On the Development of Paludina vivipara. 



293 



A renewed investigation of the development of Paludina shows that 

 Biitschli's interpretation of it is open to much criticism. His point 

 concerning the actual distance between mouth and anus remaining 

 always the same is valueless, unless the mantle cavity be regarded as 

 entirely formed by invagination. Even on the latter supposition rapid 

 development of the original right division of the mantle cavity, at a 

 time when torsion is being produced, shows that there is actually 

 great activity in Biitschli's zone of cessation of growth. 



Lm.c.- 



Plate's suggestion that the liver, by its rapid growth on one side, 

 alone brings about the asymmetry of the whole body is unsatisfactory, 

 in that while torsion goes forward most rapidly the liver is almost 

 stationary in its growth, while its chief development takes place after 

 torsion is complete. 



More fundamental than the above is the change in the position 

 of organs which takes place through ontogeny. Biitschli and Plate 

 have no explanation to offer of any displacement of organs in a vertical 

 direction. A careful study of successive stages, however, reveals an 

 orderly rotation of all the organs, whereby those originally dorsal 

 become ventral, and vice, versd. This can probably only be explained 



