Whorl (mollusc)
Encyclopedia
Spiral
In mathematics, a spiral is a curve which emanates from a central point, getting progressively farther away as it revolves around the point.-Spiral or helix:...
growth of a mollusc shell
Mollusc shell
The mollusc shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes...
. A spiral configuration of the shell is found in of numerous gastropods, but it is also found in shelled cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s including Nautilus
Nautilus
Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...
, Spirula and the large extinct subclass of cephalopods known as the ammonites.
A spiral shell can be visualized as consisting of a long conical
Cone (geometry)
A cone is an n-dimensional geometric shape that tapers smoothly from a base to a point called the apex or vertex. Formally, it is the solid figure formed by the locus of all straight line segments that join the apex to the base...
tube, the growth of which is coiled into an overall helical or planispiral shape, for reasons of both strength and compactness.
The number of whorls which exist in an adult shell of a particular species depends on mathematical factors in the geometric growth, as described in work of D’Arcy Wentworth Thompson
D'Arcy Wentworth Thompson
Sir D'Arcy Wentworth Thompson CB FRS FRSE was a Scottish biologist, mathematician, and classics scholar. A pioneering mathematical biologist, he is mainly remembered as the author of the 1917 book On Growth and Form, written largely in Dundee in 1915...
(On Growth and Form) and David Raup. The main factor is how rapidly the conical tube expands (or flares-out) over time. When the rate of expansion is low, such that each subsequent whorl is not that much wider than the previous one, then the adult shell has numerous whorls. When the mathematical factors governing the pattern of growth are such that there is a very rapid expansion of the conical shape, of the shell tube, then the adult shell has very few whorls.
The number of whorls present in an adult shell differs greatly in various taxa. The extant marine gastropod families Turritellidae
Turritellidae
Turritellidae, the tower snails, is a taxonomic family of small to medium-sized sea snails, marine gastropod molluscs in the clade Sorbeoconcha.These snails are filter feeders...
and Terebridae
Terebridae
The Terebridae, commonly referred to as auger shells or auger snails, are a group or taxonomic family of small to large predatory sea snails, marine gastropod mollusks....
, and the extinct Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
family Nerineidae
Nerineidae
The Nerineidae is an extinct taxonomic family of fossil sea snails, marine gastropod mollusks in the informal group Lower Heterobranchia.-Genera:Genera within the family Nerineidae include:* Nerinea, the type genus* Bactroptyxis...
, have very high spired
Spire (mollusc)
A spire is a descriptive term for part of the coiled shell of mollusks. The word is a convenient aid in describing shells, but it does not refer to a very precise part of shell anatomy: the spire consists of all of the whorls except for the body whorl...
shells with a large number of whorls, and a relatively small aperture
Aperture (mollusc)
The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....
.
The shells of a few genera of gastropods, and of the cephalopod genus Spirula, have whorls that are disconnected.
Counting the number of whorls
To count exactly the number of whorls in gastropods a straight line is drawn to separate the semi-circular nucleus (protoconchProtoconch
A protoconch is an embryonic or larval shell of some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod...
(1 - see image) from the younger part of the shell. An arrow placed at a 90° angle on this line, following the course of the whorl, reaches the end of the first whorl where it is parallel to its starting position. From that point all whorls are counted towards the margin of the shell, estimating the ultimate whorl with an accuracy of a quarter whorl.
Other authors (Ehrmann 1933; Richter & Seapy 1999) applied a slightly different measuring method, resulting in whorl numbers being a quarter higher.
Terminology
- Apical whorls—the whorls near the apexApex (mollusc)Apex is an anatomical term for the tip of the mollusc shell of a gastropod, scaphopod, or cephalopod mollusc.-Gastropods:The word "apex" is most often used to mean the tip of the spire of the shell of a gastropod...
or tip of the shell of gastropods - Body whorlBody whorlBody whorl is part of the morphology of a coiled gastropod mollusk.- In gastropods :In gastropods, the body whorl, or last whorl, is the most recently-formed and largest whorl of a spiral or helical shell, terminating in the aperture...
—The most recently formed whorl of a spiral shell, in which most of the body of the animal is found - Nuclear whorl(s)—small, generally smooth whorls formed within the egg, and constituting the apex of the shell
- ProtoconchProtoconchA protoconch is an embryonic or larval shell of some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod...
—a larval shell of a mollusc; also refers to protoconch whorls of an adult shell - Teleoconch—all the whorls of a shell after the protoconch whorls
Further reading
- Solem A.Alan SolemAlan Solem, full name George Alan Solem was an American malacologist, a biologist who studied mollusks.He was one of the most renown land snail experts of his time and had earned a reputation for his comprehensive revisions of mainly terrestrial pulmonates...
(1983) "Lost or kept internal whorls: ordinal differences in land snails". Journal of Molluscan Studies 49(supp. 12A): 172-178.