20G 
TORSK^N OF THE liODY. 
animal is strictly symmetrical np to the Troclios})liere stage, the pallial 
cavity being then in the rear, as in the hypothetical primitive mollusk, 
bnt is brought to the front above the neck by the cessation on the 
right side of the embryo of the growth, which is uninterruptedly con- 
tinued on the left. This torsion, eft'ected during development in the 
Streptoneures, is also manifest in the Euthyneures at the commence- 
ment of embryonic life, Imt becomes indefinite and in great part lost 
at a later stage by a movement or twisting in the opposite direction. 
The Antero-ventral torsion to which the primitive exogastric 
inclination of the liody is due, i)robably arose on account of the 
increasing size of the foot seimrating more and more widely the 
anterior and posterior openings of the alimentary canal. The lateral 
twist of the body, with which the antero-ventral torsion is practically 
combined, is owing to the continually increasing protrusion of the 
hernia-like visceral dome and the development upon it of a long, 
heavy shell, giving protection to and a means of concealment for the 
entire animal, which was gradually becoming more actively rci)tant. 
Fig. 109. 
Fig. 110. 
Diagrams illustrating the various positions which it is possible for the hypothetical, unwieldy 
elongated shell to assume (after Lang). columellar muscle ; c. clenidia or gills ; a. anus. 
Fig. 4(17 shows the shell in its hypothetical primitively upright position. 
Fig. 4(1S shows the shell forwardly inclined, the most unfavourable position for locomotion, but 
the most favourable imaginable for the exercise of the functions of the posterior pallial organs. 
1' ig. 4(11) shows the shell inclined to the rear, the most favourable position for locomotion, but the 
most unfavourable for the exercise of the functions of the organs confined within the compressed 
mantle cavity. 
P'ig. 410 shows the shell inclined to the left side, the position it would most probably assume as 
combining least interference with locomotion and with the exercise of the functions of the posteriorly 
placed pallial organs. 
As seen by the illustrative tigures, wlien the shell became so 
unwieldy from its increasingly elongate form and necessarily unstable 
e(iuilibrlum that it could no longer lie conveniently canded in an 
upright position, it would probably gradually lean over to one side, 
which, all things considered, would be the most favourable position 
it could occui)y, Imt during locomotion, it woidd naturally tend to 
become more posteriorly directed. 
In this position the compression due to the recumbent shell would 
bear heavily upon the organs 2 )laced at the left side of the body, if 
the shell leaned over to that side, and the compressed organs would 
