The umbilicus of a

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses ** Thin-shell structure Science Biology * Seashell, a hard o ...

is the axially aligned, hollow cone-shaped space within the

whorls A whorl ( or ) is an individual circle, oval, volution or equivalent in a whorled pattern, which consists of a spiral or multiple concentric objects (including circles, ovals and arcs). Whorls in nature File:Photograph and axial plane floral d ...

of a coiled mollusc shell. The term umbilicus is often used in descriptions of gastropod shells, i.e. it is a feature present on the ventral (or under) side of many (but not all) snail shells, including some species of sea snails, land snails, and freshwater snails. The word is also applied to the depressed central area on the planispiral coiled shells of '' Nautilus'' species and fossil

ammonites Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefis ...

. (These are not gastropods, but shelled

cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, a ...

In gastropods

The spirally coiled whorls of gastropod shells frequently connect to each other by their inner sides, during the natural course of its formation. This results in a more or less solid central axial pillar, known as the

columella Lucius Junius Moderatus Columella (; Arabic: , 4 – ) was a prominent writer on agriculture in the Roman Empire. His ' in twelve volumes has been completely preserved and forms an important source on Roman agriculture, together with the wo ...

. The more intimate the contact between the concave side of the whorls is, the more solid the columella becomes. On the other hand, if this connection is less intense, a hollow space inside the whorls may result, with an opening to the outside at the shell's base. This opening is known as the umbilicus. 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

. In

species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...

with a wide, open umbilicus, such as the heath snail (''

Helicella itala ''Helicella itala'' is a species of medium-sized, air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Geomitridae, the hairy snails and their allies. MolluscaBase eds. (2021). MolluscaBase. Helicella itala (Lin ...

''), 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 ''

Petasina unidentata ''Petasina unidentata'' is a species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of ...

'', to wide and shallow, such as the deep and wide depression in the rounded snail (''

Discus rotundatus ''Discus rotundatus'', common name rotund disc, is a species of small, air-breathing, land snail, a terrestrial pulmonate gastropod mollusk in the family Discidae, the disk snails. Description The shells of ''Discus rotundatus'' in the adult sta ...

''). Shells with a conspicuous umbilicus are always orthostylic, i.e. they have a poorly developed

. 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. A phaneromphalous shell has an open umbilicus. A cryptomphalous shell has the opening of the umbilicus completely plugged.

In cephalopods

References

External links

Dictionary definition

Shows the position of the umbilicus on an apple snail
{{Gastropod anatomy Mollusc shells Cephalopod zootomy