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{{Taxobox
| name = Molluscs
| fossil_range = [[Ediacaran]] or [[Cambrian]] – Recent
| image = Caribbean reef squid.jpg
| image_width = 200px
| image_caption = [[Caribbean Reef Squid]], ''Sepioteuthis sepioidea''
| regnum = [[Animal]]ia
| superphylum = [[Lophotrochozoa]]
| phylum = '''Mollusca'''
| phylum_authority = [[Carolus Linnaeus|Linnaeus]], 1758
| subdivision_ranks = [[Class (biology)|Classes]]
| subdivision =
[[Aplacophora]]<br />
[[Bivalvia]]<br />
[[Caudofoveata]]<br />
[[Cephalopoda]]<br />
[[Gastropoda]]<br />
† [[Helcionelloida]]<br />
[[Monoplacophora]]<br />
[[Polyplacophora]]<br />
† [[Rostroconchia]]<br />
[[Scaphopoda]]<br />
| diversity = about 93,000 living species<ref name="Haszprunar2001MolluscsInEncOfLifeSci" />
| diversity_link = Mollusca
}}

'''Molluscs'''<ref>Commonly spelt '''mollusk''' in the USA; however the spelling "mollusc" is preferred for the reasons given by {{bruscabrusca}}.</ref> are animals belonging to the [[Phylum (biology)|phylum]] '''Mollusca'''. There are around 93,000 recognized [[extant taxon|extant]] [[species]] within the phylum. Molluscs are a highly diverse group, with representatives of the phylum living across a wide range of environments including marine, freshwater and a variety of terrestrial habitats.

The phylum Mollusca is typically divided into nine or ten [[Taxonomy|taxonomic]] [[class (biology)|classes]], of which two are entirely [[extinct]]. [[Cephalopod]] molluscs such as [[squid]], [[cuttlefish]] and [[octopus]] are among the most [[neurobiology|neurologically-advanced]] of all invertebrates – and either the [[giant squid]] or the [[colossal squid]] is the largest known [[invertebrate]] species. The [[gastropod]]s ([[snail]]s and [[slug]]s) are by far the most numerous molluscs in terms of classified species, and account for 80% of the total.

Molluscs have such a varied range of body structures that it is difficult to find defining characteristics that apply to all modern groups. The two most universal features are a [[mantle]] with a significant cavity used for breathing and [[excretion]], and the structure of the nervous system. As a result of this wide diversity, many textbooks base their descriptions on a hypothetical "generalized mollusc". This has a single, "[[limpet]]-like" [[gastropod shell|shell]] on top, which is made of [[protein]]s and [[chitin]] reinforced with [[calcium carbonate]], and is secreted by a mantle that covers the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are [[coelomate]]s, the [[coelom]] is very small, and the main body cavity is a [[hemocoel]] through which [[blood]] circulates – molluscs' circulatory systems are mainly [[Circulatory system#Open circulatory system|open]]. The "generalized" mollusc feeding system consists of a rasping "tongue" called a [[radula]] and a complex digestive system in which exuded [[mucus]] and microscopic, muscle-powered "hairs" called [[cilia]] play various important roles.

The "generalized mollusc" has two paired [[nerve cord]]s, or three in [[bivalve]]s. The brain, in species that have one, encircles the [[esophagus]]. Most molluscs have eyes, and all have sensors that detect chemicals, vibrations and touch. The simplest type of molluscan reproductive system relies on [[external fertilization]], but there are more complex variations. All produce eggs, from which may emerge [[trochophore]] larvae, more complex [[veliger]] larvae, or miniature adults.

A striking feature of molluscs is the use of the same organ for multiple functions. For example: the heart and [[nephridia]] ("kidneys") are important parts of the reproductive system as well as the circulatory and excretory systems; in bivalves, the [[gill]]s both "breathe" and produce a water current in the mantle cavity which is important for excretion and reproduction.

There is good evidence for the appearance of gastropods, cephalopods and [[bivalve]]s in the [[Cambrian]] period {{ma|Cambrian|{{period end|Cambrian}}}}. However the evolutionary history both of molluscs' emergence from the ancestral [[Lophotrochozoa]] and of their diversification into the well-known living and [[fossil]] forms are still subjects of vigorous debate among scientists.

Molluscs have been and still are an important food source for [[anatomically modern human]]s. However there is a risk of food-poisoning from toxins that accumulate in molluscs under certain conditions, and many countries have regulations that aim to minimize this risk. Molluscs have for centuries also been the source of important luxury goods, notably [[pearl]]s, [[mother of pearl]], [[Tyrian purple]] dye, and [[sea silk]]. Their shells have also been used as a [[money]] in some pre-industrial societies, although shell "currencies" have severe limitations compared with government-backed money.

Mollusc species can also represent hazards or pests for human activities. The bite of the [[blue-ringed octopus]] is often fatal, and that of ''[[Octopus apollyon]]'' causes [[inflammation]] that can last for over a month. Stings from a few species of large tropical [[cone shell]]s can also kill, but their sophisticated though easily-produced venoms have become important tools in [[neurology|neurological]] research. [[Schistosomiasis]] (also known as bilharzia, bilharziosis or snail fever) is transmitted to humans via water snail hosts, and affects about 200&nbsp;million people. Snails and slugs can also be serious agricultural pests, and accidental or deliberate introduction of some snail species into new environments has seriously damaged some [[ecosystems]].

==Diversity==
[[Image:Cypraea chinensis with partially extended mantle.jpg| thumb | right | 200px |About 80% of all known mollusc species are [[gastropod]]s.<ref name="PonderWinstonLindberg" />]]
<!-- **************
N.B. in case some reviewer grumbles about lack of page number for Ponder & Lindberg,
could go with 100,000 living mollusc species.<ref name="RuppertFoxBarnes2004FrontEnd1">
************* -->The molluscs, with 250,000 species, are the second only to [[arthropod]]s in numbers of living animal species<ref name="PonderWinstonLindberg">{{ citation | editor=Ponder, W.F. and Lindberg, D.R.
| date=2008 | title=Phylogeny and Evolution of the Mollusca
| publisher=Berkeley: University of California Press | pages=481 | isbn=978-0520250925
}}</ref> – far behind the arthropods' 1,113,000 but well ahead of [[chordate]]s' 52,000.<ref name="RuppertFoxBarnes2004FrontEnd1">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | page=Front endpaper 1
}}</ref> There have also been an estimated 70,000 [[extinct]] species.<ref name="BruscaBrusca2003P702" /> Molluscs have more varied forms than any other animal [[phylum]] – [[snail]]s and other [[gastropod]]s, [[clam]]s and other [[bivalve]]s, [[squid]]s and other [[cephalopod]]s, and other less well-known but similarly distinctive sub-groups. The majority of species still live in the oceans, from the seashores to the [[abyssal zone]], but are also significant members of freshwater and terrestrial [[ecosystem]]s. They are extremely diverse in [[Tropics|tropical]] and [[temperate]] regions but can be found at all [[latitude]]s.<ref name="GiribetOkusuEtAlMolluscsWithSeriallyRepeatedStructures" /> About 80% of all known mollusc species are gastropods.<ref name="PonderWinstonLindberg" /> [[Cephalopod]]a such as [[squid]], [[cuttlefish]] and [[octopus]] are among the most [[neurobiology|neurologically-advanced]] of all invertebrates.<ref name="BarnesCalowEtAl2001InvertebratesSynthesis">{{ cite book
| author=Barnes, R.S.K., Calow, P., Olive, P.J.W., Golding, D.W. and Spicer, J.I. | date=2001
| title=The Invertebrates, A Synthesis | edition=3| publisher=Blackwell Science | location =UK
}}</ref> The [[giant squid]], which until recently had not been observed alive in its adult form,<ref>{{cite journal
| author=Kubodera, T. and Mori, K. date=2005
| title=First-ever observations of a live giant squid in the wild
| journal=Proceedings of the Royal Society B: Biological Sciences | volume=272| issue=1581
| pages=2583-2586 | doi=10.1098/rspb.2005.3158
| url=http://www.canarias7.es/pdf/docs/informecalamargigante.pdf | accessdate=2008-10-22
}}</ref> is one of the largest [[invertebrate]]s. However a recently-caught specimen of the [[colossal squid]], {{convert|10|m|ft}} long and weighing {{convert|500|kg|ton}}, may have overtaken it.<ref>{{cite web
| author=Richard Black | title=Colossal squid out of the freezer
| publisher=BBC News | date=April 26, 2008
| url=http://news.bbc.co.uk/1/hi/sci/tech/7367774.stm | accessdate=2008-10-01
}}</ref>

