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{{Taxobox
| fossil_range = {{fossil range|earliest=555|Cambrian|Recent}}
| image = Tonicella-lineata.jpg
| image_caption = ''[[Tonicella lineata]]'', a [[polyplacophoran]] or chiton, anterior end towards the right
| 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]]
| diversity = about 93,000 recognized living species<ref name="Haszprunar2001MolluscsInEncOfLifeSci" />
| diversity_link = Mollusca
}}

'''Molluscs'''<ref group=note>Spelled '''mollusk''' in the USA, see reasons given in Rosenberg's [http://www.conchologistsofamerica.org/articles/y1996/9609_rosenberg.asp]; for the spelling "mollusc" see the reasons given by {{Bruscabrusca}}.</ref> belong to the animal [[Phylum (biology)|phylum]] '''Mollusca'''. There are around 93,000 recognized [[extant taxon|extant]] [[species]], making it the largest [[marine biology|marine]] phylum as it contains about 23% of named marine [[organism]]s. Representatives live in marine, [[freshwater mollusc|freshwater]], and terrestrial [[habitats]]. Molluscs are highly diverse in size, in anatomical structure, in behaviour and in habitat.

The phylum is typically divided into nine or ten [[class (biology)|taxonomic classes]], of which two are [[extinct]]. The [[gastropod]]s ([[snail]]s and [[slug]]s) include by far the most [[biological classification|classified]] species, accounting for 80% of the total. [[Cephalopod]] molluscs such as [[squid]], [[cuttlefish]] and [[octopus]] are among the most [[neurobiology|neurologically advanced]] invertebrates. Either the [[giant squid]] or the [[colossal squid]] is the largest known [[invertebrate]] [[species]].

Molluscs have diverse body structures. The two most universal characteristics are a [[mantle (mollusc)|mantle]] with a significant cavity used for [[respiration (physiology)|breathing]] and [[excretion]], and the structure of the nervous system. As a result, many textbooks base their descriptions on a hypothetical "generalized mollusc", with features common to many, but not all species.

There is good evidence for the appearance of [[gastropod]]s, [[cephalopod]]s 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 vigorously debated.

Molluscs have long been an important human food source. 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 [[money]] in some pre-industrial societies.

Mollusc species can also present as hazards or pests to humans. [[Blue-ringed octopus]] bites are often fatal, and ''[[Octopus apollyon]]'' bites can cause [[inflammation]] that lasts longer than one month. Stings from a few species of large tropical [[cone shell]]s can also kill. Their sophisticated though easily-produced venoms have also 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 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 [[Winston Ponder|Ponder]] & [[David R. Lindberg|Lindberg]],
could go with 100,000 living mollusc species.<ref name="RuppertFoxBarnes2004FrontEnd1">
************* -->There are about 93,000 named mollusc species,<ref name="Haszprunar2001MolluscsInEncOfLifeSci" /> which include 23% of all named marine organisms.<ref name="HancockRecognisingResearch">{{Cite web|url=http://www.austmus.gov.au/display.cfm?id=2897|title=Recognising research on molluscs |accessdate=2009-03-09|publisher=Australian Museum|year=2008|author=Rebecca Hancock}}</ref> Molluscs are second only to [[arthropod]]s in numbers of living animal species<ref name="PonderWinstonLindberg">{{ Cite book | editor=Ponder, W.F. and Lindberg, D.R.
| year=2008 | title=Phylogeny and Evolution of the Mollusca
| publisher=Berkeley: University of California Press | pages=481 | isbn=978-0520250925 | author=edited by Winston F. Ponder, David R. Lindberg. }}</ref> – far behind the arthropods' 1,113,000 but well ahead of [[chordate]]s' 52,000.<ref name="Ruppert2004">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | year=2004}}</ref>{{rp|Front endpaper 1}} It has been estimated that there are about 200,000 living species in total,<ref name="PonderLindberg2004Phylogeny">{{Cite web|url=http://www.ucmp.berkeley.edu/museum/news/news_briefs/perth08042004.html|title=Phylogeny of the Molluscs|accessdate=2009-03-09|publisher=World Congress of Malacology|year=2004|author=Winston F. Ponder and David R. Lindberg}}</ref> and 70,000 fossil species,<ref name="BruscaBrusca2003P702" /> although the total number of mollusc species that ever existed, whether or not preserved, must be many times greater than the number alive today.<ref>{{cite book
| author=David M. Raup & Steven M. Stanley | title=Principles of Paleontology
| publisher=W.H. Freeman and Co. | edition=2 | isbn=071670220 | year=1978 | pages=4–5
}}</ref>

