Page:Proceedings of the Royal Society of London Vol 69.djvu/309

Rh 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.

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