==General description==
===Definition===
The word ''[[wikt:mollusc|mollusc]]'' is derived from the [[French language|French]] ''mollusque'', which originated from the [[Latin]] ''molluscus'', from ''[[wikt:mollis|mollis]]'', soft. ''Molluscus'' was itself an adaptation of [[Aristotle]]'s τᾲ μαλάκια, "the soft things", which he applied to [[cuttlefish]].<ref>{{citation
| contribution=Mollusca | title=Shorter Oxford English Dictionary
| editor=Little, L., Fowler, H.W., Coulson, J., and Onions, C.T.
| publisher=Oxford University press | date=1964
}}</ref> The [[science|scientific]] study of molluscs is known as '''[[malacology]]'''.<ref>{{citation
| contribution=Malacology | title=Shorter Oxford English Dictionary
| editor=Little, L., Fowler, H.W., Coulson, J., and Onions, C.T.
| publisher=Oxford University press | date=1964
}}</ref>

Molluscs have developed such a varied range of body structures that it is difficult to find [[synapomorphy| synapomorphies]] (defining characteristics) that apply to all modern groups.<ref name="GiribetOkusuEtAlMolluscsWithSeriallyRepeatedStructures">{{cite journal
| author=Giribet, G., Okusu, A., Lindgren, A.R., Huff, S.W., Schrödl, M., and Nishiguchi, M.K.
| title=Evidence for a clade composed of molluscs with serially repeated structures: Monoplacophorans are related to chitons
| journal=Proceedings of the National Academy of Sciences of the United States of America
| date=May 2006 | volume=103 | issue=20 | pages=7723-7728 | doi=10.1073/pnas.0602578103 |url=http://www.pnas.org/content/103/20/7723.full | accessdate=2008-09-30
}}</ref> The following are present in all modern molluscs:<ref name="BruscaBrusca2003P702">{{cite book
| author=Brusca, R.C., and Brusca, G.J. | title=Invertebrates | edition=2 | date=2003 | page=702
| publisher=Sinauer Associates | isbn=0878930973
}}</ref><ref name="RuppertFoxBarnes2004MolluscaGen">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=284-291
}}</ref>
*The [[Anatomical_terms_of_location#Dorsal_and_ventral | dorsal]] part of the body wall is a [[mantle]] which [[secrete]]s [[calcareous]] [[spicule]]s, plates or shells. It overlaps the body with enough spare room to form a [[mantle cavity]].
*The [[anus]] and [[genital]]s open into the mantle cavity.
*There are at least two pairs of main [[nerve cord]]s (three in [[bivalve]]s<ref name="RuppertFoxBarnes2004MolluscaBivalvia">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=367-403
}}</ref>)

Other characteristics that commonly appear in textbooks have significant exceptions:
{| class="wikitable"
|-
| &nbsp; || colspan="7" align="center" | Class
|-
!Characteristic<ref name="BruscaBrusca2003P702"/>
![[Aplacophora]]<ref name="RuppertFoxBarnes2004MolluscaAplacophora">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=291-292
}}</ref>
![[Polyplacophora]]<ref name="RuppertFoxBarnes2004MolluscaPolyplacophora">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=292-298
}}</ref>
![[Monoplacophora]]<ref name="RuppertFoxBarnes2004MolluscaMonoplacophora">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=298-300
}}</ref>
![[Gastropoda]]<ref name="RuppertFoxBarnes2004MolluscaGastropoda">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=300-343
}}</ref>
![[Cephalopoda]]<ref name="RuppertFoxBarnes2004MolluscaCephalopoda">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=343-367
}}</ref>
![[Bivalvia]]<ref name="RuppertFoxBarnes2004MolluscaBivalvia" />
![[Scaphopoda]]<ref name="RuppertFoxBarnes2004MolluscaScaphopoda">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=403-407
}}</ref>
|-
! style="text-align:left" | [[Radula]], a rasping "tongue" with [[chitin]]ous teeth
| Absent in 20% of [[Neomeniomorpha]] || Yes || Yes || Yes || Yes || No || Internal, cannot extend beyond body
|-
! style="text-align:left" | Broad, muscular foot
| Reduced or absent || Yes || Yes || Yes || Modified into arms || Yes || Small, only at "front" end
|-
! style="text-align:left" | Dorsal concentration of internal organs (visceral mass)
| Not obvious || Yes || Yes || Yes || Yes || Yes || Yes
|-
! style="text-align:left" | Large digestive [[cecum| ceca]]
| No ceca in some aplacophora || Yes || Yes || Yes || Yes || Yes || No
|-
! style="text-align:left" | Large complex [[metanephridia]] ("kidneys")
| None || Yes || Yes || Yes || Yes || Yes || Small, simple
|}

===A "generalized mollusc"===
{{see|Mollusc shell}}<!-- Keep this one, the other onewill go -->
{{Annotated image/Mollusc generalized}}
Because of the enormous variations between groups of molluscs, many text books start the subject by describing a "generalized mollusc", which some suggest ''may'' resemble very early molluscs and which is rather similar to modern [[Monoplacophora| monoplacophorans]].<ref name="RuppertFoxBarnes2004MolluscaGen" /><ref name="RuppertFoxBarnes2004MolluscaMonoplacophora"/><ref name="Healy2001MolluscaInvertebrateZoology">{{citation
| author=Healy, J.M. | chapter=The Mollusca | pages=120-171
| title=Invertebrate Zoology | date=2001 | edition=2 | editor=Anderson, D.T.
| publisher=Oxford University Press | isbn=0195513681
}}</ref><ref name="GiribetOkusuEtAlMolluscsWithSeriallyRepeatedStructures" />

The mantle secretes a shell that is mainly [[chitin]] and [[conchiolin]] (a [[protein]]) hardened with [[calcium carbonate]],<ref name="RuppertFoxBarnes2004MolluscaGen" /><ref name="Porter2007">{{cite journal
| author = Porter, S. | year = 2007
| title = Seawater Chemistry and Early Carbonate Biomineralization
| journal = Science | volume = 316 | issue = 5829 | pages = 1302 | doi = 10.1126/science.1137284
| url = http://www.sciencemag.org/cgi/content/abstract/sci;316/5829/1302 | accessdate=2008-09-30
}}</ref> except that the outermost layer is all conchiolin.<ref name="RuppertFoxBarnes2004MolluscaGen" /> The mantle cavity is a fold in the mantle that encloses a significant amount of space, and was probably at the rear in the earliest molluscs but its position now varies from group to group. In the "generalized mollusc" the [[anus]], a pair of [[osphradium| osphradia]] (chemical sensors), the hindmost pair of [[gill]]s and the exit openings of the [[nephridia]] ("kidneys") and [[gonad]]s (reproductive organs) are in the mantle cavity.<ref name="RuppertFoxBarnes2004MolluscaGen" />