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 lesser-known but similarly distinctive sub-groups. The majority of species still live in the oceans, from the seashores to the [[abyssal zone]], but some are [[freshwater molluscs|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. | year=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
| year=2005
}}</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>

Freshwater and terrestrial molluscs appear exceptionally vulnerable to extinction. Estimates of the numbers of non-marine molluscs vary widely, partly because many regions have not been thoroughly surveyed and there is a shortage of specialists in classifying these animals. However, in 2004 the [[IUCN Red List]] of Threatened Species included nearly 2,000 endangered non-marine molluscs. For comparison, the great majority of molluscs species are marine but only 41 of these appeared on the 2004 Red List. 42% of recorded extinctions since 1500 involved molluscs, almost entirely non-marine.<ref>{{cite journal|last=Lydeard|first=C.|coauthors=R. Cowie, R., Ponder, W.F., ''et al''|date=April 2004|title=The global decline of nonmarine mollusks|journal=[[BioScience]]|volume=54|pages=321–330|url=http://www.unc.edu/~keperez/lydeard_bioscience.pdf|accessdate=20 Oct 2009 | doi=10.1641/0006-3568(2004)054[0321:TGDONM]2.0.CO;2}}</ref>

==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>{{Cite book
| contribution=Mollusca | title=Shorter Oxford English Dictionary
| editor=Little, L., Fowler, H.W., Coulson, J., and Onions, C.T.
| publisher=Oxford University press | year=1964
}}</ref> The [[science|scientific]] study of molluscs is known as '''[[malacology]]'''.<ref>{{Cite book
| contribution=Malacology | title=Shorter Oxford English Dictionary
| editor=Little, L., Fowler, H.W., Coulson, J., and Onions, C.T.
| publisher=Oxford University press | year=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="Ruppert2004"/><ref name="BruscaBrusca2003P702">{{cite book
| author=Brusca, R.C., and Brusca, G.J. | title=Invertebrates | edition=2 | year=2003
| publisher=Sinauer Associates | isbn=0878930973 | pages=702
}}</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="Ruppert2004"/>{{rp|367–403}})

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="Ruppert2004"/>{{rp|291–292
}}
![[Polyplacophora]]<ref name="Ruppert2004"/>{{rp|292–298}}
![[Monoplacophora]]<ref name=Ruppert2004/>{{rp|298–300}}
![[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 | year=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 | year=2004 | pages=343–367
}}</ref>
![[Bivalvia]]<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 | year=2004 | pages=367–403
}}</ref>
![[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 | year=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 one will 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]]ns.<ref name="GiribetOkusuEtAlMolluscsWithSeriallyRepeatedStructures" /><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 | year=2004 | pages=284–291
}}</ref><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 | year=2004 | pages=298–300
}}</ref><ref name="Healy2001MolluscaInvertebrateZoology">{{Cite book
| author=Healy, J.M. | chapter=The Mollusca | pages=120–171
| title=Invertebrate Zoology | year=2001 | edition=2 | editor=Anderson, D.T.
| publisher=Oxford University Press | isbn=0195513681
}}</ref>

The following descriptions elaborate this abstraction.

