Aperture (mollusc) - Terminology

Terminology

The margin of the aperture is sometimes continuous or entire (Epitonium), or becomes continuous in the adult (Caracolus); very frequently it is interrupted, the left side of the aperture being formed only by the body-whorl.

For convenience of reference, the margin of a gastropod aperture is divided into three areas:

• The parietal wall with the outer lip (labrum): the area next to the penultimate whorl of the shell. The siphonal notch is situated at the top.
• The columellar wall with the columellar lip (labium): the wall next to the columella. The siphonal canal is situated at its base.
• The palatal wall: the outer free wall of the final whorl of the shell.

The aperture is descending or deflected, when it does not follow the spiral of the shell, but turns downwards (such as in Helix). Sometimes it departs from contact with the preceding whorl (as frequently in Cylindrella).