The underside of the body generally consists of a muscular foot, which secretes [[mucus]] as a lubricant to aid movement. The foot is attached to the underside of the shell by strong vertical muscles, which are an important defensive system: in forms that have only a top shell, such as [[limpet]]s, the foot acts a sucker attaching to the animal to a hard surface, and the vertical muscles clamp the shell down over it; in other molluscs, the vertical muscles pull the foot and other exposed soft parts into the shell.<ref name="RuppertFoxBarnes2004MolluscaGen" />

Although molluscs are [[coelomate]]s, their [[coelom]]s are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity is a [[hemocoel]] through which blood circulates and which encloses most of the other internal organs. The blood contains the [[respiratory pigment]] [[hemocyanin]] as an [[oxygen]]-carrier. The heart consists of one or more pairs of atria ([[auricle]]s) which receive oxygenated blood from the gills and pump it to the [[ventricle]], which pumps it into the [[aorta]] (main [[artery]]), which is fairly short and opens into the hemocoel.<ref name="RuppertFoxBarnes2004MolluscaGen" />

The atria of the heart also function as part of the [[excretory system]] by filtering waste products out of the blood and dumping it into the coleom as [[urine]]. A pair of [[nephridia]] ("little kidneys") to the rear of and connected to the coelom extracts any re-usable materials from the urine and dumps additional waste products into it, and then ejects it via tubes that discharge into the mantle cavity.<ref name="RuppertFoxBarnes2004MolluscaGen" />

Most molluscs have only one pair of gills, or even only one gill. Generally the gills are rather like feathers in shape, although some species have gills with filaments on only one side. They divide the mantle cavity so that water enters near the bottom and exits near the top. Their filaments have three kinds of cilia, one of which drives the water current through the mantle cavity, while the other two help to keep the gills clean. If the osphradia detect noxious chemicals or possibly [[sediment]] entering the mantle cavity, the gills' cilia may stop beating until the unwelcome intrusions have ceased. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one connected to the heart.<ref name="RuppertFoxBarnes2004MolluscaGen" />

{{Annotated image/Snail radula working}}
Most molluscs have muscular mouths with radulae, "tongues" bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. This is primarily designed to scrape [[bacteria]] and [[algae]] off rocks. Their mouths also contain [[gland]]s that secrete slimy [[mucus]], to which the food sticks. Beating [[cilia]] (tiny "hairs") drive the mucus towards the stomach, so that the mucus forms a long string. At the tapered rear end of the stomach and projecting slightly into the hindgut is the [[prostyle]], a backward-pointing cone of [[feces]] and mucus, which is rotated by further cilia so that it acts as a [[bobbin]], winding the mucus string onto itself. Before the mucus string reaches the prostyle the acidity of the stomach makes the mucus less sticky and frees particles from it. The particles are sorted by yet another group of cilia, which send the smaller particles, mainly minerals, to the prostyle so that eventually they are excreted, while the larger ones, mainly food, are sent to the stomach's [[cecum]] (a pouch with no other exit) to be digested. The sorting process is by no means perfect. Periodically circular muscles at the entrance to the hindgut pinch off a piece of the prostyle so that it is excreted, preventing the prostyle from growing too large. The anus is in the part of the mantle cavity that is swept by the outgoing "lane" of the current created by the gills. Carnivorous molluscs usually have simpler digestive systems.<ref name="RuppertFoxBarnes2004MolluscaGen" />

In general, molluscs have two pairs of main [[nerve cord]]s, the [[viscera]]l cords serving the internal organs and the pedal ones serving the foot. Both pairs run below the level of the gut, and include [[ganglia]] as local control centers in important parts of the body. Most pairs of corresponding ganglia on both sides of the body are linked by [[commissure]]s (relatively large bundles of nerves). The only ganglia above the gut are the cerebral ganglia, which sit above the [[esophagus]] (gullet) and handle "messages" from and to the eyes. The pedal ganglia, which control the foot, are just below the esophagus and their commissure and connections to the cerebral ganglia encircle the esophagus in a nerve ring.<ref name="RuppertFoxBarnes2004MolluscaGen" />

A typical mollusc has: a pair of tentacles on the head, containing chemical and mechanical sensors; a pair of eyes on the head, a pair of [[statocyst]]s in the foot which act as balance sensors; and a pair of osphradia, chemical sensors, in the incoming "lane" of the mantle cavity.<ref name="RuppertFoxBarnes2004MolluscaGen" />

{{Annotated image/Trochophore larva}}
In the simplest molluscan reproductive systems, two [[gonad]]s sit next to the coelom that surrounds the heart and shed [[ovum| ova]] or [[sperm]] into the coleom, from which the nephridia extract them and emit them into the mantle cavity. Molluscs that use such a system remain of one sex all their lives and rely on [[external fertilization]]. Some molluscs use [[internal fertilization]] and / or are [[hermaphrodite]]s, where an individual can function as both sexes; both of these methods require more complex reproductive systems.<ref name="RuppertFoxBarnes2004MolluscaGen" />

The most basic molluscan larva is a [[trochophore]] which is [[plankton]]ic and feeds on floating food particles by using the two bands of cilia round its "equator" to sweep food into the mouth, which uses more cilia to drive them into the stomach, which uses further cilia to expel undigested remains through the anus. New tissue grows in the bands of [[mesoderm]] in the interior, so that the apical tuft and anus are pushed further apart as the animal grows. The trochophore stage is often succeeded by a [[veliger]] stage in which the prototroch, the "equatorial" band of cilia nearest the apical tuft, develops into the velum ("veil"), a pair of cilia-bearing lobes with which the larva swims. Eventually the larva sinks to the seafloor and [[metamorphosis| metamorphoses]] into the adult form. In some species the newborn larvae are already veligers, and other species have [[direct development]], in which a miniature adult emerges from the egg.<ref name="RuppertFoxBarnes2004MolluscaGen" />

==Classification==
Opinions vary about the number of [[class (biology)|classes]] of molluscs – for example the table below shows eight living classes,<ref name="Haszprunar2001MolluscsInEncOfLifeSci">{{citation
| contribution=Mollusca (Molluscs)) | author=Haszprunar, G.
| title=Encyclopedia of Life Sciences | date=2001 | publisher= John Wiley & Sons, Ltd.
| doi=10.1038/npg.els.0001598
}}</ref> and two extinct ones. However some authors combine the [[Caudofoveata]] and [[Neomeniomorpha|solenogasters]] into one class, the [[Aplacophora]].<ref name="RuppertFoxBarnes2004MolluscaAplacophora" /><ref name="Healy2001MolluscaInvertebrateZoology" /> Two of the commonly-recognized classes are known only from fossils<ref name="PonderWinstonLindberg" />

