The periostracum is a thin organic coating or "skin", the outermost layer of the shell of many shelled animals, including molluscs and brachiopods. Among molluscs it is seen in snails and clams, i.e. in gastropods and bivalves, but it is found in cephalopods such as Allonautilus scrobiculatus. Periostracum is an integral part of the shell, it forms as the shell forms, along with the other shell layers. Periostracum is visible as the outer layer of the shell of many molluscan species from terrestrial and marine habitats, may be seen in land snails, river mussels and other kinds of freshwater bivalves, as well as in many kinds of marine shelled molluscs; the word "periostracum" means "around the shell", meaning that the periostracum is wrapped around what is the more calcareous part of the shell. Technically the calcareous part of the shell can be referred to as the "ostracum", but that term is only rarely used; this shell layer is composed of a type of protein known as conchiolin. Conchiolin is composed of quinone-tanned proteins, which are similar to those found in the epidermal cuticle.
The formation of a shell requires certain biological machinery. The shell is deposited within a small compartment, the extrapallial space, sealed from the environment by the periostracum, a leathery outer layer around the rim of the shell, where growth occurs; this caps off the extrapallial space, bounded on its other surfaces by the existing shell and the mantle. The periostracum acts as a framework from which the outer layer of carbonate can be suspended, but in sealing the compartment, allows the accumulation of ions in concentrations sufficient for crystallization to occur; the accumulation of ions is driven by ion pumps packed within the calcifying epithelium. The organic matrix forms the scaffold that directs crystallization, the deposition and rate of crystals is controlled by hormones produced by the mollusc; the periostracum was essential in allowing early molluscs to obtain large size with a single valve. The periostracum is secreted from a groove in the mantle, termed the periostracal groove.
When secreted, it consists of the soluble protein periostracin. Periostracum is yellowish or brownish in color. In some species it is black; the periostracum is often a different color than the underlying layer of the shell. In the shells of species which have periostracum, this shell layer is quite physically worn away or chemically eroded in the parts of the shell that are older, thus it may only still be visible in the more formed areas of the shell. Periostracum can in some cases be quite thin, smooth and transparent, such that it looks like a thin yellow varnish, or it can be thicker and more or less opaque; when it is thick it is relatively rough in texture and dull. In some species the periostracum is tufted, or forms hair-like growths which in some cases can give the fresh shell a velvety feel, see. In some species the periostracum adheres tightly to the underlying shell surface, in others the periostracum is less attached. In certain marine species, such as for example certain species of cone snails, a heavy periostracum obscures the color patterns that exist on the calcareous layer of the shell.
In many aquatic species, once a shell has been removed from the water and has had time to dry out the periostracum may become brittle and start to flake or peel off of the surface of the shell. It is not uncommon for shell collectors to deliberately remove a periostracum layer if they feel that a shell is more attractive without it; however the periostracum is an important part of the shell, is of interest to malacologists. Details of the periostracum can sometimes be helpful in identifying a species. Haired shells occur in gastropods in several species of the Stylommatophoran families Polygyridae and Hygromiidae; these families are only distantly related, suggesting that this features has evolved several times independently. Haired shells are exclusively observed in species living in moist microhabitats, like layers of fallen leaves, broad-leaved vegetation, damp meadows or wet scree; such a correlation suggests an adaptive significance of the trait in such a habitat. These hairs can reach varying lengths.
In some cases hardly visible, they confer an furry impression to the shell in others. These semi-rigid structures are part of the periostracum, a thin protein layer secreted by the snail to cover the calcareous shell. Building hairs requires the snail to have specialised glandular tissue and complex strategies to form them; this trait can be assumed to be costly and should thus present a selective advantage to its bearers in order to be conserved. Experiments by Pfenninger et al. on genus Trochulus showed an increased adherence of haired shells to wet surfaces. Haired shells appeared to be the ancestral character state, a feature most lost three times independently; the possession of hairs facilitates the adherence of the snails to their herbaceous food plants during foraging when humidity levels are high. The absence of hairs in some Trochulus species could thus be explained as a loss of the potential adaptive function linked to habitat shifts; the periostracum of brachiopods is made of chitin.
