Gastropoda

Gastropoda

Overview
The Gastropoda or gastropods, more commonly known as snails and slugs, are a large taxonomic class within the phylum Mollusca
Mollusca
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

. The class Gastropoda includes snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s and slug
Slug
Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...

s of all kinds and all sizes from microscopic to quite large. There are huge numbers of sea snail
Sea snail
Sea snail is a common name for those snails that normally live in saltwater, marine gastropod molluscs....

s and sea slug
Opisthobranchia
Opisthobranchs are a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia, but no longer considered to represent a monophyletic grouping...

s, as well as freshwater snail
Freshwater snail
A freshwater snail is one kind of freshwater mollusc, the other kind being freshwater clams and mussels, i.e. freshwater bivalves. Specifically a freshwater snail is a gastropod that lives in a watery non-marine habitat. The majority of freshwater gastropods have a shell, with very few exceptions....

s and freshwater limpet
Limpet
Limpet is a common name for a number of different kinds of saltwater and freshwater snails ; it is applied to those snails that have a simple shell which is more or less conical in shape, and either is not spirally coiled, or appears not to be coiled in the adult snails.The name limpet is most...

s, and land snail
Land snail
A land snail is any of the many species of snail that live on land, as opposed to those that live in salt water and fresh water. Land snails are terrestrial gastropod mollusks that have shells, It is not always an easy matter to say which species are terrestrial, because some are more or less...

s and land slugs.

The class Gastropoda contains a vast total of named species, second only to the insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s in overall number.
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Encyclopedia
The Gastropoda or gastropods, more commonly known as snails and slugs, are a large taxonomic class within the phylum Mollusca
Mollusca
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

. The class Gastropoda includes snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s and slug
Slug
Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...

s of all kinds and all sizes from microscopic to quite large. There are huge numbers of sea snail
Sea snail
Sea snail is a common name for those snails that normally live in saltwater, marine gastropod molluscs....

s and sea slug
Opisthobranchia
Opisthobranchs are a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia, but no longer considered to represent a monophyletic grouping...

s, as well as freshwater snail
Freshwater snail
A freshwater snail is one kind of freshwater mollusc, the other kind being freshwater clams and mussels, i.e. freshwater bivalves. Specifically a freshwater snail is a gastropod that lives in a watery non-marine habitat. The majority of freshwater gastropods have a shell, with very few exceptions....

s and freshwater limpet
Limpet
Limpet is a common name for a number of different kinds of saltwater and freshwater snails ; it is applied to those snails that have a simple shell which is more or less conical in shape, and either is not spirally coiled, or appears not to be coiled in the adult snails.The name limpet is most...

s, and land snail
Land snail
A land snail is any of the many species of snail that live on land, as opposed to those that live in salt water and fresh water. Land snails are terrestrial gastropod mollusks that have shells, It is not always an easy matter to say which species are terrestrial, because some are more or less...

s and land slugs.

The class Gastropoda contains a vast total of named species, second only to the insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s in overall number. The fossil history of this class goes all the way back to the Late Cambrian. There are 611 families of gastropods, of which 202 families are extinct, being found only in the fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 record.

Gastropoda (previously known as univalves and sometimes spelled Gasteropoda) are a major part of the phylum Mollusca and are the most highly diversified class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 in the phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

, with 60,000 to 80,000 living snail and slug species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

. The anatomy
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...

, behavior, feeding and reproductive adaptations of gastropods vary significantly from one clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 or group to another. Therefore, it is difficult to state many generalities for all gastropods.

The class Gastropoda has an extraordinary diversification of habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...

s. Representatives live in gardens, in woodland, in deserts, and on mountains; in small ditches, great rivers and lakes; in estuaries, mudflat
Mudflat
Mudflats or mud flats, also known as tidal flats, are coastal wetlands that form when mud is deposited by tides or rivers. They are found in sheltered areas such as bays, bayous, lagoons, and estuaries. Mudflats may be viewed geologically as exposed layers of bay mud, resulting from deposition of...

s, the rocky intertidal, the sandy subtidal, in the abyssal
Abyssal zone
The abyssal zone is the abyssopelagic layer or pelagic zone that contains the very deep benthic communities near the bottom of oceans. "Abyss" derives from the Greek word ἄβυσσος, meaning bottomless. At depths of 4,000 to 6,000 metres , this zone remains in perpetual darkness and never receives...

 depths of the oceans including the hydrothermal vent
Hydrothermal vent
A hydrothermal vent is a fissure in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots. Hydrothermal vents exist because the earth is both...

s, and numerous other ecological niches, including parasitic 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 shell
Gastropod shell
The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...

 large 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.