<center>
{| class="wikitable"
|width="120" | '''Class'''
|width="340" | '''Major organisms'''
|width="110" | '''Described living species'''<ref name="Haszprunar2001MolluscsInEncOfLifeSci" />
| '''Distribution'''
|-
| [[Caudofoveata]]<ref name="RuppertFoxBarnes2004MolluscaAplacophora" /> || worm-like organisms || align="center" | 120 || seabed {{convert|200|-|3000|m|ft}}
|-
| [[Aplacophora]]<ref name="RuppertFoxBarnes2004MolluscaAplacophora" /> || [[Neomeniomorpha|solenogasters]], worm-like organisms || align="center" | 200 || seabed {{convert|200|-|3000|m|ft}}
|-
| [[Polyplacophora]]<ref name="RuppertFoxBarnes2004MolluscaPolyplacophora" /> || chitons || align="center" | 1,000 || rocky tidal zone and seabed
|-
| [[Monoplacophora]]<ref name="RuppertFoxBarnes2004MolluscaMonoplacophora" /> || limpet-like organisms || align="center" | 25 || seabed {{convert|1800|-|7000|m|ft}}; one species {{convert|200|m|ft}}
|-
| [[Gastropoda]]<ref name="RuppertFoxBarnes2004MolluscaGastropodGen">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | page=300
}}</ref> || [[abalone]], [[limpet]]s, [[conch]], [[nudibranch]]s, [[sea hare]]s, [[sea butterfly]], [[snail]]s, [[slug]]s || align="center" | 70,000 || marine, freshwater, land
|-
| [[Cephalopod]]a<ref name="RuppertFoxBarnes2004MolluscaCephalopodGen">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | page=343
}}</ref> || [[squid]], [[octopus]], [[cuttlefish]], [[nautilus]] || align="center" | 900 || marine
|-
| [[Bivalvia]]<ref name="RuppertFoxBarnes2004MolluscaBivalveGen">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | page=367
}}</ref> || [[clam]]s, [[oyster]]s, [[scallop]]s, [[mussel]]s || align="center" | 20,000 || marine, freshwater
|-
| [[Scaphopoda]]<ref name="RuppertFoxBarnes2004MolluscaScaphopoda" /> || tusk shells || align="center" | 500 || marine {{convert|6|-|7000|m|ft}}
|-
| [[Rostroconchia]] †<ref>{{cite book
| author=Clarkson, E.N.K., | title=Invertebrate Palaeontology and Evolution
| publisher=Blackwell | date=1998 | isbn=0632052384 | page=221

| url=http://books.google.co.uk/books?id=g1P2VaPQWfUC&pg=PA221&dq=%22Invertebrate+Palaeontology+and+Evolution%22+rostroconchia#PPA221,M1
| accessdate=2008-10-27
}}</ref> || fossils; probable ancestors of bivalves || align="center" | [[extinct]] || marine
|-
| [[Helcionelloida]] †<ref name="RunnegarPojeta1974" /> || fossils; snail-like organisms such as ''[[Latouchella]]'' || align="center" | extinct || marine
|-
|}
</center>

==Evolution==
{{main|Evolution of Mollusca}}
===Fossil record===
{| align="right"
|- valign="top"
|[[Image:Yochelcionella water flow.png| thumb | right | The tiny [[Helcionellid]] fossil ''[[Yochelcionella]]'' is thought to be an early [[mollusc]]. This restoration shows water flowing in under the shell, over the gills and out through the "exhaust pipe". ]]
|[[Image:Neptunea despecta.jpg| thumb | right | 150px | Spirally-coiled shells are a common distinguishing feature of many [[gastropod]]s<ref name="RuppertFoxBarnes2004MolluscaGastropoda" />]]
|}
There is debate about whether some [[Ediacaran]] and Early [[Cambrian]] fossils really are molluscs. ''[[Kimberella]]'', from about {{ma|555}}, has been described as "mollusc-like",<ref name="Fedonkin1997">{{cite journal
| author = Fedonkin, M.A. | coauthors = Waggoner, B.M. | year = 1997
| title = The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism
| journal = Nature | volume = 388 | issue = 6645 | pages = 868 | doi = 10.1038/42242
}}</ref><ref name="FedonkinEtAl2007NewDataOnKimberella">{{ cite journal | title=New data on Kimberella, the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications | author=Fedonkin, M.A., Simonetta, A. and Ivantsov, A.Y. | journal=Geological Society, London, Special Publications | date=2007 | volume-286 | pages=157-179 | doi=10.1144/SP286.12 | url=http://www.geosci.monash.edu.au/precsite/docs/workshop/prato04/abstracts/fedonkin2.pdf | accessdate=2008-07-10 }}</ref> but others are unwilling to go further than "probable [[bilateria]]n".<ref name="Butterfield2006">{{cite journal
| author = Butterfield, N.J. | year = 2006
| title = Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale
| journal = Bioessays | volume = 28 | issue = 12 | pages = 1161–6 | doi = 10.1002/bies.20507
| accessdate = 2007-05-21
}}</ref> There is an even sharper debate about whether ''[[Wiwaxia]]'', from about {{ma|505}} was a mollusc, and much of this centers on whether its feeding apparatus was a type of [[radula]] or more similar to that of some [[polychaete]] worms.<ref name="CaronScheltemaSchanderRudkin2006">{{cite journal
| author = Caron, J.B. | coauthors = Scheltema, A., Schander, C., and Rudkin, D. | year = 2006
| date=[[2006-07-13]]
| title = A soft-bodied mollusc with radula from the Middle Cambrian Burgess Shale
| journal = Nature | volume = 442 | issue = 7099 | pages = 159–163 | issn = | doi = 10.1038/nature04894
| url = http://www.nature.com/nature/journal/v442/n7099/pdf/nature04894.pdf | accessdate = 2008-08-07
}}</ref><ref name="Butterfield2006" /> Nicholas Butterfield, who opposes the idea that ''Wiwaxia'' was a mollusc, has written that earlier [[microfossil]]s from {{ma|515|510}} are fragments of a genuinely mollusc-like radula.<ref name="Butterfield2008EarlyCambrianRadula">{{ cite journal
| author=Butterfield, N.J. | title=An Early Cambrian Radula
| journal=Journal of Paleontology | date=May 2008 | volume=82 | issue=3 | pages=543-554
| doi=10.1666/07-066.1
| url=http://findarticles.com/p/articles/mi_qa3790/is_200805/ai_n25501673/pg_1?tag=artBody;col1
| accessdate=2008-08-20
}}</ref>