==Anatomy==
The generalized mollusc has a single, "[[limpet]]-like" [[gastropod shell|shell]] on top. The shell is secreted by a mantle that covers the upper surface. The underside consists of a single muscular "foot".
===Mantle and mantle cavity===
The mantle cavity is a fold in the mantle that encloses a significant amount of space. The cavity was probably at the rear in the earliest molluscs but its position now varies from group to group. The [[anus]], a pair of [[osphradium|osphradia]] (chemical sensors) in the incoming "lane", 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" />

===Shell{{anchor|Shell}}===
{{Main|Mollusc shell}}

The mollusc shell is a biogenic composite material that has been the subject of much interest in materials science because of its unusual properties and its model character for biomineralization. Molluscan shells consist of 95-99% calcium carbonate by weight, while an organic component makes up the remaining 1-5%. The resulting composite has a fracture toughness ~3000 times greater than that of the crystals themselves.<ref name="Currey1999">{{cite pmid|10562511}}</ref> In the biomineralization of the mollusc shell, specialized proteins are responsible for directing crystal nucleation, phase, morphology, and growths dynamics and ultimately give the shell its remarkable mechanical strength. The application of biomimetic principles elucidated from mollusc shell assembly and structure may help in fabricating new composite materials with enhanced optical, electronic, or structural properties.

===Foot===
The underside consists of a muscular foot, which has adapted to different purposes in different classes.<ref name=mollusca11/>{{rp|4}} The foot carries a pair of [[statocyst]]s which act as balance sensors. In gastropods, it secretes [[mucus]] as a lubricant to aid movement. In forms that have only a top shell, such as [[limpet]]s, the foot acts as a sucker attaching 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" /> In bivalves, the foot is adapted for burrowing into the sediment;<ref name=mollusca11>{{mollusca|volume=11}}</ref>{{rp|4}} in cephalopods it is used for jet propulsion,<ref name=mollusca11/>{{rp|4}} and the tentacles and arms are derived from the foot.<ref>{{cite journal
| doi = 10.1002/jmor.10564
| year = 2008
| month = Jan
| author = Shigeno, S; Sasaki, T; Moritaki, T; Kasugai, T; Vecchione, M; Agata, K
| title = Evolution of the cephalopod head complex by assembly of multiple molluscan body parts: Evidence from Nautilus embryonic development.
| volume = 269
| issue = 1
| pages = 1–17
| pmid = 17654542
| journal = Journal of morphology }}</ref>

==Physiology==
A striking feature of molluscs is the use of organs 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 serves both excretion and reproduction.

===Circulation===
Molluscs' circulatory systems are mainly [[Circulatory system#Open circulatory system|open]]. 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 (heart)|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" />

===Respiration===
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 to the heart.<ref name="RuppertFoxBarnes2004MolluscaGen" />
{{-}}

===Eating, digestion, and elimination===
{{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 hindgut's entrance pinch off and excrete a piece of the prostyle, 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" />

===Nervous system===
Molluscs have two pairs of main [[nerve cord]]s (three in [[bivalve]]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" />

The brain, in species that have one, encircles the [[esophagus]]. Most molluscs have a head with eyes, and all have a pair of sensor-containing tentacles, also on the head, that detect chemicals, vibrations and touch.<ref name="RuppertFoxBarnes2004MolluscaGen" />

===Reproduction===
{{Annotated image/Trochophore larva}}
The simplest 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. 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, functioning as both sexes; both of these methods require more complex reproductive systems.<ref name="RuppertFoxBarnes2004MolluscaGen" />

===Maturation===
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. Whilst metamorphosis is the usual state in molluscs, the cephalopods differ in exhibiting direct development: the hatchling is a 'miniaturized' form of the adult.<ref name=Marin2004>{{Cite doi|10.1016/j.crpv.2004.07.009}}</ref>

==Taxonomy==
Opinions vary about the number of [[class (biology)|classes]] of molluscs – for example the table below shows eight living classes,<ref name="Haszprunar2001MolluscsInEncOfLifeSci">{{Cite book
| contribution=Mollusca (Molluscs) | author=Haszprunar, G.
| title=Encyclopedia of Life Sciences | year=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="Healy2001MolluscaInvertebrateZoology" /><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 | year=2004 | pages=291–292
}}</ref> 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">{{cite book
| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology
| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | year=2004 | pages=292–298
}}</ref> || chitons || align="center" | 1,000 || rocky tidal zone and seabed
|-
| [[Monoplacophora]]<ref name="RuppertFoxBarnes2004MolluscaMonoplacophora" /> || limpet-like organisms || align="center" | 31 || 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 | year=2004 | pages=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 | year=2004 | pages=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 | year=2004 | pages=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 | year=1998 | isbn=0632052384
| url=http://books.google.co.uk/books?id=g1P2VaPQWfUC&pg=PA221&dq=%22Invertebrate+Palaeontology+and+Evolution%22+rostroconchia#PPA221,M1
| accessdate=2008-10-27 | pages=221
}}</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==
===Fossil record===
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 | year=2007 | 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 | volume=286 }}</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><ref group=note>It is possible that ''Kimberella'' is more closely related to, for instance, the brachiopods than the molluscs</ref><ref name=Sigwart2007/> 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="Butterfield2006" /><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| doi = 10.1038/nature04894
| url = http://www.nature.com/nature/journal/v442/n7099/pdf/nature04894.pdf | accessdate = 2008-08-07
}}</ref> 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>

