A varix is an anatomical feature of the shell of certain sea snails, marine gastropod molluscs. Gastropods whose shells have varices are families and species within the taxonomic groups Littorinimorpha and Neogastropoda; the varix is a thickened axial ridge, a subcylindrical protrusion, in the shell which exists in some families of marine gastropods. It is an important shell character in generic classification. A varix is located at intervals around the whorl, is formed by considerable thickening of the outer lip during a resting stage in the growth of the shell. In other words, in gastropods whose shells have varices, the shells are characterised by episodic growth - the shell grows in spurts, during the resting phase the varix forms. In many gastropod whose shells have varices, for example the Cassinae, the varix is merely a thickening and swelling of the shell at that point, but in some genera within the family Muricidae, such as Chicoreus, Hexaplex and Pterynotus, within the genus Biplex, of the family Ranellidae, the varices are characterised by elaborate ruffles, frills or lamellae.
Some other genera, for example Murex, are armed with protective spines which may be straight or curved, which are formed by the varices closing or curling around their axis
The clausilium is a calcareous anatomical structure, found in one group of air-breathing land snails: terrestrial pulmonate gastropod mollusks in the family Clausiliidae, the door snails. The clausilium is one part of the clausilial apparatus; the presence of a clausilium is the reason for the common name "door snails", because all the snails in this family have a spoon-shaped "door" or clausilium, which can slide down to close the aperture of the shell. However, this structure is emphatically not the same thing as an operculum, non-existent in pulmonate snails, only occurring in the Amphibolidae; the exact shape of the clausilium varies from genus to genus: it can be tongue-shaped, spoon-shaped or spatula-shaped. The wide flat end of the clausilium can close the aperture of the snail shell, thus protect the soft parts against predation by animals such as carnivorous beetle larvae; the narrow end of the clausilium slides in a groove, formed by spiral folds on the inside of the shell around the columella.
Because the groove is long, the muscles that control the clausilium are long, the whole structure can be retracted into the shell. The mechanism is different, but the clausilium is vaguely reminiscent of an automated garage door opener. Iľja Michajlovič Licharev: Klauziliidy. Fauna SSSR. Molljuski. Vol. 3, Fasc. 4. 320 pp. Izdat. Akad. N)
In anatomy, a suture is a rigid joint between two or more hard elements of an organism, with or without significant overlap of the elements. Sutures are found in the skeletons or exoskeletons of a wide range of animals, in both invertebrates and vertebrates. Sutures are found in animals with hard parts from the Cambrian period to the present day. Sutures were and are formed by several different methods, they exist between hard parts that are made from several different materials; the skeletons of vertebrate animals are made of bone, in which the main rigid ingredient is calcium phosphate. The skulls of most vertebrates consist of sets of bony plates held together by cranial sutures; these sutures are held together by Sharpey's fibers which grow from each bone into the adjoining one. In the type of crurotarsal ankle, found in crocodilians and some other archosaurs, the astragalus is fixed to the tibia by a suture, the joint bends around a peg on the astragalus, which fits into a socket in the calcaneum.
The shells of most molluscs are made of calcium carbonate, of conchiolin, a protein. For more information, see Mollusc shell. In cephalopod mollusks which have external shells, the shell is divided into compartments by septa; the septa are joined to the external shell by sutures formed by repeated invagination. The sutures are visible from the outside and form complex and elaborate patterns. Nearly all snail shells can be visualized as a tube of increasing diameter, closed at the small end, spirally wrapped around a central axis. For more information, see Gastropod shell; each complete rotation of this spirally-arranged tube is called a whorl. The whorls of a snail shell overlap one another, forming a spire. Where the whorls overlap, there is a clear indentation; this indentation forms a visible line, continuous and reaches from the apex of the shell to the aperture. Details of the suture are useful in discriminating one species from another, for example, sometimes the suture is channeled; the suture provides a sort of geographic marker from which one can refer to the positioning of patterning or sculpture, where, relevant: for example some species have a darker or lighter subsutural band on the shell.