However the [[Helcionellid]]s, which first appear over {{ma|535}} in the Early Cambrian, are thought to be early [[mollusc]]s with rather snail-like shells, and possibly the ancestors of the modern [[conchifera]]ns, a group that includes all the well-known modern families – gastropods, cephalopods and bivalves.<ref name="RunnegarPojeta1974">{{ cite journal
| author=Runnegar, B. and Pojeta, J. | date=1974
| title=Molluscan phylogeny: the paleontological viewpoint
| journal=Science | volume=186 | issue=4161 | pages=311-7 | doi=10.1126/science.186.4161.311
| url=http://www.sciencemag.org/cgi/content/abstract/186/4161/311 | accessdate=2008-07-30
}}</ref><ref>{{ citation
| author=Peel, J.S. | date=1991 | contribution=Functional morphology of the Class Helcionelloida nov., and the early evolution of the Mollusca | editor=Simonetta, A.M. and Conway Morris, S
| title=The Early Evolution of Metazoa and the Significance of Problematic Taxa | isbn=0521402425
| publisher=Cambridge University Press |pages=157-177
}}</ref><ref>{{cite journal
| author=Gubanov, A.P., and Peel, J.S.
| title=The early Cambrian helcionelloid mollusc ''Anabarella'' Vostokova
| journal=Palaeontology | volume=46 | issue=5 | pages=1073-1087 | date=November 2003
| doi=10.1111/1475-4983.00334
}}</ref> Although most Helcionellid fossils are only a few millimeters long, the discovery of larger specimens in 2008 has led to suggestions that the tiny specimens were juveniles and that adults were a few centimeters long, like most modern snails.<ref name="Mus2008">{{cite journal
| author = Mus, M. M.; Palacios, T.; Jensen, S. | year = 2008
| title = Size of the earliest mollusks: Did small helcionellids grow to become large adults?
| journal = Geology | volume = 36 | issue = 2 | pages = 175 | doi = 10.1130/G24218A.1
| url = http://geology.geoscienceworld.org/cgi/content/abstract/36/2/175 | accessdate=2008-10-01
}}</ref> Fossil gastropods, with their characteristic twisted shells, have been reported from "Latest Early [[Cambrian]]" rocks in Canada – unfortunately it is impossible to give a numerical date for these rocks.<ref>{{cite journal
| author=Landing, E., Geyer, G., and Bartowski, K.E.
| title=Latest Early Cambrian Small Shelly Fossils, Trilobites, and Hatch Hill Dysaerobic Interval on
the Quebec Continental Slope
| journal=Journal of Paleontology | volume=76| issue=2 | date=March 2002 | pages=287-305
| doi=10.1666/0022-3360(2002)076<0287:LECSSF>2.0.CO;2
}}</ref>
{{clear}}
{{Annotated image | float=right | caption=Septa and [[siphuncle]] in [[nautiloid]] shell
| image=Nautiloid septa n siphuncle 01.png | width=210 | image-width=150 | height=150
| annotations=
{{Annotation|130|5|<span style{{=}}"background-color:blue; border: 1px solid blue; font-size:80%">&nbsp;&nbsp;&nbsp;</span> {{=}} Septa}}
{{Annotation|130|35|<span style{{=}}"background-color:yellow; border: 1px solid silver; font-size:80%">&nbsp;&nbsp;&nbsp;</span> {{=}} [[Siphuncle]]}}
}}
For a long time it was thought that ''[[Volborthella]]'', some fossils of which pre-date {{ma|530}}, was a cephalopod. However discoveries of more detailed fossils showed that ''Volborthella''’s shell was not secreted but built from grains of the mineral [[silicon dioxide]] (silica), and that it was not divided into a series of compartments by [[Septum (marine biology)| septa]] as those of fossil shelled cephalopods and the living ''[[Nautilus]]'' are. ''Volborthella''’s classification is uncertain.<ref>{{citation
| author=Hagadorn, J.W., and Waggoner, B.M. | date= 2002 | pages=135-150
| contribution=The Early Cambrian problematic fossil Volborthella: New insights from the Basin and Range
| editor=Corsetti, F.A.
| title=Proterozoic-Cambrian of the Great Basin and Beyond, Pacific Section SEPM Book 93
| publisher=SEPM (Society for Sedimentary Geology)
| url=http://www3.amherst.edu/~jwhagadorn/publications/volb.pdf | accessdate=2008-10-01
}}</ref> The Late Cambrian fossil ''[[Plectronoceras]]'' is now thought to be the earliest clearly cephalod fossil, as its shell had septa and a [[siphuncle]], a strand of tissue that ''Nautilus'' uses to remove water from compartments that it has vacated as it grows, and which is also visible in fossil [[ammonite]] shells. However ''Plectronoceras'' and other early cephalopods crept along the seafloor instead of swimming, as their shells contained a "ballast" of stony deposits on what is thought to be the underside and had stripes and blotches on what is thought to be the upper surface.<ref>{{cite book
| author=Vickers-Rich, P., Fenton, C.L., Fenton, M.A. and Rich, T.H.
| title=The Fossil Book: A Record of Prehistoric Life
| publisher=Courier Dover Publications | date= 1997 | isbn=0486293718 | pages=269-272
}}</ref> All cephalopods with external shells except the [[nautiloid]]s became extinct by the end of the [[Cretaceous]] period {{ma|65}}.<ref name="MarshallWard1996">{{cite journal |author=Marshall C.R., and Ward P.D. |year=1996 |title=Sudden and Gradual Molluscan Extinctions in the Latest Cretaceous of Western European Tethys |journal=Science |volume=274 |issue=5291 |pages=1360–1363 |doi=10.1126/science.274.5291.1360 |pmid=8910273
}}</ref> However the shell-less [[Coleoidea]] ([[squid]], [[octopus]], [[cuttlefish]]) are abundant to-day.

The Early Cambrian fossils ''[[Fordilla]]'' and ''[[Pojetaia]]'' are regarded as [[bivalve]]s.<ref>{{cite journal
| author=Pojeta, J. | date=2000 | title=Cambran Pelecypoda (Mollusca)
| journal=American Malacological Bulletin | volume=15 | pages=157-166
}}</ref><ref> {{cite journal
| author=Schneider, J.A. | title=Bivalve systematics during the 20th century
| journal=Journal of Paleontology | date=November 2001 | volume=75 | issue=6 | pages=1119-1127
| doi=10.1666/0022-3360(2001)075<1119:BSDTC>2.0.CO;2
| url=http://findarticles.com/p/articles/mi_qa3790/is_200111/ai_n9001371/pg_3 | accessdate=2008-10-05
}}</ref><ref>{{cite journal
| author=Gubanov, A.P., Kouchinsky, A.V. and Peel, J.S.
| title=The first evolutionary-adaptive lineage within fossil molluscs
| journal=Lethaia | volume=32 | issue=2 | pages=155-157
}}</ref><ref> {{cite journal
| author=Gubanov, A.P., and Peel, J.S.
| title=The early Cambrian helcionelloid mollusc ''Anabarella'' Vostokova
| journal=Palaeontology | volume=46 |issue=5 | pages=1073-1087 | doi=10.1111/1475-4983.00334
}}</ref> "Modern-looking" bivalves appeared in the [[Ordovician]] period, {{ma|488|443}}.<ref>{{cite journal
| author=Zong-Jie, F.
| title=An introduction to Ordovician bivalves of southern China, with a discussion of the early evolution of the Bivalvia
| journal=Geological Journal | Volume=41 | issue=3-4 | pages 303-328 | doi=10.1002/gj.1048
}}</ref> One bivalve group, the [[rudist]]s, became major [[reef]]-builders in the Cretaceous, but became extinct in the [[Cretaceous-Tertiary extinction]].<ref>{{cite journal
| author=Raup, D.M., and Jablonski, D.
| title=Geography of end-Cretaceous marine bivalve extinctions | journal=Science | date=1993
| volume=260 | issue=5110 | pages=971–973 |doi=10.1126/science.11537491 | pmid=11537491
}}</ref> However bivalves are now abundant and diverse.

