Spire (mollusc)
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A spire is a part of the coiled
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 ...
of
mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is esti ...
s. The spire consists of all of 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 ...
except for the body whorl. Each spire whorl represents a rotation of 360°. A spire is part of the shell of a
snail A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gastro ...
, a
gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...
mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is esti ...
, a
gastropod shell The gastropod shell is part of the body of a Gastropoda, gastropod or snail, a kind of mollusc. The shell is an exoskeleton, which protects from predators, mechanical damage, and dehydration, but also serves for muscle attachment and calcium s ...
, and also the whorls of the shell in
ammonite 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 cuttlefish) ...
s, which are
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
shelled
cephalopod 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 ...
s. In textbook illustrations of gastropod shells, the tradition (with a few exceptions) is to show most shells with the spire uppermost on the page. The spire, when it is not damaged or eroded, includes the
protoconch A protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod. In older texts it is also called ...
(also called the nuclear whorls or the larval shell), and most of the subsequent teleoconch whorls (also called the postnuclear whorls), which gradually increase in area as they are formed. Thus the spire in most gastropods is pointed, the tip being known as the "
apex The apex is the highest point of something. The word may also refer to: Arts and media Fictional entities * Apex (comics), a teenaged super villainess in the Marvel Universe * Ape-X, a super-intelligent ape in the Squadron Supreme universe *Apex, ...
". The word "spire" is used, in an analogy to a church
spire A spire is a tall, slender, pointed structure on top of a roof of a building or tower, especially at the summit of church steeples. A spire may have a square, circular, or polygonal plan, with a roughly conical or pyramidal shape. Spires are ...
or
rock spire A pinnacle, tower, spire, needle or natural tower (german: Felsnadel, ''Felsturm'' or ''Felszinne'') in geology is an individual column of rock, isolated from other rocks or groups of rocks, in the shape of a vertical shaft or spire. Examples are ...
, a high, thin, pinnacle. The "spire angle" is the angle, as seen from the apex, at which a spire increases in area. It is an angle formed by imaginary lines tangent to the spire. Some gastropod shells have very high spires (the shell is much higher than wide), some have low spires (the shell is much wider than high), and there are all possible grades between. In a few gastropod families the shells are not
helical Helical may refer to: * Helix, the mathematical concept for the shape * Helical engine, a proposed spacecraft propulsion drive * Helical spring, a coilspring * Helical plc, a British property company, once a maker of steel bar stock * Helicoil A t ...
in their coiling, but instead are
planispiral Planispiral is a condition in which a tubicolous shell is coiled in a single horizontal plane and the diameter increases away from the axis of coiling. This term is usually used for describing gastropod shells. Many cephalopod A cephalopod is ...
, flat-coiled. In these shells, the spire does not have a raised point, but instead is sunken. Snails with high spires tend to prefer vertical surfaces while those with low spires prefer horizontal surfaces. This is thought to aid in reducing competition between high and low-spired species in a habitat. Snails with middle-height spires show little preference to surface angle. Gastropod shells that are not spirally coiled (for example shells of
limpet Limpets are a group of aquatic snails that exhibit a conical shell shape (patelliform) and a strong, muscular foot. Limpets are members of the class Gastropoda, but are polyphyletic, meaning the various groups called "limpets" descended indep ...
s) have no
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 ...
.


Decollated shells

In some species as high-spired shells become adult the soft parts of the animal cease to occupy the upper parts of the cavity of the shell. The space thus vacated is sometimes filled with solid shell, as in ''
Magilus ''Magilus'' is a genus of sea snails, marine gastropod mollusks in the family Muricidae Muricidae is a large and varied taxonomic family of small to large predatory sea snails, marine gastropod mollusks, commonly known as murex snails or ...
''; or it is partitioned off, as in ''
Vermetus ''Vermetus'' is a genus of sea snails, marine gastropod mollusks in the family Vermetidae, the worm snails or worm shells. The genus ''Vermetus'' is very ancient: it occurs in the fossil record from the Jurassic to the Quaternary (age range: ...
'', ''
Euomphalus ''Euomphalus'' is a genus of fossil marine gastropods known to have lived from the Silurian to the Middle Permian.J.B.Knight,et al 1960. Treatise on Invertebrate Paleontology, Part I, Mollusca 1, ch on systematic descriptions. Geol Soc of Amer ...
'', ''
Turritella ''Turritella'' is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae.Vos, C.; Gofas, S. (2013). Turritella Lamarck, 1799. Accessed through: World Register of Marine Species at http://www.m ...
'',
Triton Triton commonly refers to: * Triton (mythology), a Greek god * Triton (moon), a satellite of Neptune Triton may also refer to: Biology * Triton cockatoo, a parrot * Triton (gastropod), a group of sea snails * ''Triton'', a synonym of ''Triturus' ...
or ''
Caecum The cecum or caecum is a pouch within the peritoneum that is considered to be the beginning of the large intestine. It is typically located on the right side of the body (the same side of the body as the appendix, to which it is joined). The wo ...
''. The empty
apex The apex is the highest point of something. The word may also refer to: Arts and media Fictional entities * Apex (comics), a teenaged super villainess in the Marvel Universe * Ape-X, a super-intelligent ape in the Squadron Supreme universe *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 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 ...
of the spire closely wound and not increasing much in diameter. A typical example is the
decollate snail The decollate snail, scientific name ''Rumina decollata'', is a medium-sized predatory land snail, a species of terrestrial pulmonate gastropod mollusk in the family Achatinidae. It is a European species that has been introduced in a number o ...
(''Rumina decollata'').