The marine shelled species of gastropod include edible species such as abalone
Abalone
Abalone , from aulón, are small to very large-sized edible sea snails, marine gastropod molluscs in the family Haliotidae and the genus Haliotis...

, conch
Conch
A conch is a common name which is applied to a number of different species of medium-sized to large sea snails or their shells, generally those which are large and have a high spire and a siphonal canal....

es, periwinkle
Common Periwinkle
The common periwinkle or winkle, scientific name Littorina littorea, is a species of small edible sea snail, a marine gastropod mollusk which has gills and an operculum, and is classified within the family Littorinidae, the periwinkles....

s, whelk
Whelk
Whelk, also spelled welk or even "wilks", is a common name used to mean one or more kinds of sea snail. The species, genera and families referred to using this common name vary a great deal from one geographic area to another...

s, and numerous other sea snails that produce seashell
Seashell
A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer created by an animal that lives in the sea. The shell is part of the body of the animal. Empty seashells are often found washed up on beaches by beachcombers...

s which are coiled in the adult stage, even though in some cases the coiling may not be very visible, for example in cowries. There are also a number of families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 of species such as all the various limpet
Limpet
Limpet is a common name for a number of different kinds of saltwater and freshwater snails ; it is applied to those snails that have a simple shell which is more or less conical in shape, and either is not spirally coiled, or appears not to be coiled in the adult snails.The name limpet is most...

s, where the shell is coiled only in the larval stage, and is a simple conical structure after that.

Etymology


The word "gastropod" is derived from the Ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 words γαστήρ (gastér, stem: gastr-) "stomach", and πούς (poús, stem: pod-) "foot", hence stomach-foot. This is an anthropomorphic name, based on the fact that to humans it appears as if snails and slugs crawl on their bellies
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...

. In reality, snails and slugs have their stomach, the rest of their digestive system
Digestive system of gastropods
The digestive system of gastropods has evolved to suit almost every kind of diet and feeding behavior. Gastropods as the largest taxonomic class of the mollusca are very diverse indeed: the group includes carnivores, herbivores, scavengers, filter feeders, and even parasites.In particular, the...

 and all the rest of their viscera in a hump on the opposite, dorsal
Dorsum (biology)
In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...

 side of the body. In most gastropods this visceral hump is covered by, and contained within, the shell.

In the scientific literature, gastropods were described under the vernacular (French) name "gasteropodes" by Georges Cuvier
Georges Cuvier
Georges Chrétien Léopold Dagobert Cuvier or Jean Léopold Nicolas Frédéric Cuvier , known as Georges Cuvier, was a French naturalist and zoologist...

 in 1795. The name was later Latinized
Latinisation (literature)
Latinisation is the practice of rendering a non-Latin name in a Latin style. It is commonly met with for historical personal names, with toponyms, or for the standard binomial nomenclature of the life sciences. It goes further than Romanisation, which is the writing of a word in the Latin alphabet...

.

The earlier name univalve means "one valve" or shell, in contrast to bivalve applied to mollusks such as clams and meaning that those animals possess two valves or shells.

Diversity


At all taxonomic levels, gastropods are second only to the insects in terms of their diversity.

Gastropods are the class of molluscs which have the greatest numbers of named species. However estimates of the total number of gastropod species varies widely, depending on the 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 brackish gastropods. The total number of recent species of freshwater snails is about 4,000.

There are 444 recently extinct species of gastropods (extinct since the year 1500), 18 species that are now extinct in the wild
Extinct in the Wild
Extinct in the Wild is a conservation status assigned to species or lower taxa, the only known living members of which are being kept in captivity or as a naturalized population outside its historic range.-Examples:...

 (but still existing in captivity) and 69 "possibly extinct" species.

The number of prehistoric (fossil) species of gastropods is at least 15,000 species.

