Air-breathing land gastropod, the Roman snailKingdom:Phylum:Class:Gastropoda, 179565,000 to 80,000 speciesThe gastropods ( ), more commonly known as snails and slugs, belong to a large of within the, called Gastropoda. This class comprises and from saltwater, from freshwater, and from the land. There are many thousands of species of and, as well as, freshwater, and and.The class Gastropoda contains a vast total of named species, second only to the in overall number.
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The fossil history of this class goes back to the. As of 2017, 721 of gastropods are known, of which 245 are and appear only in the record, while 476 are currently or without a fossil record.Gastropoda (previously known as univalves and sometimes spelled 'Gasteropoda') are a major part of the phylum Mollusca, and are the most highly diversified in the, with 65,000 to 80,000 living snail and slug.
The, behavior, feeding, and reproductive adaptations of gastropods vary significantly from one or group to another. Therefore, it is difficult to state many generalities for all gastropods.The class Gastropoda has an extraordinary diversification of.
Representatives live in gardens, woodland, deserts, and on mountains; in small ditches, great rivers and lakes; in, the rocky, the sandy subtidal, in the depths of the oceans including the, and numerous other ecological niches, including ones.Although the name 'snail' can be, and often is, applied to all the members of this class, commonly this word means only those species with an external big enough that the soft parts can withdraw completely into it. Those gastropods without a shell, and those with only a very reduced or internal shell, are usually known as slugs; those with a shell into which they can partly but not completely withdraw are termed semi-slugs.The marine shelled species of gastropod include species such as, and numerous other sea snails that produce that are coiled in the adult stage—though in some, the coiling may not be very visible, for example in. In a number of of species, such as all the various, the shell is coiled only in the stage, and is a simple conical structure after that. Contents.Etymology In the scientific literature, gastropods were described under 'gasteropodes' by in 1795.The earlier name univalve means 'one valve' or shell, in contrast to applied to other molluscs such as clams and meaning that those animals possess two or shells.Diversity At all levels, gastropods are second only to the in terms of their.Gastropods have the greatest numbers of named species.
However, estimates of the total number of gastropod species vary widely, depending on cited sources. The number of gastropod species can be ascertained from estimates of the number of described species of Mollusca with accepted names: about 85,000 (minimum 50,000, maximum 120,000). But an estimate of the total number of Mollusca, including undescribed species, is about 240,000 species. The estimate of 85,000 molluscs includes 24,000 described species of terrestrial gastropods.Different estimates for aquatic gastropods (based on different sources) give about 30,000 species of marine gastropods, and about 5,000 species of freshwater and gastropods. Many deep-sea species remain to be discovered as only 0.0001% of the deep-sea floor has been studied biologically.
The total number of living species of freshwater snails is about 4,000.There are 444 recently species of gastropods (extinct since the year 1500), 18 species that are now (but still existing in captivity) and 69 'possibly extinct' species.The number of prehistoric (fossil) species of gastropods is at least 15,000 species.In marine habitats, the and the are home to the highest diversity of marine gastropods, while the continental shelf and abyssal depths have a low diversity of marine gastropods. Main articles:, andSome of the more familiar and better-known gastropods are (the land snails and slugs). Some live in freshwater, but the majority of all named species of gastropods live in a marine environment.Gastropods have a worldwide distribution, from the near and zones to the tropics. They have become adapted to almost every kind of existence on earth, having colonized nearly every available medium.In habitats where there is not enough to build a really solid shell, such as on some acidic soils on land, there are still various species of slugs, and also some snails with a thin translucent shell, mostly or entirely composed of the protein.Snails such as and have adapted to desert conditions. Other snails have adapted to an existence in ditches, near deepwater hydrothermal vents, the pounding surf of, and many other diverse areas.Gastropods can be accidentally transferred from one habitat to another by other animals, e.g.
The smallest bird species reported to carry a gastropod was a ( Parus major), as a hairy snail was found in the plumage of a wintering great tit in in 2010. The anatomy of an aquatic snail with a gill, a male gastropod. Note that much of this anatomy does not apply to gastropods in other.Light yellow - bodyBrown - andGreen - digestive systemLight purple -Yellow -Red - heartPink -Dark violet -1. Cerebral ganglion3.4. Pleural ganglion8. Atrium of heart9.
Visceral ganglion10. Tentacle (chemosensory, 2 or 4)16. (everted, normally internal)18. Esophageal nerve ring19.
Pedal ganglion20. Lower commissura21.22. Pallial cavity / cavity / respiratory cavity23. Parietal ganglion24.
Anus25.26.27.28.Snails are distinguished by an anatomical process known as, where the visceral mass of the animal rotates 180° to one side during development, such that the is situated more or less above the head. This process is unrelated to the coiling of the shell, which is a separate phenomenon.
