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The best documented source of gastropod asymmetry is the developmental process known as
torsion. Like other molluscs, gastropods pass through a trochophore stage, and then form
a characteristic stage of development known as the veliger. During the veliger stage a
180° rotation of the mantlepallial cavity from posterior to anterior places the anus, and
renal openings over the head, and twists organ systems that pass through the snailıs
waistı (the area between the foot and visceral mass) into a figure eight. This rotation
is accomplished by a combination of differential growth and muscular contraction. In some
taxa the contribution of each process is about 50:50, but in other taxa the entire rotation
is accomplished by differential growth. Although the results of torsion are the best-known
asymmetries in gastropods, numerous other asymmetries appear independent of the torsion
process. Anopedal flexure, sometimes considered a feature of torsion, is widely distributed
in the Mollusca; it is present in the extinct hyoliths as well as in the Scaphopoda and
Cephalopoda (and to a lesser extent in the Bivalvia).
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