THE DEVELOPMENT OF PALUDTNA YIVTPAPA. 125 



which, for reasons connected with the equilibrium of the 

 body, lies with its apex pointing towards the right (see his 

 figs. F to Hj pp. 185 and 187). This it is, he believes, which 

 causes the approximation of the mouth and anus on the right 

 side. As coiling proceeds this process would be accentuated, 

 and so, npparently, the condition which obtains in the adult 

 Gasteropod is reached without the aid of the late develop- 

 ment of the mantle cavity relied upon by Biitschli. Though 

 this is put forward merely as a phylogenetic theory, Plate 

 believes that the facts of development will fully bear it out, 

 and it is only from this point of view that we can deal with 

 it here. 



Pelseneer (14) was the first to put aside the old point of 

 view. To him it seemed that embryologically two distinct 

 processes took place, both of which had for their object, as it 

 were, the approxiiuation of mouth and anus. The first of 

 these, which ho calls " torsion ventrale," leaves the embryo 

 still symmetrical, but with the alimentary canal bent sharply 

 so that the anus lies far forward ventrally. The mouth and 

 anus being prevented from approaching nearer along this 

 line on account of the outgrowth of the foot, the second 

 process comes into play. This is a " torsion verticale," which 

 takes place at right angles to the last, and has the result of 

 all the organs contained in the shell undergoing a rotation 

 through 180°, the ventral anus thereby becoming dorsal, the 

 organs of the original right side being carried over to the 

 left, and those of the original left to the right. 



More recently Amaudrut (1) has approached the same 

 problems from the point of view of comparative anatomy, 

 and, from a study of the oesophagus and adjacent organs of 

 a number of Gasteropods, has come to the conclusion that 

 the region between the head and the visceral hump has 

 undergone a twist through 180°. This, of course, would fit 

 in well with Pelseneer's observations, for, if the whole visceral 

 hump has undergone torsion with regard to the head, the 

 oesophagus must needs be twisted. 



Finally, Boutan (2) has brought out a paper on the 



