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A siphon is an anatomical structure which is part of the body of aquatic
mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is est ...
s in three
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...
es:
Gastropoda The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. The ...
,
Bivalvia Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
and
Cephalopoda A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
(members of these classes include saltwater and
freshwater snail Freshwater snails are gastropod mollusks which live in fresh water. There are many different families. They are found throughout the world in various habitats, ranging from ephemeral pools to the largest lakes, and from small seeps and springs ...
s,
clams Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two she ...
,
octopus An octopus ( : octopuses or octopodes, see below for variants) is a soft-bodied, eight- limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, ...
,
squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fittin ...
and relatives). Siphons in molluscs are tube-like structures in which water (or, more rarely, air) flows. The water flow is used for one or more purposes such as locomotion, feeding, respiration, and
reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...
. The siphon is part of the
mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...
of the mollusc, and the water flow is directed to (or from) the mantle cavity. A single siphon occurs in some gastropods. In those bivalves which have siphons, the siphons are paired. In
cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, a ...
, there is a single siphon or funnel which is known as a
hyponome A siphon is an anatomical structure which is part of the body of aquatic molluscs in three Class (biology), classes: Gastropoda, Bivalvia and Cephalopoda (members of these classes include saltwater and freshwater snails, clams, octopus, squid and ...
.


In gastropods

In some (but not all)
sea snail Sea snail is a common name for slow-moving marine gastropod molluscs, usually with visible external shells, such as whelk or abalone. They share the taxonomic class Gastropoda with slugs, which are distinguished from snails primarily by the a ...
s,
marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...
gastropod molluscs, the animal has an anterior extension of the
mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...
called a siphon, or inhalant siphon, through which water is drawn into the mantle cavity and over the gill for respiration.Örstan A. 13 April 2007
''Melongena's siphon''
Snail's Tales.
This siphon is a soft fleshy tube-like structure equipped with
chemoreceptors A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorecept ...
which "smell" or "taste" the water, in order to hunt for food. Marine gastropods that have a siphon are either
predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill t ...
s or scavengers. Although in gastropods the siphon functions perfectly well as a tube, it is not in fact a hollow organ, it is simply a flap of the mantle that is rolled into the shape of a tube. In many marine gastropods where the siphon is particularly long, the structure of the shell has been modified in order to house and protect the soft tissue of the siphon. This shell modification is known as the
siphonal canal The siphonal canal is an anatomical feature of the shells of certain groups of sea snails within the clade Neogastropoda. Some sea marine gastropods have a soft tubular anterior extension of the mantle called a siphon through which water is d ...
. For a gastropod whose shell has an exceptionally long siphonal canal, see Venus comb murex. In the case of some other marine gastropod shells, such as the volute and the ''Nassarius'' pictured to the right, the shell has a simple "siphonal notch" at the anterior edge of the
aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An ...
instead of a long siphonal canal. The ''Aplysia'' gill and siphon withdrawal reflex is a defensive
reflex In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...
which is found in sea hares of the genus '' Aplysia''; this reflex has been much studied in
neuroscience Neuroscience is the science, scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions and disorders. It is a Multidisciplinary approach, multidisciplinary science that combines physiology, an ...
.


Siphon as a snorkel

Freshwater
apple snail Ampullariidae, commonly known as the apple snails, is a family of large freshwater snails, aquatic gastropod mollusks with a gill and an operculum. These snails simultaneously have a gill and a lung as functional respiratory structures, which ...
s in the genera ''
Pomacea ''Pomacea'' is a genus of freshwater snails with gills and an operculum, aquatic gastropod mollusks in the family Ampullariidae, the apple snails. The genus is native to the Americas; most species in this genus are restricted to South America. ...
'' and '' Pila'' have an extensible siphon made from a flap of the left mantle cavity. They use this siphon in order to breathe air while they are submerged in water which has a low oxygen content so they cannot effectively use their gill. Apple snails use the siphon in a way that is reminiscent of a human swimmer using a snorkel, except that the apple snail's siphon can be retracted completely, or extended to various lengths as needed.''Respiratory system''
The apple snail website, http://www.applesnail.net, accessed 26 February 2009.
For these freshwater snails, the siphon is an anti-predator adaptation. It reduces their vulnerability to being attacked and eaten by birds because it enables the apple snails to breathe without having to come all the way up to the surface, where they are easily visible to predators. The shells of these freshwater snails have simple round apertures; there is no special notch for the siphon.


Paired siphons of bivalves

Those bivalves that have siphons, have two of them. Not all bivalves have siphons however: those that live on or above the substrate, as is the case in
scallop Scallop () is a common name that encompasses various species of marine bivalve mollusks in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related families ...
s,
oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not ...
s, etc., do not need them. Only those bivalves that burrow in
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...
, and live buried in the sediment, need to use these tube-like structures. The function of these siphons is to reach up to the surface of the sediment, so that the animal is able to respire, feed, and excrete, and also to reproduce. The deeper a bivalve species lives in the sediment, the longer its siphons are. Bivalves which have extremely long siphons, like the geoducks pictured here, live very deeply buried, and are hard to dig up when clamming. Many bivalves that have siphons can withdraw them completely into the shell when needed, but this is not true of all species. Bivalves that ''can'' withdraw the siphons into the shell have a "pallial sinus", a sort of pocket, into which the siphons can fit when they are withdrawn, so that the two shell valves can close properly. The existence of this pocket shows even in an empty shell, as a visible indentation in the pallial line, a line which runs along parallel to the ventral margin of the shell. The bivalve's two siphons are situated at the posterior edge of the mantle cavity. There is an inhalant or incurrent siphon, and an exhalant or excurrent siphon. The water is circulated by the action of the
gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
s. Usually water enters the mantle cavity through the inhalant siphon, moves over the gills, and leaves through the exhalant siphon. The water current is utilized for respiration, but also for filter feeding,
excretion Excretion is a process in which metabolic waste is eliminated from an organism. In vertebrates this is primarily carried out by the lungs, kidneys, and skin. This is in contrast with secretion, where the substance may have specific tasks after ...
, and reproduction.


Feeding

Depending on the species and family concerned, some bivalves utilize their inhalant siphon like the hose of a vacuum cleaner, and actively suck up food particles from the marine substrate. Most other bivalves ingest microscopic phytoplankton as food from the general water supply, which enters via the inhalant siphon and reaches the mouth after passing over the gill. Please also see pseudofeces.


Hyponome of cephalopods

The hyponome or siphon is the organ used by
cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
s to expel water, a function that produces a locomotive force. The hyponome developed from the foot of the molluscan ancestor.''Class Cephalopoda: the Head-Feet''
Accessed 21 November 2008.
Water enters the mantle cavity around the sides of the funnel, and subsequent contraction of the hyponome expands and then contracts, expelling a jet of water. In most cephalopods, such as
octopus An octopus ( : octopuses or octopodes, see below for variants) is a soft-bodied, eight- limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, ...
,
squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fittin ...
, and
cuttlefish Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of ...
, the hyponome is a muscular tube. The hyponome of the
nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in ...
differs however, in that it is a one-piece flap that is folded over. Whether
ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefis ...
s possessed a hyponome and if so what form it may have taken, is as yet not known.''Discussion''
http://palaeo-electronica.org/ Accessed 21 November 2008.


References


External links







{{Use dmy dates, date=May 2017 Bivalve anatomy Cephalopod zootomy Gastropod anatomy Invertebrate respiratory system Articles containing video clips