New cells on the edges of the brachiopod mantle secrete material that extends the periostracum, but are displace
Anisus septemgyratus is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Planorbidae, the ram's horn snails. Glöer considered Anisus septemgyratus as a junior synonym of Anisus leucostoma. Glöer & Meier-Brook used name Anisus septemgyratus again. Horsák et al. consider Anisus calculiformis as a synonym of Anisus septemgyratus. This species occurs in countries and islands including: Czech Republic - in Moravia, critically endangered Slovakia Poland British Isles... The number of prostate diverticles ranges from 30 to more than 50. Anisus septemgyratus at Animalbase
The Nerineidae is an extinct taxonomic family of fossil sea snails, marine gastropod mollusks in the informal group Lower Heterobranchia. Genera within the family Nerineidae include: Nerinea, the type genus Bactroptyxis The Taxonomicon
The body whorl is part of the morphology of the shell in those gastropod mollusks that possess a coiled shell. The term is sometimes used in a similar way to describe the shell of a cephalopod mollusk. In gastropods, the body whorl, or last whorl, is the most formed and largest whorl of a spiral or helical shell, terminating in the aperture, it is called the "body whorl" because most of the body of the soft parts of the animal fits into this whorl. The proportional size of the body whorl in gastropod shells differs according to the actual shell morphology. For shells in which the rate of whorl expansion of each revolution around the axis is high, the aperture and the body whorl are large, the shell tends to be low spired; the shell of the abalone is a good example of this kind of shell. The opposite tendency can sometimes create a high spire with little whorl increase per revolution. In these instances, e.g. in the shell of Turritella species, both the body whorl and the aperture are small. In mollusc shells where there is no elevation at all to the spire, only moderate whorl expansion, the body whorl can sometimes still represent a large part of the shell, e.g. in some species in the family Planorbidae, such as the genus Segmentina.
The body chamber or living chamber in shelled cephalopod mollusks is an equivalent space, is sometimes called the body whorl. It is the last chamber in the shell of a nautiloid or ammonoid; the body of the animal occupies the living chamber, apart from the siphuncle which extends through the rest of the septa to provide buoyancy
The gladius, or pen, is a hard internal bodypart found in many cephalopods of the superorder Decapodiformes and in a single extant member of the Octopodiformes, the vampire squid. It is so named for its superficial resemblance to the Roman short sword of the same name, is a vestige of the ancestral mollusc shell, external; the gladius is located dorsally within the mantle and extends for its entire length. Composed of chitin, it lies within the shell sac, responsible for its secretion. Gladii are known from a number of extinct cephalopod groups, including teudopseids and prototeuthids. Gladii are shaped in many distinctive ways and vary between species, though are like a feather or leaf; some examples are shown below. Cuttlebone Belemnoidea Argonaut Nautilus Mollusc shell Bizikov, V. A.. Squid gladius: its use for the study of growth, intraspecies structure and evolution. Ph. D. Thesis. Institute of Oceanology, SSSR Academy of Sciences, Moscow. 513 pp. Toll, R. B.. The comparative morphology of the gladius in the order Teuthoidea in relation to systematics and phylogeny.
Ph. D. Thesis. University of Miami, Coral Gables, Florida. 390 pp. Toll, R. B.. The gladius in teuthoid systematics. Smithsonian Contributions to Zoology 586: 55–67
D'Arcy Wentworth Thompson
Sir D'Arcy Wentworth Thompson CB FRS FRSE was a Scottish biologist and classics scholar. He was a pioneer of mathematical biology, travelled on expeditions to the Bering Strait and held the position of Professor of Natural History at University College, Dundee for 32 years at St Andrews for 31 years, he was elected a Fellow of the Royal Society, was knighted, received the Darwin Medal and the Daniel Giraud Elliot Medal. Thompson is remembered as the author of the 1917 book On Growth and Form, which led the way for the scientific explanation of morphogenesis, the process by which patterns and body structures are formed in plants and animals. Thompson's description of the mathematical beauty of nature and the mathematical basis of the forms of animals stimulated thinkers as diverse as Julian Huxley, C. H. Waddington, Alan Turing, Claude Lévi-Strauss, Eduardo Paolozzi, Le Corbusier, Christopher Alexander and Mies van der Rohe. Thompson was born at 3 Brandon Street in Edinburgh to Fanny Gamgee and D'Arcy Wentworth Thompson, Classics Master at Edinburgh Academy and Professor of Greek at Queen's College, Galway.
