Columella (mollusk)
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The columella (meaning little column), is an anatomical feature of a coiled snail shell or gastropod shell. It is often only clearly visible as a structure when the shell is broken, or sliced in half vertically.
If you visualize a coiled snail shell as being a cone of shelly material which is successively wrapped in a spiral way around a central axis, then the columella occupies the space where the geometric axis of the shell is. The columella runs from the apex of the shell to the midpoint of the undersurface of the shell.
In the case of widely-umbilicate shells, the columella is hollow, but it is still wrapped around the central axis.
The columella of certain gastropod shells can have plications of various kinds, which are usually visible when looking into the aperture of the shell. These features of the columella are often useful in identifying the family, genus, or species of the gastropod.
[edit] Columellar muscles
The soft parts of the body of the gastropod are held in place in the shell by the columellar muscles. These muscles are attached strongly to the columella itself rather far up in the shell. These muscles are what the animal uses to withdraw the foot, head, and other soft parts into the shell for protection from drying out, and from predators.
In very large gastropods such as the Queen Conch, Eustrombus gigas, once the columellar muscles are cut with a knife, the soft parts of the animal fall out of the shell easily. Conch fishermen break a small hole in the spire of the shell, cut the columellar muscles, and harvest the live meat of this species. Often the fisherman dump the empty shell back into the water again.
