwisted ancestor had a gut 

 going straight from front to back. This underwent — 



1. A ventral flexure, giving the gut a cephalopod-like 

 disposition. 



2. A lateral torsion through 180° in a counter-clockwise 

 direction, affecting all the animal except the head and foot. 

 As a result of this torsion the branchial cavity and anus, 

 previously postero- ventral, became antero-dorsal. 



It is generally allowed that the pre-torsional position of 

 the branchial cavity militated against its efficiency in a form 

 possessing a creeping foot, for it would be pressed down 

 against the top of the foot by the weight of the shell. 

 Natural selection, therefore, led to the upward shifting of 

 the cavity by survival of npward variations of its position. 

 As far as I am aware, no one has yet shown how it is that 

 the twist is counter-clockwise, and this has been an undoubted 

 weakness in the theory ; but the difficulty is, I think, 

 removed by a consideration of the excretory organs and 

 ctenidia on the lines suggested in this paper. 



Let it be granted that in all Gastropods which have under- 

 gone the torsion the right is the excretory side of the 

 branchial cavity, the left being more particularly devoted to 

 respiration. 



Then it is at least possible that this differentiation was 

 already established during or before the torsion. It is not 

 sufficient to argue that this is unlikely because the two 