===Phylogeny===
<div style="float:right; width:30em; border:solid 1px silver; padding:2px; margin:2px; font-size:80%;">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
|label1=[[Lophotrochozoa]]
|1={{clade
|1=[[Brachiopod]]s
|2={{clade
|1={{clade
|1={{clade
|1={{clade
|1=[[Bivalve]]s
|2=[[Monoplacophora]]ns<br />("limpet-like", "living fossils")
|3={{clade
|1=[[Gastropod]]s<br />([[snail]]s, [[slug]]s, [[limpet]]s, [[sea hare]]s)
|2={{clade
|1=[[Cephalopod]]s<br />([[nautiloid]]s, [[ammonites]], [[squid]], etc.)
|2=[[Scaphopod]]s (tusk shells)
}}
}}
}}
}}
|2={{clade
|1=[[Aplacophora]]ns<br />(spicule-covered, worm-like)
|2=[[Polyplacophora]]ns (chitons)
}}
}}
|2={{clade
|label1=[[Halwaxiid]]s
|1={{clade
|1=''[[Wiwaxia]]''
|2=''[[Halkieria]]''
}}
|2=''[[Orthrozanclus]]''
}}
|3=''[[Odontogriphus]]''
}}
}}
}}
</div>A possible "family tree" of molluscs (2007).<ref name="SigwartSutton2007DeepMolluscanPhylogeny" /><ref>{{cite web
| title=The Mollusca | publisher=University of California Museum of Paleontology
| url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php | accessdate=2008-10-02
}}</ref> Does not include [[annelid]] worms as the analysis concentrated on fossilizable "hard" features.<ref name="SigwartSutton2007DeepMolluscanPhylogeny" /></div>
The [[phylogeny]] (evolutionary "family tree") of molluscs is a controversial subject. In addition to the debates about whether ''[[Kimberella]]'' and any of the "[[halwaxiid]]s" were molluscs or closely related to molluscs,<ref name="FedonkinEtAl2007NewDataOnKimberella" /><ref name="Butterfield2006" /><ref name="CaronScheltemaSchanderRudkin2006" /><ref name="Butterfield2008EarlyCambrianRadula" /> there are debates about the relationships between the classes of living molluscs.<ref name="SigwartSutton2007DeepMolluscanPhylogeny">{{cite journal
| author=Sigwart, J.D., and Sutton, M.D. | date=October 2007
| title=Deep molluscan phylogeny: synthesis of palaeontological and neontological data
| journal=Proceedings of The Royal Society: Biology | volume=274 | issue=1624 | pages=2413-2419
| doi=10.1098/rspb.2007.0701
}} For a summary, see {{cite web
| title=The Mollusca | publisher=University of California Museum of Paleontology
| url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php | accessdate=2008-10-02
}}</ref>

Molluscs are members of the [[Lophotrochozoa]],<ref name="SigwartSutton2007DeepMolluscanPhylogeny" /> a group defined by having [[trochophore]] larvae and, in the case of living [[brachiopod]]s, a feeding structure called a [[lophophore]]. The other members of the Lophotrochozoa are the [[annelid]] worms and seven marine [[phylum| phyla]].<ref>{{cite web
| title=Introduction to the Lophotrochozoa
| publisher=University of California Museum of Paleontology
| url=http://www.ucmp.berkeley.edu/phyla/lophotrochozoa.html | accessdate=2008-10-02
}}</ref>

The diagram on the right summarizes a phylogeny presented in 2007. Two other widely-supported reconstructions of the evolutionary relationships ''within'' the molluscs are:
{| cellspacing="0" cellpadding="0"
|- valign="top"
|<div style="float:right; width:auto; border:solid 1px silver; padding:2px; margin:2px; font-size:80%;"><div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
|label1=Molluscs
|1={{clade
|label1=[[Aculifera]]
|1={{clade
|1={{clade
|1=[[Solenogastres]]
|2=[[Caudofoveata]]
}}
|2=[[Polyplacophora]]ns
}}
|label2=[[Conchifera]]
|2={{clade
|1=[[Monoplacophora]]ns
|2={{clade
|1=[[Bivalve]]s
|2=[[Scaphopod]]s
|3=[[Gastropod]]s
|4=[[Cephalopod]]s
}}
}}
}}
}}
</div>The "[[Aculifera]]" hypothesis<ref name="SigwartSutton2007DeepMolluscanPhylogeny"/></div>
|<div style="float:right; width:auto; border:solid 1px silver; padding:2px; margin:2px; font-size:80%;"><div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
|label1=Molluscs
|1={{clade
|1={{clade
|1=[[Solenogastres]]
|2=[[Caudofoveata]]
|label3=[[Testaria]]
|3={{clade
|1=[[Polyplacophora]]ns
|2={{clade
|1=[[Monoplacophora]]ns
|2={{clade
|1=[[Bivalve]]s
|2=[[Scaphopod]]s
|3=[[Gastropod]]s
|4=[[Cephalopod]]s
}}
}}
}}
}}
}}
}}
</div>The "[[Testaria]]" hypothesis<ref name="SigwartSutton2007DeepMolluscanPhylogeny"/></div>
|}
{{clear}}

==Interactions with humans==
===Uses by humans===
{{see|Seashell}}
<!--[[Image:Wool techelet.jpg|thumb|right|1500px|Some wool dipped in [[techelet]] solution, from the ''[[Murex trunculus]]'', turning blue in the sunlight outside P'til Techelet in [[Israel]].]]-->
[[Image:2005mollusc.PNG|thumb|right|300px|Mollusc output in 2005]]
Molluscs, especially bivalves such as [[clam]]s and [[mussel]]s, have been an important food source for many different peoples around the world at least since the appearance of anatomically modern humans – and this has often resulted in over-fishing.<ref>{{cite journal
| author=Mannino, M.A., and Thomas, K.D.
| title=Depletion of a resource? The impact of prehistoric human foraging on intertidal mollusc communities and its significance for human settlement, mobility and dispersal
| journal=World Archaeology | volume 33 | issue=3 | date=February 2002 | pages=452-474
| doi=10.1080/00438240120107477
}}</ref> Other molluscs commonly eaten include [[octopus]]es and [[squid]]s, [[whelk]]s, [[oysters]], and [[scallops]].<ref>{{cite book
| author=Garrow, J.S., Ralph, A., and James, W.P.T. | title=Human Nutrition and Dietetics | page=370
| publisher=Elsevier Health Sciences | date=2000 | isbn=0443056277
}}</ref> In 2005, China accounted for 80% of the global mollusc catch, netting almost 11 million tonnes. Within Europe, France remained the industry leader.<ref>{{cite web
| url=http://www.fao.org/figis/servlet/TabLandArea?tb_ds=Capture&tb_mode=TABLE&tb_act=SELECT&tb_grp=COUNTRY | accessdate=2008-10-03
| title=China catches almost 11&nbsp;m tonnes of molluscs in 2005 | publisher=[[FAO]]
}}</ref> However some countries have strict regulations about the importation and handling of molluscs and other [[seafood]], mainly to minimize the risk that humans may be poisoned by [[toxin]]s that have accumulated in the animals.<ref>{{cite web
| title=Importing fishery products or bivalve molluscs | publisher=Food Standards Agency
| location=[[United Kingdom]]
| url=http://www.food.gov.uk/foodindustry/imports/want_to_import/fisheryproducts/
| accessdate=2008-10-02
}}</ref>

[[Image:Pearl.jpg| thumb | left | 200px | Saltwater [[pearl oyster]] farm in Seram, [[Indonesia]] ]]
Most molluscs that have shells can produce pearls, but only the pearls of [[bivalve]]s and some [[gastropod]]s whose shells are lined with [[nacre]] are valuable.<ref name="RuppertFoxBarnes2004MolluscaBivalvia" /><ref name="RuppertFoxBarnes2004MolluscaGastropoda" /> The best natural pearls are produced by the [[pearl oyster]]s ''[[Pinctada]] margaritifera'' and ''Pinctada mertensi'', which live in the [[tropical]] and [[sub-tropical]] waters of the [[Pacific Ocean]]. Natural pearls form when a small foreign object gets stuck between the [[Mantle (mollusc)|
mantle]] and shell. There are two methods of culturing pearls, by inserting either "seeds" or beads into oysters. The "seed" method uses grains of ground shell from freshwater [[mussel]]s, and over-harvesting for this purpose has [[endangered]] several freshwater mussel species in the southeastern [[USA]].<ref name="RuppertFoxBarnes2004MolluscaBivalvia" /> The pearl industry is so important in some areas that significant sums of money are spent on monitoring the health of farmed molluscs.<ref>{{cite journal
| author=Jones, J.B., and Creeper, J.
| title=Diseases of Pearl Oysters and Other Molluscs: a Western Australian Perspective
| journal=Journal of Shellfish Research | volume=25 | issue=1 | date=April 2006 | pages=233–238
| doi=10.2983/0730-8000(2006)25[233:DOPOAO]2.0.CO;2
}}</ref>