When an angulation of the whorls occurs, the space between it and the suture above it constitutes the area known as the "shoulder" of the shell. The shoulder angle may be simple or keeled, may sometimes have nodes or spines A trilobite's carapace consisted of calcite and calcium phosphate deposited on a lattice of chitin; the trilobite body is divided into three major sections: a cephalon with eyes and sensory organs such as antennae. In many species the cephalon had sutures running from back to front round the outside edges of the eyes; these sutures divided the cephalon into 3 pieces. The sutures in trilobites' cephalons were unusual because it seems their main function was to create weaknesses which made it easy for this part of the carapace to split when the animal needed to molt
A mollusc valve is each articulating part of the shell of a mollusc. Each part is known as a valve or in the case of chitons, a "plate". Members of two classes of molluscs: the Bivalvia and the Polyplacophora have valves. Species within one family of unusual small sea snails, marine opisthobranch gastropods in the family Juliidae have two articulating shells or valves, which resemble those of a bivalve; this exceptional family is known as the bivalved gastropods. Gastropods in general are sometimes called "univalves", because in those that have a shell, the shell is in one part; the valves of chitons are eight dorsal, articulated shell plates, which are coloured and sculpted. After death the girdle that holds the plates together disintegrates and the plates separate, thus individual plates can be found washed up in beach drift, as shown in the image at the top of this article. Bivalve molluscs have a shell, composed of two separate but articulating parts; each one of these two parts is known as a "valve".
The two valves are known as the "right valve" and the "left valve". In many, but not all, the two valves are more or less symmetrical and thus look like mirror images of one another; the great majority of shelled gastropods or snails have a shell in one part, hence the older name "univalve". The gastropod operculum, when present when it is composed of calcium carbonate, is not considered to be a valve. In contrast, species within one family of small sea snails, the Juliidae, opisthobranch gastropod molluscs, have a hinged shell, composed of two parts joined by a ligament; these two parts closely resemble the two valves of a bivalve. This group of species are referred to as "bivalved gastropods"; these are sacoglossans in several genera including Julia, Midorigai, Edenttellina and Candinida. These bivalve gastropods were for a long time known only from dead material; because of this, they had been described as being somewhat atypical bivalves. It was not until living individuals were found in the late 20th century that it was understood that these are in fact unusual gastropods.
Nacre known as mother of pearl, is an organic-inorganic composite material produced by some molluscs as an inner shell layer. It is strong and iridescent. Nacre is found in some of the most ancient lineages of bivalves and cephalopods. However, the inner layer in the great majority of mollusc shells is porcellaneous, not nacreous, this results in a non-iridescent shine, or more in non-nacreous iridescence such as flame structure as is found in conch pearls; the outer layer of pearls and the inside layer of pearl oyster and freshwater pearl mussel shells are made of nacre. Other mollusc families that have a nacreous inner shell layer include marine gastropods such as the Haliotidae, the Trochidae and the Turbinidae. Nacre is composed of hexagonal platelets of aragonite 10–20 µm wide and 0.5 µm thick arranged in a continuous parallel lamina. Depending on the species, the shape of the tablets differ. Whatever the shape of the tablets, the smallest units they contain are irregular rounded granules.
These layers are separated by sheets of organic matrix composed of elastic biopolymers. This mixture of brittle platelets and the thin layers of elastic biopolymers makes the material strong and resilient, with a Young's modulus of 70 GPa. Strength and resilience are likely to be due to adhesion by the "brickwork" arrangement of the platelets, which inhibits transverse crack propagation; this structure, at multiple length sizes increases its toughness, making it as strong as silicon. The statistical variation of the platelets has a negative effect on the mechanical performance because statistical variation precipitates localization of deformation. However, the negative effects of statistical variations can be offset by interfaces with large strain at failure accompanied by strain hardening. On the other hand, the fracture toughness of nacre increases with moderate statistical variations which creates tough regions where the crack gets pinned. But, higher statistical variations generates weak regions which allows the crack to propagate without much resistance causing the fracture toughness decreases.
