Asiphon is an anatomical structure which is part of the body of aquaticmolluscs in threeclasses:Gastropoda,Bivalvia andCephalopoda (members of these classes include saltwater andfreshwater snails,clams,octopus,squid 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 aslocomotion,feeding,respiration, andreproduction. The siphon is part of themantle of the mollusc, and the water flow is directed to (or from) themantle cavity.
A single siphon occurs in some gastropods. In those bivalves which have siphons, the siphons are paired. Incephalopods, there is a single siphon or funnel which is known as ahyponome.
In some (but not all)sea snails,marine gastropod molluscs, the animal has an anterior extension of themantle called a siphon, or inhalant siphon, through which water is drawn into the mantle cavity and over the gill for respiration.[1]
This siphon is a soft fleshy tube-like structure equipped withchemoreceptors which "smell" or "taste" the water, in order to hunt for food.[2][3][4] Marine gastropods that have a siphon are eitherpredators orscavengers.[5]
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.[1]
In many marine gastropods where the siphon is particularly long, the structure of theshell has been modified in order to house and protect the soft tissue of the siphon. This shell modification is known as thesiphonal canal. For a gastropod whoseshell has an exceptionally long siphonal canal, seeVenus comb murex.
In the case of some other marine gastropod shells, such as the volute and theNassarius pictured to the right, the shell has a simple "siphonal notch" at the anterior edge of theaperture instead of a long siphonal canal.
TheAplysia gill and siphon withdrawal reflex is a defensivereflex which is found in sea hares of the genusAplysia; this reflex has been much studied inneuroscience.
Freshwaterapple snails in the generaPomacea andPila 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.[6]
Apple snails use the siphon in a way that is reminiscent of a human swimmer using asnorkel, except that the apple snail's siphon can be retracted completely, or extended to various lengths as needed.[6]
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.[6]
The shells of these freshwater snails have simple roundapertures; there is no special notch for the siphon.
Those bivalves that have siphons, have two of them. Not all bivalves have siphons however: those that live on or above thesubstrate, as is the case inscallops,oysters, etc., do not need them. Only those bivalves that burrow insediment, 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.[7][8]
The deeper a bivalve species lives in the sediment, the longer its siphons are. Bivalves which have extremely long siphons, like thegeoducks pictured here, live very deeply buried, and are hard to dig up whenclamming.[9]
Many bivalves that have siphons can withdraw them completely into the shell when needed, but this is not true of all species. Bivalves thatcan 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.[10]
The bivalve's two siphons are situated at the posterior edge of themantle cavity.[11] There is an inhalant or incurrent siphon, and an exhalant or excurrent siphon.[12] The water is circulated by the action of thegills. 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 forfilter feeding,excretion, and reproduction.
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 marinesubstrate. 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.[13]
Please also seepseudofeces.
Thehyponome or siphon is the organ used bycephalopods to expel water, a function that produces alocomotive force. The hyponome developed from the foot of the molluscan ancestor.[14]
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 asoctopus,squid, andcuttlefish, the hyponome is a muscular tube. The hyponome of thenautilus differs however, in that it is a one-piece flap that is folded over. Whetherammonites possessed a hyponome and if so what form it may have taken, is as yet not known.[15]
Cephalopod anatomy | ||||||||||||||
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| Shell |
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| Mantle & funnel |
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| Head & limbs |
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| General | ||||||||||||||
Developmental stages:Spawn →Paralarva (Doratopsis stage) → Juvenile → Subadult → Adult •Egg fossils •Protoconch (embryonic shell) | ||||||||||||||