Habitat



Some of the more familiar and better-known gastropods are terrestrial gastropods (the land snails and slugs) and some live in freshwater, but more than two thirds of all named species live in a marine environment.

Gastropods have a worldwide distribution from the near Arctic and Antarctic zones to the tropics. They have become adapted to almost every kind of existence on earth, having colonized every medium available except the air.

In habitats where there is not enough calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 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 conchiolin
Conchiolin
Conchiolin and perlucin are complex proteins which are secreted by a mollusc's outer epithelium ....

.

Snails such as Sphincterochila boissieri
Sphincterochila boissieri
Sphincterochila boissieri is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Sphincterochilidae...

and Xerocrassa seetzeni
Xerocrassa seetzeni
Xerocrassa seetzeni is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Hygromiidae.- Distribution :...

have adapted to desert conditions, other snails have adapted to an existence in ditches, near deepwater hydrothermal vents, the pounding surf of rocky shores, caves, and many other diverse areas.

Anatomy




Snails are distinguished by an anatomical process known as torsion
Torsion (gastropod)
Torsion is an anatomical event which takes place during the very early part of the life of snails and slugs of all kinds. In other words, torsion is a gastropod synapomorphy which occurs in all gastropods during larval development. Torsion is the rotation of the visceral mass, mantle and shell 180˚...

, where the visceral mass of the animal rotates 180° to one side during development, such that the anus 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 opisthobranch gastropods are secondarily de-torted to various degrees.

Torsion occurs in two mechanistic stages. The first is muscular and the second is mutagenetic. The effects of torsion are primarily physiological - the organism develops an asymmetrical nature with the majority of growth occurring on the left side. This leads to the loss of right-paired appendages (e.g. ctenidia
Ctenidium
Ctenidium may be:* Ctenidium , , a comb-like gill, part of the respiratory system of gastropods and bivalves* Ctenidium, , a row of peg-like spines in some insects...

 (comb-like respiratory apparatus), gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

s, nephridia
Nephridium
A Nephridium is an invertebrate organ which occurs in pairs and function similar to kidneys. Nephridia remove metabolic wastes from an animal's body. They are present in many different invertebrate lines. There are two basic types, metanephridia and protonephridia, but there are other...

, 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 head with two or four sensory tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...

s with eyes, and a ventral foot, which gives them their name (Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 gaster, stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

, and poda, feet
Foot
The foot is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made up of one or more segments or bones, generally including claws...

). The foremost division of the foot is called the propodium. Its function is to push away sediment as the snail crawls. The larval shell of a gastropod is called a protoconch
Protoconch
A protoconch is an embryonic or larval shell of some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod...

.

The shell



Most shelled gastropods have a one piece shell
Gastropod shell
The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...

, typically coiled or spiraled. This coiled shell usually opens on the right-hand side (as viewed with the shell apex
Apex (mollusc)
Apex is an anatomical term for the tip of the mollusc shell of a gastropod, scaphopod, or cephalopod mollusc.-Gastropods:The word "apex" is most often used to mean the tip of the spire of the shell of a gastropod...

 pointing upward). Numerous species have an operculum
Operculum (gastropod)
The operculum, meaning little lid, is a corneous or calcareous anatomical structure which exists in many groups of sea snails and freshwater snails, and also in a few groups of land snails...

, 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 sea slugs
Opisthobranchia
Opisthobranchs are a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia, but no longer considered to represent a monophyletic grouping...

 are very brightly colored. This serves either as a warning, when they are poisonous or contain stinging cells, or to camouflage
Camouflage
Camouflage is a method of concealment that allows an otherwise visible animal, military vehicle, or other object to remain unnoticed, by blending with its environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly...

 them on the brightly-colored hydroids, sponges and seaweeds on which many of the species are found.

Lateral outgrowths on the body of nudibranchs are called cerata
Cerata
Cerata are anatomical structures found in nudibranch sea slugs, marine opisthobranch gastropod mollusks.They are dorsal and lateral outgrowths on the upper surfaces of the body....

. These contain a part of digestive gland, which is called the diverticula
Diverticula (mollusc)
Diverticula is an anatomical term for a set of organs which are visible from the outside on a group of sea slugs known as nudibranchs, which are marine opisthobranch gastropod molluscs....

.