Torsion is present in all gastropods, but the are secondarily de-torted to various degrees.Torsion occurs in two stages. The first, mechanistic stage, is muscular, and the second is. The effects of torsion are primarily physiological - the organism develops an asymmetrical growth, with the majority occurring on the left side. This leads to the loss of right-paired appendages (e.g., (comb-like respiratory apparatus), etc.).
Furthermore, the anus becomes redirected to the same space as the head. This is speculated to have some evolutionary function, as prior to torsion, when retracting into the shell, first the posterior end would get pulled in, and then the anterior. Now, the front can be retracted more easily, perhaps suggesting a defensive purpose.However, this 'rotation hypothesis' is being challenged by the 'asymmetry hypothesis' in which the gastropod mantle cavity originated from one side only of a bilateral set of mantle cavities.Gastropods typically have a well-defined with two or four sensory with eyes, and a ventral foot, which gives them their name ( gaster, and poda, ). The foremost division of the foot is called the propodium. Its function is to push away sediment as the snail crawls.
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The larval shell of a gastropod is called a.The principal characteristic of the Gastropoda is the asymmetry of their principal organs. The essential feature of this asymmetry is that the anus generally lies to one side of the median plane.; The (gill-combs), the (olfactory organs), the (or pallial mucous gland), and the of the heart are single or at least are more developed on one side of the body than the other; Furthermore, there is only one orifice, which lies on the same side of the body as the anus. Of, a small land snail, has dextral coiling, which is typical (but not universal) in gastropod shells.Upper image: dorsal view of the shell, showing the apexCentral image: lateral view showing the spire and aperture of the shellLower image: basal view showing the umbilicusMost shelled gastropods have a one piece, typically coiled or spiraled, at least in the larval stage. This coiled shell usually opens on the right-hand side (as viewed with the shell pointing upward).
Numerous species have an, which in many species acts as a trapdoor to close the shell. This is usually made of a horn-like material, but in some molluscs it is calcareous. In the land slugs, the shell is reduced or absent, and the body is streamlined.Body wall Some are very brightly colored.
This serves either as a warning, when they are or contain, or to them on the brightly colored, and seaweeds on which many of the species are found.Lateral outgrowths on the body of are called. These contain an outpocketing of digestive gland called the.Sensory organs and nervous system. Main articles: andThe of gastropods include, eyes,. Gastropods have no hearing.In terrestrial gastropods (land snails and slugs), the olfactory organs, located on the tips of the four, are the most important sensory organ.
The chemosensory organs of marine gastropods are called.The majority of gastropods have simple visual organs, eye spots either at the tip or base of the tentacles. However, 'eyes' in gastropods range from simple that only distinguish light and dark, to more complex, and even to. In land snails and slugs, vision is not the most important sense, because they are mainly animals.The nervous system of gastropods includes the and the. The central nervous system consist of connected by nerve cells. It includes paired ganglia: the cerebral ganglia, pedal ganglia, ganglia, pleural ganglia, parietal ganglia and the visceral ganglia.
There are sometimes also buccal ganglia. Digestive system. Main articles: andThe of a gastropod is usually adapted to the food that a species eats. The simplest gastropods are the and, herbivores that use their hard to rasp at on rocks.Many marine gastropods are burrowers, and have a that extends out from the edge.
Sometimes the shell has a to accommodate this structure. A siphon enables the animal to draw water into their and over the gill. They use the siphon primarily to 'taste' the water to detect prey from a distance. Gastropods with siphons tend to be either predators or scavengers.Respiratory system.
Main articles: andAlmost all marine gastropods breathe with a, but many freshwater species, and the majority of terrestrial species, have a pallial. The respiratory protein in almost all gastropods is, but one freshwater family, the, have as the respiratory protein.In one large group of sea slugs, the gills are arranged as a rosette of feathery plumes on their backs, which gives rise to their other name,. Some nudibranchs have smooth or backs with no visible gill mechanism, such that respiration may likely take place directly through the skin.Circulatory system. See also:is a part of the behavior of gastropods with some families of land snails creating and utilizing, the throwing of which have been identified as a form of.The main aspects of the life cycle of gastropods include:. Egg laying and the eggs of gastropods. The of gastropods.
The larvae or larval stadium: some gastropods may be and/or. and (each of these are present in some gastropods only). The growth of gastropods. Courtship and mating in gastropods: is internal or external according to the species. External fertilization is common in marine gastropods.Feeding behavior. Snail eating a dandelion flowerThe diet of gastropods differs according to the group considered.
Marine gastropods include some that are, feeders, and also a few ciliary feeders, in which the is reduced or absent. Land-dwelling species can chew up leaves, bark, fruit and decomposing animals while marine species can scrape algae off the rocks on the sea floor. In some species that have evolved into endoparasites, such as the, many of the standard gastropod features are strongly reduced or absent.A few are herbivores and some are carnivores. The carnivorous habit is due to specialisation. Many gastropods have distinct dietary preferences and regularly occur in close association with their food species.Some predatory carnivorous gastropods include, for example:, and others.Genetics Gastropods exhibit an important degree of variation in organization when compared to other animals. Main events of occurred at the origin of and, whereas fewer changes occurred between the ancestors of (only D, C and N) and (a large single, and of the tRNAs D and N). Within Heterobranchia, gene order seems relatively conserved, and gene rearrangements are mostly related with of tRNA genes.
