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Primary spirals may appear in regular succession on either side of the first primary, which generally becomes the shoulder angle if angulation occurs. Secondary spirals may appear by intercalation between the primary ones, and generally are absent in the young shell, except in some highly accelerated types.
This shell has nine whorls. A whorl is a single, complete 360° revolution or turn in the spiral or whorled growth of a mollusc shell. A spiral configuration of the shell is found in numerous gastropods, but it is also found in shelled cephalopods including Nautilus, Spirula and the large extinct subclass of cephalopods known as the ammonites.
The form of the shell of a gastropod is usually regular in coiling, and is normally a cone curved into a spiral, and descending in a screw-like manner from the apex or initial whorl to the aperture. The shell grows in a regular geometrical progression in its normal pattern, although these modes vary among themselves widely.
Botanical whorls: sepals, petals, leaves, or branches radiating from a single point (photo of flower of Friesodielsia desmoides, family Annonaceae, juxtaposed with diagram of axial cross-section) Mollusc whorls : Each complete 360° turn in the spiral growth of the shell of the mollusc Anisus septemgyratus , family Planorbidae .
An Archimedean spiral is, for example, generated while coiling a carpet. [5] A hyperbolic spiral appears as image of a helix with a special central projection (see diagram). A hyperbolic spiral is some times called reciproke spiral, because it is the image of an Archimedean spiral with a circle-inversion (see below). [6]
The body whorl shows in this side view as the last complete turn of the shell spiral, the part of the shell that leads up to the aperture. The body whorl is part of the morphology of the shell in those gastropod mollusks that possess a coiled shell. The term is also sometimes used in a similar way to describe the shell of a cephalopod mollusk.
Shell of marine snail Lunella torquata with the calcareous operculum in place Gastropod shell of the freshwater snail Viviparus contectus with corneous operculum in place. An operculum (Latin for 'cover, covering'; pl. opercula or operculums) is a corneous or calcareous anatomical structure like a trapdoor that exists in many (but not all) groups of sea snails and freshwater snails, and also ...
The umbilicus of a shell is the axially aligned, hollow cone-shaped space within the whorls 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 ...