Sensory organs and nervous system


Sensory organs
Sense
Senses are physiological capacities of organisms that provide inputs for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology , and philosophy of perception...

 of gastropods include olfactory organs
Olfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...

, eyes, statocyst
Statocyst
The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...

s and mechanoreceptor
Mechanoreceptor
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. There are four main types in the glabrous skin of humans: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini corpuscles...

s. Gastropods have no hearing.

In terrestrial gastropods (land snails and slugs), the olfactory organs, located on the tips of the 4 tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...

s, are the most important sensory organ, The chemosensory organs of opisthobranch marine gastropods are called rhinophore
Rhinophore
A rhinophore is one of a pair of club-shaped structures which are the most prominent part of the external head anatomy of a group of sea slugs, marine gastropod opisthobranch mollusks in the order Nudibranchia, the nudibranchs, specifically the dorid nudibranchs.- Etymology :The name relates to the...

s.

The majority of gastropods have simple visual organs, eye spots, that are situated either at the tip of the tentacles or the base of the tentacles. However "eyes" in gastropods range from these simple ocelli which cannot process an image being only able to distinguish light and dark, to more complex pit eyes, and even to lens eyes. In land snails and slugs, vision is not the most important sense, because they are mainly nocturnal animals.

The nervous system of gastropods includes the peripheral nervous system
Peripheral nervous system
The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain...

 and the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

. The central nervous system consist of ganglia connected by nerve cells. It includes paired ganglia: the cerebral ganglia, pedal ganglia, osphradial ganglia, pleural ganglia, parietal ganglia and the visceral ganglia. There are sometimes also buccal ganglia.

Digestive system



The radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...

 of a gastropod is usually adapted to the food that a species eats. The simplest gastropods are the limpet
Limpet
Limpet is a common name for a number of different kinds of saltwater and freshwater snails ; it is applied to those snails that have a simple shell which is more or less conical in shape, and either is not spirally coiled, or appears not to be coiled in the adult snails.The name limpet is most...

s and abalone
Abalone
Abalone , from aulón, are small to very large-sized edible sea snails, marine gastropod molluscs in the family Haliotidae and the genus Haliotis...

s, herbivores that use their hard radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...

 to rasp at seaweed
Seaweed
Seaweed is a loose, colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae...

s on rocks.

Many marine gastropods are burrowers, and have a siphon that extends out from the mantle
Mantle (mollusc)
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...

 edge. Sometimes the shell has a siphonal canal
Siphonal canal
Some sea marine gastropods have a soft tubular anterior extension of the mantle called a siphon through which water is drawn into the mantle cavity and over the gill and which serves as a chemoreceptor to locate food. In many carnivorous snails, where the siphon is particularly long, the structure...

 to accommodate this structure. A siphon enables the animal to draw water into their mantle cavity 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



Almost all marine gastropods breathe with a gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

, but many freshwater species, and the majority of terrestrial species, have a pallial lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

. Gastropods with a lung belong to one group with common descent, the Pulmonata, however, gastropods with gills are paraphyletic. The respiratory protein in almost all gastropods is hemocyanin
Hemocyanin
Hemocyanins are respiratory proteins in the form of metalloproteins containing two copper atoms that reversibly bind a single oxygen molecule . Oxygenation causes a color change between the colorless Cu deoxygenated form and the blue Cu oxygenated form...

, but a pulmonate family Planorbidae
Planorbidae
Planorbidae, common name the ramshorn snails or ram's horn snails, is a family of air-breathing freshwater snails, aquatic pulmonate gastropod mollusks....

 have hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

 as 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, nudibranch
Nudibranch
A nudibranch is a member of what is now a taxonomic clade, and what was previously a suborder, of soft-bodied, marine gastropod mollusks which shed their shell after their larval stage. They are noted for their often extraordinary colors and striking forms...

s. Some nudibranchs have smooth or warty backs and have no visible gill mechanism, such that respiration may likely take place directly through the skin.

Circulatory system



Gastropods have open circulatory system and the transport fluid is hemolymph
Hemolymph
Hemolymph, or haemolymph, is a fluid in the circulatory system of some arthropods and is analogous to the fluids and cells making up both blood and interstitial fluid in vertebrates such as birds and mammals...

