368 , Proceedings of Boyal Society of Edinburgh. [sess. 
the gill has disappeared, while the respiratory function is taken up 
by a number of lamellae, into which the edge of the mantle has 
split up (compare a similar phenomenon in Patella). This is the 
case in Pleurophyllidia, the relative position of whose pallial organs 
agrees with that in Pleurobranchea. From this the further step 
towards the condition in Tritonia is easy — where the mantle pro- 
cesses become divided up into typical l^udibranch appendages, the 
mantle organs still retaining the lateral linear position. 
Conclusions. 
We may sum up what we have now arrived at concerning the 
torsion of the Molluscan body looked at from this negative point of 
view of the untwisting after torsion. With the disappearance of 
the osphradium there is a turning backwards of the pallial complex 
(Bulla), and with the falling backwards of the visceral mass there 
is a dragging backwards of the heart, and further retreat of the 
pallial organs, with diminution of the pallial cavity (Philine). 
When the shell becomes flattened out or rudimentary it ceases to 
exercise any distorting influence on the body, and the foot and 
mantle become the modifying factors, leading to characteristic 
variations. In the Nudibranchs the original bilateral symmetry of 
the Mollusca (c/. fig. 6) is apparently resumed, but the lost organs 
of the one side do not again reappear. 
It will have been noted that in all these variations there is 
usually present an obvious mechanical cause (coming into play only 
during post-embryonic life), such as the inclination of the heavy 
shell to one side, causing a one-sided pallial cavity ; the dragging 
forward of the mantle by a sensory organ, and backward by excre- 
tory organs, both under the control of and exercised by the animal ; 
the falling backwards of the shell and visceral mass, due to mode 
of life; the consequent dragging backwards of the heart; the 
becoming rudimentary of the shell, due to change in the position of 
its generating organ, the mantle. Natural selection with indefinite 
variation will, of course, account for these changes, as it will for 
any change that cannot be proved to be positively injurious to the 
animal or the species. Such a proof is perhaps impossible, yet 
every case of possible definite variation with a possible mechanical 
