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Shells of two different species of sea snail: on the left is the normally sinistral (left-handed) shell of Neptunea angulata, on the right is the normally dextral (right-handed) shell of Neptunea despecta The shell of a large land snail (probably Helix pomatia) with parts broken off to show the interior structure.
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.
For <, spiral-ring pattern; =, regular spiral; >, loose spiral. R is the distance of spiral starting point (0, R) to the center. R is the distance of spiral starting point (0, R) to the center. The calculated x and y have to be rotated backward by ( − θ {\displaystyle -\theta } ) for plotting.
The name "ammonite", from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams' horns. Pliny the Elder ( d. 79 AD near Pompeii) called fossils of these animals ammonis cornua (" horns of Ammon ") because the Egyptian god Ammon ( Amun ) was typically ...
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 .
The shell-less aplacophora have a chitinous cuticle that has been likened to the shell framework; it has been suggested that tanning of this cuticle, in conjunction with the expression of additional proteins, could have set the evolutionary stage for the secretion of a calcareous shell in an aplacophoran-like ancestral mollusc.
The empty apex in these shells is sometimes very thin, and becomes brittle. In some species it breaks away, leaving the shell truncated or decollated. Decollated shells usually have the whorls of the spire closely wound and not increasing much in diameter. [2] A typical example is the decollate snail (Rumina decollata).
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]