[[Image:Meister von San Vitale in Ravenna 004.jpg| thumb | right | 100px | [[Byzantine]] Emperor [[Justinian I]] clad in [[Tyrian purple]] ]]
Other luxury and high-[[Social status|status]] products have been made from molluscs. [[Tyrian purple]], made from the ink glands of [[murex]] shells, "...&nbsp;fetched its weight in silver" in the fourth-century [[Before Christ|BC]], according to [[Theopompus]].<ref>The fourth-century [[Before Christ|BC]] historian [[Theopompus]], cited by Athenaeus (12:526) around 200 BC ; according to {{cite book
| author=Gulick, C.B. | date=1941| title=Athenaeus, The Deipnosophists | location=Cambridge, Mass.
| publisher=Harvard University Press
}}</ref> The discovery of large numbers of Murex shells on [[Crete]] suggests that the [[Minoans]] may have pioneered the extraction of "Imperial purple" during the Middle Minoan period in the 20th&ndash;18th century BC, centuries before the [[Tyrians]].<ref>{{cite journal
| author=Reese, D.S. | date=1987
| title=Palaikastro Shells and Bronze Age Purple-Dye Production in the Mediterranean Basin
| journal=Annual of the British School of Archaeology at Athens | volume=82 | pages=201-6
}}</ref><ref>{{cite journal
| author=Stieglitz, R.R. | date=1994 | title=The Minoan Origin of Tyrian Purple
| journal=Biblical Archaeologist | volume=57 | pages=46-54
}}</ref> [[Sea silk]] is a fine, rare and valuable [[textile|fabric]] produced from the long silky threads ([[byssus]]) secreted by several bivalve molluscs, particularly ''[[Pinna nobilis]]'', to attach themselves to the sea bed.<ref>''Webster's Third New International Dictionary (Unabridged)'' 1976. G. & C. Merriam Co., p. 307.</ref> [[Procopius]], writing on the Persian wars circa 550 [[Common Era|CE]], "stated that the five hereditary satraps (governors) of Armenia who received their insignia from the Roman Emperor were given [[chlamys]] (or cloaks) made from ''lana pinna'' (Pinna "wool," or byssus). Apparently only the ruling classes were allowed to wear these chlamys."<ref>{{cite journal
| author=Turner, R.D., and Rosewater, J. | title=The Family Pinnidae in the Western Atlantic
| journal=Johnsonia | volume=3 | issue=38 | date=June 1958 | page=294
}}</ref>

Mollusc shells, including those of [[cowrie]]s, were used as a kind of [[money]] in several pre-industrial societies. However these "currencies" generally differed in important ways from the standardized government-backed and -controlled money familiar to industrial societies. Some shell "currencies" were not used for commercial transactions but mainly as [[social status]] displays at important occasions such as weddings.<ref>{{cite journal
| author=Maurer­, B. | title=The Anthropology of Money
| journal=Annual Review of Anthropology | volume=35| pages=15-36 | date=October 2006
| doi=10.1146/annurev.anthro.35.081705.123127
| url=http://www.anthro.uci.edu/faculty_bios/maurer/Maurer-AR.pdf | accessdate=2008-10-23
}}</ref> When used for commercial transactions they functioned as [[commodity]] money, in other words as a tradable commodity whose value differed from place to place, often as a result of difficulties in transport, and which was vulnerable to incurable [[inflation]] if more efficient transport or "goldrush" behavior appeared.<ref>{{cite book
| title=The Shell Money of the Slave Trade | author=Hogendorn, J., and Johnson, M.
| publisher=Cambridge University Press | date=2003 | isbn=052154110
}} Particulalrl chapters "Boom and slump for the cowrie trade" (pages 64-79) and "The cowrie as money: transport costs, values and inflation" (pages 125-147)</ref>

===Threats to humans===
====Stings and bites====
[[Image:Hapalochlaena lunulata.JPG|thumb|100px|The [[blue-ringed octopus]]'s rings are a warning signal – this octopus is alarmed, and its bite can kill.<ref name="AVRU_BlueRinged" />]]
Apart from the risks of food-poisoning or [[seafood]] [[allergy| allergies]], which can be fatal, a few species of molluscs in the wild can present a serious risk to humans, when handled. To put this into correct perspective however, deaths from mollusc venoms are less than 10% of the number of deaths from [[jellyfish]] stings.<ref name="WilliamsonFennerEtAl1996VenomousMarine Animals">{{cite book
| author=Williamson, J.A., Fenner, P.J., Burnett, J.W., and Rifkin, J.
| title=Venomous and Poisonous Marine Animals: A Medical and Biological Handbook
| publisher=UNSW Press | date=1996 | isbn=0868402796 | pages=65-68
| url=http://books.google.co.uk/books?id=YsZ3GryFIzEC&pg=PA75&lpg=PA75&dq=mollusc+venom+fatal&source=web&ots=tBDHW2xdlx&sig=PAvTz2z3hnwUyE4lfU8fvzwNbD4&hl=en&sa=X&oi=book_result&resnum=2&ct=result#PPA57,M1 | accessdate=2008-10-03
}}</ref>

All octopuses are venomous<ref name="Anderson1995GiantPacificOctopus">Anderson, R.C. (1995) Aquarium husbandry of the giant Pacific octopus. ''Drum and Croaker'' '''26''':14-23</ref> but only a few species pose a significant threat to humans. The blue-ringed octopuses in the genus ''[[Blue-ringed octopus|Hapalochlaena]]'', which live around Australia and New Guineau, bite humans only if severely provoked,<ref name="AVRU_BlueRinged">{{cite web
| title=Blue ringed octopus | author=Alafaci, A. | publisher=Australian Venom Research Unit
| url=http://www.avru.org/compendium/biogs/A000060b.htm | accessdate=2008-10-03
}}</ref> but their venom kills 25% of human victims. Another tropical species, ''[[Octopus apollyon]]'', causes severe [[inflammation]] that can last for over a month even if treated correctly.<ref>{{cite journal
| author=Brazzelli, V., Baldini, F., Nolli, G., Borghini, F., and Borroni, G.
| title=''Octopus apollyon'' bite
| journal=Contact Dermatitis | volume=40 | issue=3 | pages=169-170 | date=1999
| doi=10.1111/j.1600-0536.1999.tb06025.x
}}</ref>

