1894 - 95 .] Dr Gilchrist on Torsion of the Molluscan Body, 363 
and wider, a fact no doubt due to the diminished rate of growth of 
the shell in proportion to the growth of the animal ; and this, again, 
is connected with the enveloping by the mantle. 
The fact that there is present a shell still capable of containing 
part of the viscera has an important bearing on the position of the 
mantle cavity. Viewing the animal from above, as in the diagrams, 
it would appear as if the mantle and organs associated with it were 
shifted to the posterior of the body. This, however, is not to be 
considered as actually the case if we are to compare these forms 
with others in which the shell is quite flattened out, and where the 
viscera formerly contained in the coiled shell now descend into the 
foot. Here the shell and visceral mass have merely fallen or 
been dragged backwards, and there is not a true morphological 
translation of parts. This position, which it is difficult to regard as 
other than the result of a purely mechanical strain, is also reflected 
in the dragging backward and twisting of the heart (see figs.). 
The habits and environment of the animals are quite in keeping 
with this, as they are found (especially Doridium and Philine) 
crawling amongst debris and mud or sand. 
Before leaving the Cephalaspidea we may mention a form — 
Gasteropteron — belonging to this group, but which does not fit in 
very well either to this division or those following, owing to the fact 
that it is adapted to a pelagic or semipelagic mode of life. It 
presents nothing of much interest in the torsion of the body. The 
pallial organs lie on the right side ; the shell and mantle are much 
reduced, especially the former. 
With the exception of this form, we have found that the shell in 
the Cephalaspidea is still an important structure to which the rest 
of the organism must be conformed. When, however, we pass 
over into the group where the shell is quite flattened out or dis- 
appears, these two organs — the mantle and the foot — begin to 
show peculiar specialisations, and become, as the shell formerly 
was, important factors in determining the topography of the 
body. There are two extremes : — 1st, the foot may be much en- 
larged, and specialised into locomotory and respiratory organs, while 
the mantle remains small or disappears ; 2nd, or vice versa^ the mantle 
may enlarge and the foot remain stationary in its development. 