{| align="right"
|- valign="top"
|[[Image:Yochelcionella water flow.png| thumb | right | 120px | The tiny [[Helcionellid]] fossil ''[[Yochelcionella]]'' is thought to be an early [[mollusc]]<ref name="RunnegarPojeta1974" />]]
|[[Image:Neptunea despecta.jpg| thumb | right | 120px | Spirally-coiled shells are appear in many [[gastropod]]s<ref name="RuppertFoxBarnes2004MolluscaGastropoda" />]]
|}
However, the [[Helcionellid]]s, which first appear over {{ma|540}} in Early Cambrian rocks from [[Siberia]] and [[China]],<ref name="Parkhaev 2007">{{cite journal |author= P. Yu. Parkhaev |year=2007 |title=The Cambrian ‘basement’ of gastropod evolution |journal=Geological Society, London, Special Publications |volume=286 |pages=415–421 |dio= 10.1144/SP286.31 |url= http://books.google.com/books?id=GA7-8JIh9IwC&pg=PA415 |accessdate=2009-11-01}}</ref><ref name="Steiner2007">{{cite journal |author=Michael Steiner, Guoxiang Li, Yi Qian, Maoyan Zhu and Bernd-Dietrich Erdtmann |year=2007 |title= Neoproterozoic to early Cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the Yangtze Platform (China) |journal= Palaeogeography, Palaeoclimatology, Palaeoecology |volume=254 |pages=67–99 |doi= 10.1016/j.palaeo.2007.03.046}}</ref> are thought to be early molluscs with rather snail-like shells. Shelled molluscs therefore predate the earliest [[trilobite]]s.<ref name="RunnegarPojeta1974">{{ cite journal
| author=Runnegar, B. and Pojeta, J. | year=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
| pmid=17839855
}}</ref> Although most helcionellid fossils are only a few millimeters long, specimens a few centimeters long have also been found, most with more [[limpet]]-like shapes. There have been suggestions that the tiny specimens were juveniles and the larger ones adults.<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>

Some analyses of helcionellids concluded that these were the earliest [[gastropod]]s.<ref>{{cite doi|10.1666/0022-3360(2002)076<0287:LECSSF>2.0.CO;2}}</ref> However other scientists are not convinced that Early Cambrian fossils show clear signs of the [[Torsion (gastropod)|torsion]] that identifies modern gastropods twists the internal organs so that the anus lies above the head.<ref name="RuppertFoxBarnes2004MolluscaGastropoda" /><ref name="FrýdaNützelWagner2008">{{cite book|last=Frýda|first=J.|coauthors=Nützel, A. and Wagner, P.J.|title=Phylogeny and evolution of the Mollusca|editor=Ponder, W.F., and Lindberg, D.R.|pages=239–264|chapter=Paleozoic Gastropoda|publisher=California Press|year=2008|isbn=0520250923|url=http://books.google.co.uk/books?id=nm0IZAQQ6S0C&pg=RA1-PA239&dq=earliest+gastropod+fossil#v=onepage&q=earliest%20gastropod%20fossil&f=false|accessdate=4 Nov 2009}}</ref><ref>{{cite journal|last=Kouchinsky|first=A.|year=2000|title=Shell microstructures in Early Cambrian molluscs|journal=Acta Palaeontologica Polonica|volume=45|issue=2|pages=119–150|url=http://app.pan.pl/archive/published/app45/app45-119.pdf|accessdate=4 Nov 2009}}</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>{{Cite book
| author=Hagadorn, J.W., and Waggoner, B.M. | year= 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 cephalopod 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 | year= 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 today.<ref>{{cite web|url=http://www.thecephalopodpage.org/evolution.php|title=A Broad Brush History of the Cephalopoda|last=Monks|first=N.|accessdate=2009-03-21}}</ref>