The form of a shell

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 The apex is the highest point of something. The word may also refer to: Arts and media Fictional entities * Apex (comics), a teenaged super villainess in the Marvel Universe * Ape-X, a super-intelligent ape in the Squadron Supreme universe *Apex, ...
or initial
whorl 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 ...
to the
aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...
. The shell grows in a regular geometrical progression in its normal pattern, although these modes vary among themselves widely. Thus we have the simple depressed cone of ''
Patella The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as m ...
'', all aperture and no spire. From it there is every gradation, from the ''
Haliotis ''Haliotis'', common name abalone, is the only genus in the family Haliotidae. This genus once contained six subgenera. These subgenera have become alternate representations of ''Haliotis''. The genus consists of small to very large, edible, h ...
'', almost equally depressed and broad, the result, however, of a very rapidly enlarging spiral, to the long, many-whorled ''
Turritella ''Turritella'' is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae.Vos, C.; Gofas, S. (2013). Turritella Lamarck, 1799. Accessed through: World Register of Marine Species at http://www.m ...
'' or ''
Vermetus ''Vermetus'' is a genus of sea snails, marine gastropod mollusks in the family Vermetidae, the worm snails or worm shells. The genus ''Vermetus'' is very ancient: it occurs in the fossil record from the Jurassic to the Quaternary (age range: ...
'', which is a ''Turritella'' partially unrolled into a simple long tube — the opposite of the ''Patella''.


Different types of spire

''See also'' : Gastropod shell#Shape of the shell * Turbinoform or Turbinate: having a broadly conical spire and a convex base, as in ''
Turbo In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pro ...
'', turban-shaped. * Turriform: with a many-whorled, slender spire, as in ''
Turritella ''Turritella'' is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae.Vos, C.; Gofas, S. (2013). Turritella Lamarck, 1799. Accessed through: World Register of Marine Species at http://www.m ...
''.


Chirality

In most spiral shells the spire normally curves to the right, that is to say, placing the shell with its
apex The apex is the highest point of something. The word may also refer to: Arts and media Fictional entities * Apex (comics), a teenaged super villainess in the Marvel Universe * Ape-X, a super-intelligent ape in the Squadron Supreme universe *Apex, ...
turned upward from the observer and its
aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...
in view : the aperture will be on the right hand side. In others the volutions proceed in the opposite direction with such regularity as to be eminently characteristic of some species and genera (''
Physa ''Physa'' is a genus of small, left-handed or sinistral, air-breathing freshwater snails, aquatic pulmonate gastropod mollusks in the subfamily Physinae of the family Physidae.MolluscaBase eds. (2021). MolluscaBase. Physa Draparnaud, 1801. Ac ...
'', ''
Clausilia ''Clausilia'' is a European genus of small, air-breathing land snails, terrestrial pulmonate gastropod mollusks in the family Clausiliidae, the door snails, all of which have a clausilium.Bank, R.; Bouchet, P. (2017). Clausilia Draparnaud, 1805. ...
'', etc.). However, in certain genera, it is found that species normally dextral will exceptionally produce sinistrally coiled shells, and vice versa. This abnormal growth probably is caused by disturbance of the relations of the embryo with its initial shell.
George Washington Tryon George Washington Tryon Jr. (20 May 1838 – 5 February 1888) was an American malacologist who worked at the Academy of Natural Sciences in Philadelphia. Biography George Washington Tryon was the son of Edward K. Tryon and Adeline Savidt ...
, Structural and systematic conchology, p.40; 1884


References

{{Gastropod anatomy Mollusc shells Gastropod anatomy Cephalopod zootomy