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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 but may have a remnant within the mantle, or in some cases the shell is reduced such that the body cannot be retracted within it .
The gastropod theory is now considered incorrect on the basis that there were no shell traces, nor many of the features usually found in gastropod shells such as ridges, and coils tapering to a point. Furthermore, the size of Dinocochlea is much larger than any other gastropod living or extinct. The coprolite theory is also considered incorrect ...
In other words, in gastropods whose shells have varices, the shells are characterised by episodic growth - the shell grows in spurts, and during the resting phase the varix forms. In many gastropod whose shells have varices, for example the Cassinae , the varix is essentially merely a thickening and swelling of the shell at that point.
The peristome is the margin of the aperture of a gastropod shell. It is the edge of the lip of the shell. This part is sometimes reflected (turned back) or thickened once the snail reaches adult size, and these qualities of the peristome can be diagnostic features of the shell which may aid in identification of the species.
Turritella is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae. [3] They have tightly coiled shells, whose overall shape is basically that of an elongated cone. The name Turritella comes from the Latin word turritus meaning "turreted" or "towered" and the diminutive suffix -ella. [4]
The shell of the sea snail Epitonium scalare has a sculpture of sharp vertical ribs known as "costae" The shell of the freshwater snail Bulinus forskalii has strong transverse ribs which form an angle at the shoulder. Sculpture is a feature of many of the shells of mollusks.
The shell of Semicassis pyrum has a large parietal callus, at the top in this image The shell of Cymatium pileare has a narrow parietal callus around the surface of the aperture nearest the columella, on the left of the shell opening as it is shown here. A parietal callus is a feature of the shell anatomy of some groups of snails, i.e. gastropods.
The length of the shell attains 2.4 mm. (Original description) Shell: Small, obovate, with a scarcely noticeable spire, strong and porcellanous white, lustrous, with an exceedingly narrow aperture. The outer lip is truncate but not emarginate, varixed, and toothless, while the columella has four small folds ending in a sharp point.