362 Proceedings of Eoyal Soeiety of Edinburgh. [sess. 
touch at the entrance of the pallial cavity being sufficient to cause 
it to be abruptly closed. Perhaps also to be explained by this is 
the fact that the genital opening has now considerably changed its 
position, so that, when this contraction occurs, it is completely shut 
out from the pallial cavity. This is the first indication of the wide 
separation of the genital opening from the others, which is so 
marked a feature of many other forms among the Opisthobranchs. 
In Acera, also, we find the first trace of the disappearance of the 
shell. It is thin and membraneous. This condition is evidently to 
be attributed to the change in the position of its secreting organ, the 
mantle, — for it, like the foot and the animal generally, shows a ten- 
dency to come out of the shell. It lies partially reflected over the 
outside of the shell. We do not therefore necessarily require to 
call in the aid of natural selection to account for the disappearance 
of the shell. 
Such forms as Doridium (fig. 4) or Philine (practically fig. 4) are 
illustrations of further progress in the same direction. Here the 
pallial organs are turned much further backward ; the shell is now 
completely enveloped by the mantle; the visceral region is about 
equal to the cephalic region. 
The forms we have been considering are all animals in which the 
first stages only of the disappearance of the shell are seen, viz., it 
becomes enveloped by the mantle ; the coiling becomes less marked 