Nacre appears iridescent because the thickness of the aragonite platelets is close to the wavelength of visible light. These structures interfere constructively and destructively with different wavelengths of light at different viewing angles, creating structural colours; the crystallographic c-axis points perpendicular to the shell wall, but the direction of the other axes varies between groups. Adjacent tablets have been shown to have different c-axis orientation randomly oriented within ~20° of vertical. In bivalves and cephalopods, the b-axis points in the direction of shell growth, whereas in the monoplacophora it is the a-axis, this way inclined; the interlocking of bricks of nacre has large impact on both the deformation mechanism as well as its toughness. In addition, the mineral–organic interface results in enhanced resilience and strength of the organic interlayers. Nacre formation is not understood; the initial onset assembly, as observed in Pinna nobilis, is driven by the aggregation of nanoparticles within an organic matrix that arrange in fibre-like polycrystalline configurations.
The particle number increases successively and, when critical packing is reached, they merge into early-nacre platelets. Nacre growth is mediated by organics, controlling the onset and form of crystal growth. Individual aragonite "bricks" are believed to grow to the full height of the nacreous layer, expand until they abut adjacent bricks; this produces the hexagonal close-packing characteristic of nacre. Bricks may nucleate on randomly dispersed elements within the organic layer, well-defined arrangements of proteins, or may grow epitaxially from mineral bridges extending from the underlying tablet. Nacre differs from fibrous aragonite – a brittle mineral of the same form – in that the growth in the c-axis is slow in nacre, fast in fibrous aragonite. Nacre is secreted by the epithelial cells of the mantle tissue of various molluscs; the nacre is continuously deposited onto the inner surface of the shell, the iridescent nacreous layer known as mother of pearl. The layers of nacre smooth the shell surface and help defend the soft tissues against parasites and damaging debris by entombing them in successive layers of nacre, forming either a blister pearl attached to the interior of the shell, or a free pearl within the mantle tissues.
The process is called encystation and it continues as long as the mollusc lives. The form of nacre varies from group to group. In bivalves, the nacre layer is formed of single crystals in a hexagonal close packing. In gastropods, crystals are twinned, in cephalopods, they are pseudohexagonal monocrystals, which are twinned; the main commercial sources of mother of pearl have been the pearl oyster, freshwater pearl mussels, to a lesser extent the abalone, popular for their sturdiness and beauty in the latter half of the 19th century. Used for pearl buttons during the 1900s, were the shells of the great green turban snail Turbo marmoratus and the large top snail, Tectus niloticus; the international trade in mother of pearl is governed by the Convention on International Trade in Endangered Species of Wild Fauna and Flora, an agreement signed by more than 170 countries. Nacre has been used for centuries for a variety o
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, the whorls of the shell in ammonites, which are fossil shelled cephalopods. In textbook illustrations of gastropod shells, the tradition 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, most of the subsequent teleoconch whorls, which 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, pinnacle. The "spire angle" is the angle, it is an angle formed by imaginary lines tangent to the spire. Some gastropod shells have high spires, some have low spires, 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 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. The empty apex in these shells is sometimes thin, becomes brittle. In some species it breaks away, leaving the shell decollated. Decollated shells have the whorls of the spire wound and not increasing much in diameter. A typical example is the decollate snail; the form of the shell of a gastropod is regular in coiling, is a cone curved into a spiral, 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 all aperture and no spire. From it there is every gradation, from the Haliotis equally depressed and broad, the result, however, of a rapidly enlarging spiral, to the long, many-whorled Turritella or Vermetus, a Turritella unrolled into a simple long tube — the opposite of the Patella.
See also: Gastropod shell#Shape of the shell Turbinoform or Turbinate: having a broadly conical spire and a convex base, as in Turbo, turban-shaped. Turriform: with a many-whorled, slender spire, as in Turritella. In most spiral shells the spire curves to the right, 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. However, in certain genera, it is found that species dextral will exceptionally produce sinistrally coiled shells, vice versa; this abnormal growth is caused by disturbance of the relations of the embryo with its initial shell