[[Image:Textile cone.JPG| thumb | left| 100px | [[Cone shell]]s are dangerous to bathers but useful to [[neurology]] researchers<ref name="New Scientist19October1996DoctorSnail" />]]
[[Cone shell]]s, carnivorous [[gastropod]]s that feed on marine invertebrates and fish, produce a huge array of [[toxin]]s, some fast-acting and others slower but deadlier – they can afford to do this because their toxins are relatively cheap to make compared with those of snakes or spiders.<ref name="New Scientist19October1996DoctorSnail">{{cite journal
| author=Concar, D. | journal=New Scientist | date=19 October 1996
| title=Doctor snail – Lethal to fish and sometimes even humans, cone snail venom contains a pharmacopoeia of precision drugs
| http://environment.newscientist.com/article/mg15220523.900-doctor-snail--lethal-to-fish-and-sometimes-even-humans-cone-snail-venom-contains-apharmacopoeia-of-precision-drugs-itdavid-concarit-finds-out-how-the-toxinstarget-nerve-cells.html | accessdate=2008-10-03
}}</ref> Many painful stings have been reported and a few fatalities, although some of the reported fatalities may be exaggerations.<ref name="WilliamsonFennerEtAl1996VenomousMarine Animals" /> Only the few species that can kill fish are likely to be seriously dangerous to humans.<ref>{{cite web
| url=http://grimwade.biochem.unimelb.edu.au/cone/deathby.html
| title=Cone Shell Mollusc Poisoning, with Report of a Fatal Case | author=Livett, B.
| publisher=Department of Biochemistry and Molecular Biology, University of [[Melbourne]]
}}</ref> The effects of individual cone shell toxins on victims' nervous systems are so precise that they are useful tools for research in [[neurology]], and the small size of their [[molecule]]s makes it easy to synthesize them.<ref name="New Scientist19October1996DoctorSnail" /><ref>{{cite journal
| author=Haddad, V.(junior), de Paula Neto, J.B., and Cobo, V.J.
| title=Venomous mollusks: the risks of human accidents by ''Conus'' snails (Gastropoda: Conidae) in Brazil
| journa=Revista da Sociedade Brasileira de Medicina Tropical | volume=39 | issue=(5) | pages=498-500
| date=September-October 2006
| url=http://www.scielo.br/pdf/rsbmt/v39n5/a15v39n5.pdf | accessdate=2008-10-03
}}</ref>

The traditional belief that a [[giant clam]] can trap the leg of a person between its valves, thus drowning them, is a myth.<ref>{{cite book
| author=Cerullo, M.M., Rotman, J.L., and Wertz, M.
| title=The Truth about Dangerous Sea Creatures
| publisher=Chronicle Books | date=2003 | isbn=0811840506 | page=10
| url=http://books.google.co.uk/books?hl=en&lr=&id=1MOxNDmFLd4C&oi=fnd&pg=PA1&dq=giant+clam+trap+foot&ots=lYe_I4NhIO&sig=0KwRdmEoB4rt6zk1lvxPdiH_jkE#PPA10,M1 | accessdate=2008-10-03
}}</ref>

====Pests====
[[Image: Schistosomiasis itch.jpeg| thumb | right | 200px
|Skin [[vesicle]]s created by the penetration of [[Schistosoma]]. Source: [[Centers for Disease Control and Prevention]] ]]
[[Schistosomiasis]] (also known as bilharzia, bilharziosis or snail fever) is "second only to malaria as the most devastating parasitic disease in tropical countries. An estimated 200 million people in 74 countries are infected with the disease — 100 million in Africa alone."<ref>{{cite web
| title=The Carter Center Schistosomiasis Control Program |publisher=The [[Carter Center]]
| url=http://www.cartercenter.org/health/schistosomiasis/index.html | accessdate=2008-10-03
}}</ref> The parasite has 13 known species, of which two infect humans. The parasite itself is not a mollusc, but all the species have freshwater snails as [[Host (biology)|intermediate hosts]].<ref>{{cite book
| author=Brown, D.S. | title=Freshwater Snails of Africa and Their Medical Importance
| publisher=CRC Press | date=1994 | isbn=0748400265 | page=305
}}</ref>

Despite its name, ''[[Molluscum contagiosum]]'' is a [[virus|viral]] disease, and does not actually have anything to do with molluscs.<ref>{{cite web
| title=Molluscum (Molluscum Contagiosum): Frequently Asked Questions for Everyone
| publisher=[[Centers for Disease Control and Prevention]]
| url=http://www.cdc.gov/ncidod/dvrd/molluscum/faq/everyone.htm | accessdate=2008-10-03
}}</ref>

Some species of molluscs, particularly certain snails and [[slugs]], can be serious crop pests,<ref>{{cite book
| author=Barker, G.M. | title=Molluscs As Crop Pests | publisher=CABI Publications | date=2002
| isbn=0851993206
}}</ref> and snails or slugs introduced into new environments can unbalance local [[ecosystem]]s. One such pest, the giant African snail ''[[Achatina fulica]]'', has been introduced to many parts of Asia, as well as to many islands in the [[Indian Ocean]] and [[Pacific Ocean]]. In the 1990s this species reached the [[West Indies]]. Attempts to control it by introducing the predatory snail ''[[Euglandina rosea]]'' proved disastrous, as the predator ignored ''Achatina fulica'' and went on to extirpate several native snail species instead.<ref>{{cite journal
| author=Civeyrel, L., and Simberloff, D.
| title=A tale of two snails: is the cure worse than the disease?
| journal=Biodiversity and Conservation | volume=5 | issue=10 | date=October 1996 | pages=1231-1252
| doi=10.1007/BF00051574
}}</ref>

==References==
{{reflist|2}}

===Further reading===
{{refbegin}}
* {{cite book|author = Starr & Taggart|year = 2002|title = Biology: The Unity and Diversity of Life|location= Pacific Grove, California | publisher=Thomson Learning}}
* Nunn, J.D., Smith, S.M., Picton, B.E. and McGrath, D. 2002. ''Checklist, atlas of distribution and bibliography for the marine mollusca of Ireland.'' in. Marine Biodiversity in Ireland and Adjacent Waters. Ulster Museum. publication no. 8.
{{refend}}

==External links==
{{Wikicommons|Mollusca}}
{{Wikispecies|Mollusca}}
{{Wikibookspar|Dichotomous Key|Mollusca}}
* [http://www.gastropods.com/ Hardy's Internet Guide to Marine Gastropods]
* [http://www.tafi.org.au/zooplankton/imagekey/mollusca/index.html Planktonic mollusca fact sheets]

[[Category:Molluscs|Molluscs]]
[[Category:Phyla]]


[[ar:رخويات]]
[[zh-min-nan:Nńg-thé tōng-bu̍t]]
[[bs:Mekušci]]
[[bg:Мекотели]]
[[ca:Mol·lusc]]
[[cs:Měkkýši]]
[[cy:Molwsg]]
[[da:Bløddyr]]
[[de:Weichtiere]]
[[et:Limused]]
[[el:Μαλάκια]]
[[es:Mollusca]]
[[eo:Molusko]]
[[eu:Mollusca]]
[[fa:نرم‌تنان]]
[[fr:Mollusca]]
[[gl:Molusco]]
[[ko:연체동물]]
[[hr:Mekušci]]
[[io:Molusko]]
[[id:Mollusca]]
[[ia:Mollusco]]
[[is:Lindýr]]
[[it:Mollusca]]
[[he:רכיכות]]
[[ka:მოლუსკები]]
[[la:Mollusca]]
[[lv:Moluski]]
[[lb:Weechdéieren]]
[[lt:Moliuskai]]
[[hu:Puhatestűek]]
[[mk:Мекотели]]
[[nl:Weekdieren]]
[[ja:軟体動物]]
[[no:Bløtdyr]]
[[nn:Blautdyr]]
[[oc:Mollusca]]
[[pl:Mięczaki]]
[[pt:Moluscos]]
[[ro:Moluscă]]
[[qu:Llamp'u uywa]]
[[ru:Моллюски]]
[[simple:Mollusc]]
[[sk:Mäkkýše]]
[[sl:Mehkužci]]
[[sr:Мекушци]]
[[fi:Nilviäiset]]
[[sv:Blötdjur]]
[[ta:மெல்லுடலி]]
[[te:మొలస్కా]]
[[th:หอย]]
[[vi:Động vật thân mềm]]
[[tr:Yumuşakçalar]]
[[uk:Молюски]]
[[zh:软体动物]]

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