Umbilicus (mollusk)

The umbilicus of a shell is the axially aligned, hollow cone-shaped space within the whorls of a coiled mollusk shell. The term umbilicus is often used in descriptions of gastropod shells, i.e. it is a feature present in many snail shells.

The word is also applied to the depressed central area on the planispiral coiled shells of "Nautilus" species and fossil ammonites. (These are not gastropods, but shelled cephalopods.)

Another way of characterizing the umbilicus in gastropods is as the hole around which the inner surface of the shell is coiled when that space is not filled by a columella.

In species with a wide, open umbilicus, such as the Heath Snail ("Helicella itala"), the spiral of the whorls can be clearly viewed by looking into the umbilicus.

An umbilicus can vary from very narrow and punctured, as found in "Trichia unidentata", to wide and shallow, such as the deep and wide depression in the Rounded Snail ("Discus rotundatus"). Shells with a conspicuous umbilicus are always orthostylic, i.e. they have a poorly developed columella.

Sometimes there is a dimple or funnel-shaped depression, known as the umbilical region or the umbilical field, next to or at the basal hollow of the columella, when the walls of successive whorls are not closely wound against each other.

References

External links

* Dictionary definition [http://www.thefreedictionary.com/umbilicus]
* Shows the position of the umbilicus on an apple snail [http://www.applesnail.net/content/anatomy/shell.php]
* Umbilicus on a top snail [http://fieldtrip.britishecologicalsociety.org/rocky%20tour%201/rocky%20shore%20tour%20web/topwinkcomp.htm]