His mother, Fanny Gamgee died 9 days after his birth as a result of complications and he was brought up by his maternal grandfather Joseph Gamgee, a veterinary surgeon. He lived with his grandfather and uncle, John Gamgee, at 12 Castle Terrace, facing north onto Edinburgh Castle, he was nephew to Sampson Gamgee. From 1870 to 1877 he attended The Edinburgh Academy and won the 1st Edinburgh Academical Club Prize in 1877. In 1878, he matriculated at the University of Edinburgh to study medicine. Two years he moved south to study zoology at Trinity College in the University of Cambridge; as a student in Cambridge, D'Arcy Thompson was first a sizar received a scholarship. He translated Hermann Müller's work on the fertilization of flowers, to earn money on the side, because it appealed to him, it was published in 1883 with an introduction by Charles Darwin. He speculated that if he had chosen to translate Wilhelm Olbers Focke's hybridization of flowers, he "might have anticipated the discovery of Mendel by twenty years".
He graduated with a Bachelor of Arts degree in Natural Science in 1883. From 1883-1884, Thompson stayed in Cambridge as so-called Junior Demonstrator in physiology, teaching students. In 1884, he was appointed Professor of Biology at University College, Dundee, a post he held for 32 years. One of his first tasks was to create a Zoology Museum for teaching and research, now named after him. In 1885 he was elected a Fellow of the Royal Society of Edinburgh, his proposers were Frank W. Young, William Evans Hoyle and Daniel John Cunningham, he served as Vice President to the Society from 1916 to 1919 and as President from 1934 to 1939. In 1896 and 1897, he went on expeditions to the Bering Straits, representing the British Government in an international inquiry into the fur seal industry to assess the fur seal's declining numbers. "Thompson's diplomacy avoided an international incident" between Russia and the United States which both had hunting interests in this area. His final report for the government drew attention to the near extinction of the sea otter and whale populations.
He became one of the first to press for conservation agreements, his recommendations contributed to the issuing of species protection orders. He subsequently was appointed Scientific Adviser to the Fisheries Board of Scotland and representative to the International Council for the Exploration of the Sea, he took the opportunity to collect many valuable specimens for his museum, one of the largest in the country at the time, specialising in Arctic zoology, through his links to the Dundee whalers. The D'Arcy Thompson Zoology Museum still has the Japanese spider crab that he collected, the rare skeleton of a Steller's Sea Cow. Whilst in Dundee, Thompson sat on the committee of management of the Dundee Private Hospital for Women, he was a founder member of the Dundee Social Union and pressed for it "to buy four slum properties in the town", which they renovated so that "the poorest families of Dundee could live there."In 1917, aged 57, Thompson was appointed to the Chair of Natural History at St Andrews University, where he remained for the last 31 years of his life.
In 1918 he delivered the Royal Institution Christmas Lecture on The Fish of the Sea. The German British mathematician Walter Ledermann described in his memoir how, as an assistant in Mathematics, he met the biology Professor Thompson at St Andrews in the mid 1930s and how Thompson "was fond of exercising his skills as an amateur of mathematics", that "he used quite sophisticated mathematical methods to elucidate the shapes that occur in the living world" and "differential equations, a subject which evidently lay outside d'Arcy Thompson's fields of knowledge at that time." Ledermann wrote how on one occasion he helped him and writing out the answer to his question. In Country Life magazine in October 1923, he wrote: "This is but a little town, our lives are somewhat narrow who dwell therein; the stones cry out to us as we pass.... Only last week I went down to the little ancient church of Saint-Julien-le-Pauvre in Paris, passed through it to stand for a moment in the deserted garden whence one looks across the river and gets the finest view of all of Notre Dame.... here have been civilization and learning for a few short centuries longer than in St Andrews....