Geological history and evolution. See also: andThe first gastropods were exclusively marine, with the earliest representatives of the group appearing in the (, ), though their only gastropod character is a coiled shell, so they could lie in the stem lineage, if they are gastropods at all. Earliest organisms like, and are no longer considered gastropodsand the tiny coiled of earliest time is probably not even a mollusk.
As such, it's not until the Ordovician that the first crown-group members arise. By the period the gastropods were a varied group present in a range of aquatic habitats.
Commonly, gastropods from the rocks of the early era are too poorly preserved for accurate identification. Still, the genus contains fifteen identified species. Fossil gastropods were less common during the era than.Most of the gastropods of the Palaeozoic era belong to primitive groups, a few of which still survive. By the period many of the shapes seen in living gastropods can be matched in the fossil record, but despite these similarities in appearance the majority of these older forms are not directly related to living forms. It was during the era that the ancestors of many of the living gastropods evolved.One of the earliest known terrestrial (land-dwelling) gastropods is, which is found in the of the period in Europe, but relatives of the modern land snails are rare before the period, when the familiar first appeared.
: another European pulmonate land snail, which has been introduced to many other countriesIn rocks of the era, gastropods are slightly more common as fossils; their shells are often well preserved. Their fossils occur in ancient beds deposited in both freshwater and marine environments. The ' of the period and the ' of the early Cretaceous period, which both occur in southern England, are containing the tightly packed remains of the pond snail.Rocks of the era yield very large numbers of gastropod fossils, many of these fossils being closely related to modern living forms. The diversity of the gastropods increased markedly at the beginning of this era, along with that of the.Certain trail-like markings preserved in ancient sedimentary rocks are thought to have been made by gastropods crawling over the soft mud and sand. Although these are of debatable origin, some of them do resemble the trails made by living gastropods today.Gastropod fossils may sometimes be confused with or other shelled. An example of this is from the limestones of the period in Europe, the shell of which is planispirally coiled and can be mistaken for the shell of a cephalopod. Gastropods are one of the groups that record the changes in fauna caused by the advance and retreat of the during the epoch.Cladogram A showing the phylogenic relationships of Gastropoda with example species.
Microphoto (35x) of Gastropoda sp. From Holocene sediments of Amuq Plain SSE TurkeySince, biological taxonomy has attempted to reflect the of organisms, i.e., the. The classifications used in taxonomy attempt to represent the precise interrelatedness of the various taxa. However, the taxonomy of the Gastropoda is constantly being revised and so the versions shown in various texts can differ in major ways.In the older classification of the gastropods, there were four subclasses:. (gills to the right and behind the heart). (no shell). (gills in front of the heart).
(with a lung instead of gills)The of the Gastropoda is still under revision, and more and more of the old taxonomy is being abandoned, as the results of studies slowly become clearer. Nevertheless, a few of the older terms such as 'opisthobranch' and 'prosobranch' are still sometimes used in a descriptive way.New insights based on DNA sequencing of gastropods have produced some revolutionary new taxonomic insights. In the case of the Gastropoda, the taxonomy is now gradually being rewritten to embody strictly groups (only one lineage of gastropods in each group). Integrating new findings into a working remain challenging. Consistent ranks within the taxonomy at the level of subclass, superorder, order, and suborder have already been abandoned as unworkable.
Ongoing revisions of the higher taxonomic levels are expected in the near future., which appears to exist at especially high frequency in gastropods, may account for the observed differences between the older phylogenies, which were based on morphological data, and more recent gene-sequencing studies.made sweeping changes in the, resulting in a taxonomy that is a step closer to the evolutionary history of the. The Bouchet & Rocroi classification system is based partly on the older systems of classification, and partly on new research.In the past, the taxonomy of gastropods was largely based on morphological characters of the taxa. The recent advances are more based on molecular characters from and research.
This has made the taxonomical ranks and their hierarchy controversial. The debate about these issues is not likely to end soon.In the Bouchet, Rocroi et al. Taxonomy, the authors have used unranked for taxa above the rank of superfamily (replacing the ranks suborder, order, superorder and subclass), while using the traditional for all taxa below the rank of superfamily. Whenever has not been tested, or is known to be paraphyletic or polyphyletic, the term 'group' or 'informal group' has been used. The classification of families into subfamilies is often not well resolved, and should be regarded as the best possible hypothesis.In 2004, Brian Simison and showed possible origins of the Gastropoda based on gene order and analyses of complete genes.In the 2017 issue of ' journal (available online from 4 January 2018) new much updated version of 2005 'Bouchet & Rocroi' taxonomy was published: 'Revised Classification, Nomenclator and Typification of Gastropod and Monoplacophoran Families'.
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