. Hemocyanin
Hemocyanin
Hemocyanins are respiratory proteins in the form of metalloproteins containing two copper atoms that reversibly bind a single oxygen molecule . Oxygenation causes a color change between the colorless Cu deoxygenated form and the blue Cu oxygenated form...

 is present in the hemolymph
Hemolymph
Hemolymph, or haemolymph, is a fluid in the circulatory system of some arthropods and is analogous to the fluids and cells making up both blood and interstitial fluid in vertebrates such as birds and mammals...

 as the respiratory pigment.

Excretory system



The primary organs of excretion in gastropods are nephridia
Nephridium
A Nephridium is an invertebrate organ which occurs in pairs and function similar to kidneys. Nephridia remove metabolic wastes from an animal's body. They are present in many different invertebrate lines. There are two basic types, metanephridia and protonephridia, but there are other...

, which produce either ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 or uric acid
Uric acid
Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates such as ammonium acid urate. Uric acid is created when the body breaks down purine nucleotides. High blood concentrations of uric acid...

 as a waste product. The nephridium also plays an important role in maintaining water balance in freshwater and terrestrial species. Additional organs of excretion, at least in some species, include pericardial glands in the body cavity, and digestive glands opening into the stomach.

Reproductive system



Courtship is a part of mating behavior in some gastropods including some of the Helicidae
Helicidae
The Helicidae, sometimes known as the typical snails, are a taxonomic family of small to large, air-breathing, land snails. In other words, they are terrestrial pulmonate gastropod mollusks....

. Again, in some land snails, an unusual feature of the reproductive system of gastropods
Reproductive system of gastropods
The reproductive system of gastropods varies greatly from one group to another within this very large and diverse taxonomic class of animals...

 is the presence and utilization of love dart
Love dart
A love dart is a hard, long, sharp, calcareous or chitinous dart which some hermaphroditic land snails and slugs create. Love darts are made in sexually mature animals only, and are used as part of the sequence of events during courtship, before actual mating takes place...

s.

In many marine gastropods other than the opisthobranchs, there are separate sexes; most land gastropods however are hermaphrodite
Hermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...

s.

Life cycle




The main aspects of the life cycle of gastropods include:
  • Egg laying and the eggs of gastropods
  • The Embryonic development of gastropods
  • The larvae or larval stadium: some gastropods may be trochophore
    Trochophore
    A trochophore is a type of free-swimming planktonic marine larva with several bands of cilia.By moving their cilia rapidly, a water eddy is created. In this way they control the direction of their movement...

     and/or veliger
    Veliger
    A veliger is the planktonic larva of many kinds of marine and freshwater gastropod molluscs, as well as most bivalve mollusks.- Description :...

  • Estivation
    Estivation
    Aestivation is a state of animal dormancy, characterized by inactivity and a lowered metabolic rate, that is entered in response to high temperatures and arid conditions...

     and hibernation
    Hibernation
    Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...

     (each of these are present in some gastropods only)
  • The growth of gastropods
  • Courtship of gastropods and mating of gastropods: fertilisation
    Fertilisation
    Fertilisation is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo...

     is internal or external according to the species. External fertilisation is common in marine gastropods.

Feeding behavior


Marine gastropods include some that are herbivore
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...

s, detritus
Detritus
Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...

 feeders, predatory carnivore
Carnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...

s, scavenger
Scavenger
Scavenging is both a carnivorous and herbivorous feeding behavior in which individual scavengers search out dead animal and dead plant biomass on which to feed. The eating of carrion from the same species is referred to as cannibalism. Scavengers play an important role in the ecosystem by...

s, parasites, and also a few ciliary feeders, in which the radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...

 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 Parenteroxenos doglieli, many of the standard gastropod features are strongly reduced or absent.

A few sea slugs are herbivores and some are carnivores. Many have distinct dietary preferences and regularly occur in close association with their food species.

Some predatory carnivorous gastropods include, for example: Cone shells, Testacella
Testacella
Testacella is genus of small to medium-large, predatory, air-breathing, land slugs. They are terrestrial gastropod mollusks in the family Testacellidae, the shelled slugs. They are not often seen because they live underground....