Yet these two spots have a like influence on my mind and rejoice my heart with a train of shadowy memories." On 4 July 1901 Thompson
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, dehydration, but serves for muscle attachment and calcium storage; some gastropods appear shell-less but may have a remnant within the mantle, or the shell is reduced such that the body cannot be retracted within. Some snails 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, 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 available at the Hathi Trust Digital Library; the gastropod shell has three major layers secreted by the mantle. The calcareous central layer, tracum, is made of calcium carbonate precipitated into an organic matrix known as conchiolin.
The outermost layer is the periostracum, resistant to abrasion and provides most shell coloration. The body of the snail contacts the innermost smooth layer that may be composed of mother-of-pearl or shell nacre, a dense horizontally packed form of conchiolin, layered upon the periostracum as the snail grows. Gastropod shell morphology is quite constant among individuals of a species. Controlling variables are: The rate of growth per revolution around the coiling axis. High rates give wide-mouthed forms such as the abalone, low rates give coiled forms such as Turritella or some of the Planorbidae; the shape of the generating curve equivalent to the shape of the aperture. It may be round, for instance in the turban shell, elongate as in the cone shell or have an irregular shape with a siphonal canal extension, as in the Murex; the rate of translation of the generating curve along the axis of coiling, controlling how high-spired the resulting shell becomes. This may range from a flat planispiral shell, to nearly the diameter of the aperture.
Irregularities or "sculpturing" such as ribs, spines and varices made by the snail changing the shape of the generating curve during the course of growth, for instance in the many species of Murex. Ontologic growth changes as the animal reaches adulthood. Good examples are the inward-coiled lip of the cowry; some of these factors can be modelled mathematically and programs exist to generate realistic images. Early work by David Raup on the analog computer revealed many possible combinations that were never adopted by any actual gastropod; some shell shapes are found more in certain environments, though there are many exceptions. Wave-washed high-energy environments, such as the rocky intertidal zone, are inhabited by snails whose shells have a wide aperture, a low surface area, a high growth rate per revolution. High-spired and sculptured forms become more common in quiet water environments; the shell of burrowing forms, such as the olive and Terebra, are smooth and lack elaborate sculpture, in order to decrease resistance when moving through sand.
On land, high-spired forms are associated with vertical surfaces, whereas flat-shelled snails tend to live on the ground. A few gastropods, for instance the Vermetidae, cement the shell to, grow along, solid surfaces such as rocks, or other shells. Most gastropod shells are spirally coiled; the majority of gastropod species have dextral shells, but a small minority of species and genera are always sinistral, a few species show a mixture of dextral and sinistral individuals. There occur aberrantly sinistral forms of dextral species and some of these are sought by shell collectors. If a coiled gastropod shell is held with the spire pointing upwards and the aperture more or less facing the observer, a dextral shell will have the aperture on the right-hand side, a sinistral shell will have the aperture on the left-hand side; this chirality of gastropods is sometimes overlooked when photographs of coiled gastropods are "flipped" by a non-expert prior to being used in a publication. This image "flipping" results in a normal dextral gastropod appearing to be a rare or abnormal sinistral one.
Sinistrality arose independently 19 times among marine gastropods since the start of the Cenozoic. This left-handedness seems to be more common in land pulmonates, but still the dextral living species in gastropods seem to account for 99% of the total number. The chirality in gastropods appears in the gene NODAL is involved. A more recent study correlates the asymmetric coiling of the shell by the left-right asymmetric expression of the decapentaplegic gene in the mantle. In a few cases, both left- and right-handed coiling are found in the same population. Sinistral mutants of dextral species and dextral mutants of sinistral species are rare but well documented occurrences among land snails in general. Populations or species with mixed coiling are much rarer, and, so far as is known, are confined, with one exception, to a few genera of arboreal tropical snails. Besides Amphidromus, the Cuban Liguus vittatus, Haitian Liguus virgineus, some Hawaiian Partulina and many Hawaiian Achatinella, as well as several species of Pacific islands Partula, are known to have mixed dextral-sinistral populations.
A possible exception may concern some of the European clausiliids of the subfamily Alopiinae. They are ob