The Early Cambrian fossils ''[[Fordilla]]'' and ''[[Pojetaia]]'' are regarded as [[bivalve]]s.<ref>{{cite journal
| author=Pojeta, J. | year=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
| year=2003
}}</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 | doi=10.1002/gj.1048
| pages=303–328
| year=2006
}}</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 | year=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">{{cite journal
| author=Sigwart, J.D., and Sutton, M.D. | date=October 2007
| title=Deep molluscan phylogeny: synthesis of palaeontological and neontological data
| pmc = 2274978
| 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><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="Sigwart2007"/> In fact some groups traditionally classifed as molluscs may have to be redefined as distinct but related.<ref name="GoloboffEtAl2009" />

Molluscs are generally regarded members of the [[Lophotrochozoa]],<ref name="SigwartSutton2007DeepMolluscanPhylogeny" /> a group defined by having [[trochophore]] larvae and, in the case of living [[Lophorata]], 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.

Because the relationships between the members of the family tree are uncertain, it difficult to identify the features inherited from the last common ancestor of all molluscs.<ref name="Henry2004">{{cite doi|10.1016/j.ydbio.2004.04.027}}</ref> For example it is uncertain whether the ancestral mollusc was [[metamerism (biology)|metameric]] (composed of repeating units) - if it was, that would suggest an origin from an [[annelid]]-like worm.<ref name=Jacobs2000>{{cite doi|10.1046/j.1525-142x.2000.00077.x}}</ref> Scientists disagree about this: Giribet and colleagues concluded in 2006 that the repetition of gills and of the foot's retractor muscles were later developments,
<ref name="GiribetOkusuEtAlMolluscsWithSeriallyRepeatedStructures" /> while in 2007 Sigwart concluded that the ancestral mollusc was metameric, and that it had a foot used for creeping and a "shell" that was mineralized.<ref name=Sigwart2007>{{cite doi|10.1098/rspb.2007.0701}}</ref> In one particular one branch of the family tree, the shell of [[conchifera]]ns is thought to have evolved from the [[spicule]]s (small spines) of [[aplacophora]]ns; however this is difficult to reconcile with the [[embryology|embryological]] origins of spicules.<ref name=Henry2004/>

The molluscan shell appears to have originated from a mucus coating, which eventually stiffened into a [[cuticle]]. This would have been impermeable and thus forced the development of more sophisticated respiratory apparatus in the form of gills.<ref name=RunnegarPojeta1974/> Eventually, the cuticle would have become mineralized,<ref name=RunnegarPojeta1974/> <!-- this mineralization may have happened one or many times,{{Verify source|date=March 2009}} but uses -->using the same genetic machinery (''[[engrailed (gene)|engrailed]]'') as most other bilaterian [[skeleton]]s.<ref name=Jacobs2000/> The first mollusc shell almost certainly was reinforced with the mineral [[aragonite]].<ref>{{cite doi|10.1126/science.1137284}}</ref>

The evolutionary relationships ''within'' the molluscs are also debated, and the diagrams below show two widely-supported reconstructions:
{| 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}}

However, an analysis in 2009 that used both [[morphology (biology)|morphological]] and [[molecular phylogenetics]] comparisons concluded that the molluscs are not monophyletic; in particular, that [[Scaphopoda]] and [[Bivalvia]] are both separate, monophyletic lineages unrelated to the remaining molluscan classes&nbsp;–in other words that the traditional phylum Mollusca is [[polyphyletic]], and that it can only be made monophyletic if scaphopods and bivalves are excluded.<ref name="GoloboffEtAl2009">{{cite journal
| author=Goloboff, P.A. et al. | date=April 2009
| title=Phylogenetic analysis of 73,060 taxa corroborates major eukaryotic groups
| journal=Cladistics | volume=25 | issue=3 | pages=211–230 | doi=10.1111/j.1096-0031.2009.00255.x
}}</ref>

