Spire (mollusc)

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.

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 (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 size 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 size. 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.

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

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. A typical example is the decollate snail (Rumina decollata).

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Wikipedia