TORSION OF THE BODY. 
■207 
gradually become displaced and move towards the right side of the 
body, which would be favourably influenced for their reception by 
the strain of the shell upon the opposite side, and the shell would 
become more and more posteriorly placed according as the organs 
beneath moved round towards the right, to attain the most favourable 
position for the free exercise of their functions, which would be the 
side of the body opposite to that upon which the shell was recumbent, 
until owing to the constant tendency of the shell to take up a position 
in the rear as that most favourable to locomotion, and the equally 
constant and con-elated movement of the organs to attain a position 
opposite to it as offering least interference with their functions, the 
l»allial organs eventually occupy the anterior part of the body 
opposite the shell, which has at length 
deflnitely assumed the posterior position 
most favourable to locomotion, and also 
to the free action of the now anteriorly 
placed respiratoi-y and excretory organs. 
But when the shell became long and 
unwieldy and tended to fall over to the 
side of the animal, the body and shell 
could no longer retain their simple conical 
shape, as owing to the lateral inclination 
of the shell the base of the visceral dome 
would be uncovered at the right side if 
the inclination of the shell was to the left, 
and the gTOwth necessary to protect the part thus uncovered would 
necessitate a quicker increase there, which would consequently be the 
point of maximum growth and hecome convex, while the opposite or 
underside would be the point of minimum gTowth and become concave. 
If the shell retained the same position during growth a discoid shell 
would be produced, but if the growing shell on account of its shape, 
increasing weight, or other cause becomes altered in position towards 
the right the point of maximum growth would be changed and the 
spirally coiled dextral shell and visceral sac would result, which 
the now asymmetrical pallial cavity has assisted to perpetuate. 
The long continuance of the unfavourable conditions to which the 
primitively left organs of the pallial complex were subjected by the 
compression due to the heavy external shell, eventually brought about 
their more or less complete degeneration and loss, thus leaving vacant 
Fig. 411. — Diagram illustrating 
the graduation of the compression or 
strain to which the visceral dome is 
subject if the shell be inclined to the 
left (after Lang). The thickest lines 
indicate the point of greatest com- 
pression and the thinnest lines the 
point of least compression. The 
arrows indicate the direction in which 
the twisting takes place, owing to the 
position of the shell on the left side. 