==Relations 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|alt=]]
Molluscs, especially bivalves such as [[clam]]s and [[mussel]]s, have been an important food source since at least the advent 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 | issue=3 | date=February 2002 | pages=452–474
| doi=10.1080/00438240120107477
| volume=33
}}</ref> Other commonly eaten molluscs 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
| publisher=Elsevier Health Sciences | year=2000 | isbn=0443056277 | pages=370
}}</ref> In 2005, China accounted for 80% of the global mollusc catch, netting almost {{convert|11000000|t}}. 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> Some countries regulate importation and handling of molluscs and other [[seafood]], mainly to minimize the poison risk from [[toxin]]s that accumulate 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]] |alt=Photo of three circular metal cages in shallows, with docks, boathouses and palm trees in background]]
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="RuppertFoxBarnes2004MolluscaGastropoda" /><ref name="RuppertFoxBarnes2004MolluscaBivalvia" /> The best natural pearls are produced by [[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
| year=2006
}}</ref>

[[Image:Meister von San Vitale in Ravenna 004.jpg| thumb | right | 100px | [[Byzantine]] Emperor [[Justinian I]] clad in [[Tyrian purple]]|alt=Mosaic of mustachioed, curly-haired man wearing crown and surrounded by halo ]]
Other luxury and high-[[Social status|status]] products were 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. | year=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. | year=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. | year=1994 | title=The Minoan Origin of Tyrian Purple
| journal=Biblical Archaeologist | volume=57 | pages=46–54
| doi=10.2307/3210395
}}</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 | pages=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 | year=2003 | isbn=052154110
}} Particularly 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" />]]
When handled alive, a few species of molluscs can sting or bite, and for some species, this can present a serious risk to the human who is handling the animal. To put this into the 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 | year=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. Blue-ringed octopuses in the genus ''[[Blue-ringed octopus|Hapalochlaena]]'', which live around Australia and New Guinea, 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 | year=1999
| doi=10.1111/j.1600-0536.1999.tb06025.x
}}</ref>

[[Image:Textile cone.JPG| thumb | left| 100px | Live [[Cone shell|cone snail]]s can be dangerous to shell-collectors but are useful to [[neurology]] researchers<ref name="New Scientist19October1996DoctorSnail" />|alt=Photo of cone on ocean bottom]]
[[Cone snail]]s, carnivorous [[gastropod]]s that feed on marine invertebrates (and in the case of larger species on 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 require less time and energy to be produced 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 | accessdate=2008-10-03
| url=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
}}</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 larger species of cone snail that can capture and 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 | year=2003 | isbn=0811840506
| 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 | pages=10
}}</ref>

====Pests====
[[Image:Schistosomiasis itch.jpeg|thumb|right|[[Vesicle (dermatology)|Skin vesicles]] 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 | year=1994 | isbn=0748400265 | pages=305
}}</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 | year=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>

Despite its name, ''[[Molluscum contagiosum]]'' is a [[virus|viral]] disease, and is unrelated to 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>

==Notes==
<references group="note"/>
==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}}
{{Wikibooks|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]

{{Animalia}}

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

[[ar:رخويات]]
[[zh-min-nan:Nńg-thé tōng-bu̍t]]
[[bs:Mekušci]]
[[br:Blotvil]]
[[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:Moluska]]
[[ia:Mollusco]]
[[is:Lindýr]]
[[it:Mollusca]]
[[he:רכיכות]]
[[ka:მოლუსკები]]
[[la:Mollusca]]
[[lv:Moluski]]
[[lb:Weechdéieren]]
[[lt:Moliuskai]]
[[hu:Puhatestűek]]
[[mk:Мекотели]]
[[ms:Moluska]]
[[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:มอลลัสกา]]
[[tr:Yumuşakçalar]]
[[uk:Молюски]]
[[vi:Động vật thân mềm]]
[[zh:软体动物]]

Revision as of 16:59, 25 February 2010

hi Jake

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