, Daudebardia
Daudebardia
Daudebardia of small air-breathing land snails or semi-slugs, terrestrial pulmonate gastropods in the family Oxychilidae, the glass snails.-Species:Species within the genus Daudebardia include:** Daudebardia brevipes...

, Ghost slug
Ghost slug
The ghost slug, Selenochlamys ysbryda, is a species of predatory air-breathing land slug. It is a shell-less pulmonate gastropod mollusc in the family Trigonochlamydidae....

 and others.

Genetics


Gastropods exhibit an important degree of variation in mitochondrial gene organization when compared to other animals. Main events of gene rearrangement
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

 occurred at the origin of Patellogastropoda
Patellogastropoda
Patellogastropoda, common name true limpets and historically called Docoglossa, is a major phylogenetic group of marine gastropods, seen either as a clade or as a taxonomic order.-Taxonomy:Patellogastropoda was proposed by David R...

 and Heterobranchia
Heterobranchia
Heterobranchia, the heterobranchs or Euthyneura, is a taxonomic clade of snails and slugs, which includes species from the sea, the land and freshwater; marine, aquatic and terrestrial gastropod mollusks....

, whereas fewer changes occurred between the ancestors of Vetigastropoda
Vetigastropoda
Vetigastropoda is a major taxonomic group of sea snails, marine gastropod mollusks that form a very ancient lineage. Taxonomically Vetigastropoda are sometimes treated as an order although they are a clade in Bouchet and Rocroi, 2005....

 (only tRNAs D, C and N) and Caenogastropoda
Caenogastropoda
Caenogastropoda is a taxonomic clade of a large diverse group of mostly marine gastropods.Caenogastropoda contains a majority of the families of shelled marine molluscs including such families as the periwinkles, cowries, wentletraps, moon snails, and cone snails.About 60% of all living gastropods...

 (a large single inversion
Chromosomal inversion
An inversion is a chromosome rearrangement in which a segment of a chromosome is reversed end to end. An inversion occurs when a single chromosome undergoes breakage and rearrangement within itself. Inversions are of two types: paracentric and pericentric.Paracentric inversions do not include the...

, and translocation
Chromosomal translocation
In genetics, a chromosome translocation is a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes. A gene fusion may be created when the translocation joins two otherwise separated genes, the occurrence of which is common in cancer. It is detected on...

s of the tRNAs D and N). Within Heterobranchia, gene order seems to be relatively conserved and gene rearrangements are mostly related with transposition
Transposon
Transposable elements are sequences of DNA that can move or transpose themselves to new positions within the genome of a single cell. The mechanism of transposition can be either "copy and paste" or "cut and paste". Transposition can create phenotypically significant mutations and alter the cell's...

 of tRNA genes.

Geological history




The first gastropods were exclusively marine, with the earliest representatives of the group appearing in the Late Cambrian (Chippewaella
Chippewaella
Chippewaella is a genus of very primitive snail-like mollusc from the Late Cambrian. According to Wagner it is the most basal gastropod.-External links:* ----- [no date]....

, Strepsodiscus
Strepsodiscus
†Strepsodiscus is an extinct genus of very primitive snail-like mollusc from the early part of the Late Cambrian of North America...

). Early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 forms like Helcionella
Helcionella
Helcionella is the archetypical genus of helcionellid. Its shells are around about a centimetre in size; they are strongly endogastric, with the tip of the shell extending beyond the rear extremity of the shell's aperture....

and Scenella
Scenella
Scenella is an extinct genus generally classified as a mollusc; it has been aligned with the gastropods, monoplacophorans, and helcionellids, although a firm association with either class has not been established. An affinity with the hydrozoa has been considered, although some authors oppose...

are no longer considered gastropods, and the tiny coiled Aldanella of earliest Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 time is probably not even a mollusk. By the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 period the gastropods were a varied group present in a range of aquatic habitats. Commonly, fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 gastropods from the rocks of the early Palaeozoic era are too poorly preserved for accurate identification. Still, the Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 genus Poleumita
Poleumita
Poleumita is an extinct genus of medium-sized sea snails, fossil marine gastropods in the family Euomphalidae. They are known from the Silurian period.- External links :* Paleobiology database * Photo of one species and more info...

contains fiftean identified species. Fossil gastropods were less common during the Palaeozoic era than bivalves.

