Spire (mollusc)

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Apertural view of the shell of adult Tarebia granifera showing its pale brown body whorl and dark spire. Tarebia granifera shell.png
Apertural view of the shell of adult Tarebia granifera showing its pale brown body whorl and dark spire.
Very high-spired shells of the sea snail species Turritella communis Turritella communis Turmschnecke.jpg
Very high-spired shells of the sea snail species Turritella communis
Medium-spired shell (live individual) of a European land snail, probably Trochulus hispidus Little snail.JPG
Medium-spired shell (live individual) of a European land snail, probably Trochulus hispidus
Very low-spired shells of the land snail species Xerolenta obvia Xerolenta obvia3pl.jpg
Very low-spired shells of the land snail species Xerolenta obvia
The sinistral shell of the freshwater snail Planorbarius corneus, a view of the sunken spire, which is held facing downwards in life Planorbarius corneus top.jpg
The sinistral shell of the freshwater snail Planorbarius corneus , a view of the sunken spire, which is held facing downwards in life

A spire is a part of the coiled shell of molluscs. The spire consists of all of the whorls except for the body whorl. Each spire whorl represents a rotation of 360°. A spire is part of the shell of a snail, a gastropod mollusc, a gastropod shell, and also the whorls of the shell in ammonites, which are fossil shelled cephalopods.

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In textbook illustrations of gastropod shells, the tradition (with a few exceptions) is to show the majority of shells with the spire uppermost on the page.

The spire, when it is not damaged or eroded, includes the protoconch (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 word "spire" is used, in an analogy to a church spire or rock spire, 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 in their coiling, but instead are planispiral, 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 competion between high and low-spired species in a habitat. Snails with middle-height spires show little preference to surface angle. [1]

Gastropod shells that are not spirally coiled (for example shells of limpets) have no columella.

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 ; or it is partitioned off, as in Vermetus , Euomphalus , Turritella , Triton or Caecum . 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).

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 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. Thus we have the simple depressed cone of Patella , all aperture and no spire. From it there is every gradation, from the Haliotis , almost equally depressed and broad, the result, however, of a very rapidly enlarging spiral, to the long, many-whorled Turritella or Vermetus , which is a Turritella partially unrolled into a simple long tube — the opposite of the Patella. [2]

Different types of spire

See also : Gastropod shell#Shape of the shell

Chirality

In most spiral shells the spire normally curves to the right, that is to say, placing the shell with its apex turned upward from the observer and its aperture 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 , Clausilia , 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. [2]

Related Research Articles

Aperture (mollusc) The main opening of the shell, where the head-foot part of the body of the animal emerges

The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where the head-foot part of the body of the animal emerges for locomotion, feeding, etc.

Body whorl

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.

Whorl (mollusc)

A whorl is a single, complete 360° revolution or turn in the spiral growth of a mollusc shell. A spiral configuration of the shell is found in of numerous gastropods, but it is also found in shelled cephalopods including Nautilus, Spirula and the large extinct subclass of cephalopods known as the ammonites.

Umbilicus (mollusc)

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 side of many snail shells, including some species of sea snails, land snails, and freshwater snails.

Gastropod shell part of the body of a gastropod or snail

The gastropod shell is part of the body of a 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 storage. Some gastropods appear shell-less (slugs) but may have a remnant within the mantle, or the shell is reduced such that the body cannot be retracted within (semi-slug). Some snails also possess an operculum that seals the opening of the shell, known as the aperture, which provides further protection. The study of mollusc shells is known as conchology. The biological study of gastropods, and other molluscs in general, is malacology. Shell morphology terms vary by species group. An excellent source for terminology of the gastropod shell is "How to Know the Eastern Land Snails" by John B. Burch now freely available at the Hathi Trust Digital Library.

<i>Helicella itala</i> species of mollusc

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. The common Heath snail is a West Palearctic species found in the British Isles, France, Spain, Belgium, Netherlands, Switzerland, Denmark, Germany, Austria, Czech Republic, and Poland. English common name is Heath snail.

The following is a glossary of common English language and scientific terms used in the description of gastropods.

<i>Borsonella barbarensis</i> species of mollusc

Borsonella barbarensis is a species of sea snail, a marine gastropod mollusk in the family Borsoniidae.

<i>Borsonella diegensis</i> species of mollusc

Borsonella diegensis is a species of sea snail, a marine gastropod mollusk in the family Borsoniidae.

<i>Tiariturris libya</i> species of mollusc

Tiariturris libya is a species of small sea snail, a marine gastropod mollusk in the family Pseudomelatomidae.

Echinogurges clavatus is a species of sea snail, a marine gastropod mollusk in the family Eucyclidae.

<i>Ethminolia vitiliginea</i> species of mollusc

Ethminolia vitiliginea, common name the depressed top shell, is a species of sea snail, a marine gastropod mollusk in the family Trochidae, the top snails.

<i>Pseudostomatella coccinea</i> species of mollusc

Pseudostomatella coccinea is a species of small sea snail, a marine gastropod mollusk in the family Trochidae, the top snails.

<i>Trochus radiatus</i> species of mollusc

Trochus radiatus, common name the radiate top shell, is a species of sea snail, a marine gastropod mollusk in the family Trochidae, the top snails.

<i>Madagasikara johnsoni</i> species of mollusc

Madagasikara johnsoni is a species of tropical freshwater snail with a gill and an operculum, an aquatic gastropod mollusc in the family Pachychilidae.

<i>Iphiana tenuisculpta</i> species of mollusc

Iphiana tenuisculpta is a species of sea snail, a marine gastropod mollusk in the family Pyramidellidae, the pyrams and their allies.

<i>Phasianotrochus rutilus</i> species of mollusc

Phasianotrochus rutilus, common name the pink-tipped kelp shell, is a species of sea snail, a marine gastropod mollusk in the family Trochidae, the top snails.

<i>Leptothyra rubricincta</i> species of mollusc

Leptothyra rubricincta is a species of small sea snail with calcareous opercula, a marine gastropod mollusc in the family Colloniidae.

<i>Potamides archiaci</i> species of mollusc (fossil)

Potamides archiaci is an extinct species of sea snail, marine gastropod molluscs in the family Potamididae.

Hindsiclava calligonoides is an extinct species of sea snail, a marine gastropod mollusc in the family Pseudomelatomidae, the turrids and allies.

References

  1. Cook, L. M.; Jaffar, W. N. (1984). "Spire index and preferred surface orientation in some land snails". Biological Journal of the Linnean Society. 21 (3): 307–313. doi:10.1111/j.1095-8312.1984.tb00368.x. ISSN   1095-8312.
  2. 1 2 3 George Washington Tryon, Structural and systematic conchology, p.40; 1884