Most of the gastropods of the Palaeozoic era belong to primitive groups, a few of which still survive today. By the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 period many of the shapes we see 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 Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

 era that the ancestors of many of the living gastropods evolved.

One of the earliest known terrestrial (land-dwelling) gastropods is Maturipupa, which is found in the Coal Measures of the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 period in Europe, but relatives of the modern land snails are rare before the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 period, when the familiar Helix first appeared.
In rocks of the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

 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 "Purbeck Marble
Purbeck Marble
Purbeck Marble is a fossiliferous limestone quarried in the Isle of Purbeck, a peninsula in south-east Dorset, England.It is one of many kinds of Purbeck Limestone, deposited in the late Jurassic or early Cretaceous periods....

" of the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 period and the "Sussex Marble
Sussex Marble
West Sussex has a good concentration of relatively thin layers of Sussex Marble within the Weald Clay, a freshwater limestone referred to as "marble" as it takes a polish. It is not a geologically described one as it has not been subject to metamorphosis...

" of the early Cretaceous period, which both occur in southern England, are limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

s containing the tightly packed remains of the pond snail Viviparus
Viviparus
Viviparus, common name river snails, is a genus of large, freshwater snails with an operculum, aquatic gastropod mollusks. They are primitive members of the superorder Caenogastropoda.-Distribution:This genus is palaearctic in distribution....

.

Rocks of the Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

 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 bivalves.

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 trails are of debatable origin, some of them do resemble the trails made by living gastropods today.

Gastropod fossils may sometimes be confused with ammonite
Ammonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...

s or other shelled cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

s. An example of this is Bellerophon from the limestones of the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 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 Ice Sheets during the Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

 epoch.

Taxonomy


Since Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

, biological taxonomy has tried to reflect the presumed phylogeny of organisms, i.e. the tree of life
Tree of life
The concept of a tree of life, a many-branched tree illustrating the idea that all life on earth is related, has been used in science , religion, philosophy, mythology, and other areas...

. The classifications used in taxonomy attempt to represent the precise interrelatedness of the various species. The taxonomy of the Gastropoda as shown in various texts can differ in major ways.

In the older classification of the gastropods, there were four subclasses:
  • Opisthobranchia
    Opisthobranchia
    Opisthobranchs are a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia, but no longer considered to represent a monophyletic grouping...

     (gills to the right and behind the heart).
  • Gymnomorpha (no shell)
  • Prosobranchia
    Prosobranchia
    Not to be confused with the bivalve order Protobranchia.Prosobranchia was a large taxonomic subclass of sea snails, land snails and freshwater snails. This taxon of gastropods dates back to the 1920s...

     (gills in front of the heart).
  • Pulmonata
    Pulmonata
    The Pulmonata, or "pulmonates", are an informal group of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills...

     (with a lung instead of gills)

The taxonomy
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

 of the Gastropoda is under constant revision, and more and more of the old taxonomy is being abandoned, as the results of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 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 monophyletic groups (only one lineage of gastropods in each group). Integrating new findings into a working taxonomy
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

 will continue to be a challenge in coming years. 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 to be expected in the near future.

Convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

, which appears to exist at especially high frequency in the class Gastropoda class, may account for the observed differences between the older phylogenies which were based on morphological data, and more recent gene-sequencing studies.
Bouchet & Rocroi (2005) made sweeping changes in the systematics, resulting in a taxonomy that is a step closer to the evolutionary history of the phylum.

The Bouchet & Rocroi classification system is based partly on the older systems of classification, and partly on new cladistic research. In the past, the taxonomy of gastropods was largely based on phenetic morphological characters of the taxa. The recent advances are more based on molecular characters from DNA and RNA 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 clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

s for taxa above the rank of superfamily (replacing the ranks suborder, order, superorder and subclass), while using the traditional Linnaean approach for all taxa below the rank of superfamily. Whenever monophyly 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 David R. Lindberg
David R. Lindberg
David R. Lindberg is an American malacologist and professor of integrative biology at the University of California, Berkeley. He is also the Curator for the University of California Museum of Paleontology and co-editor of the journal Molecular Systematics and Phylogeography of Mollusks.Much of his...

 showed possible diphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...

 origins of the Gastropoda based on mitochondrial gene order and amino acid sequence analyses of complete genes.

External links