vinita singh moll.docx

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The molluscsor mollusks[note 1]

/mlsks/,compose the largephylumofinvertebrateanimals known as

the phylum Mollusca. Around 85,000extantspeciesof molluscs are recognized. Molluscs are the

largestmarinephylum, comprising about 23% of all the named marineorganisms.Numerous molluscs

also live infreshwaterand terrestrialhabitats.They are highly diverse, not only in size and

inanatomicalstructure, but also in behaviour and in habitat. Thephylumis typically divided into 9 or

10taxonomicclasses,of which two are entirelyextinct.Cephalopodmolluscs, such

assquid,cuttlefishandoctopus,are among the mostneurologically advancedof allinvertebratesand

either thegiant squidor thecolossal squidis the largest known invertebrate species.

Thegastropods(snailsandslugs)are by far the most numerous molluscs in terms of classifiedspecies,

and account for 80% of the total. The scientific study of molluscs is called malacology.[1]

Mollusc species

can also represent hazards or pests for human activities. The bite of the blue-ringed octopusis often fatal,

and that ofOctopus apollyoncausesinflammationthat can last for over a month. Stings from a few

species of large tropicalcone shellscan also kill, but their sophisticated, though easily produced, venoms

have become important tools inneurologicalresearch.Schistosomiasis(also known as bilharzia,

bilharziosis or snail fever) is transmitted to humans via water snail hosts, and affects about 200 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 someecosystems.

Mantle and mantle cavity[edit]

The mantle cavity, a fold in the mantle, encloses a significant amount of space. It is lined with epidermis,

and is exposed, according tohabitat,to sea, fresh water or air. The cavity was at the rear in the earliest

molluscs, but its position now varies from group to group. Theanus,a pair ofosphradia(chemical

sensors) in the incoming "lane", the hindmost pair ofgillsand the exit openings of

thenephridia("kidneys") andgonads(reproductive organs) are in the mantle cavity.[8]

The whole soft

body of bivalves lies within an enlarged mantle cavity.[6]

Shell[edit]

Main article:Mollusc shell

The mantle edge secretes a shell (secondarily absent in a number of taxonomic groups, such as

thenudibranchs[6]

)that consists of mainlychitinandconchiolin(aproteinhardened withcalcium

carbonate),[8][29]

except the outermost layer in almost all cases is all conchiolin

(seeperiostracum).[8]

Molluscs never use phosphate to construct their hard parts,[30]

with the questionable

exception ofCobcrephora.[31]

While most mollusc shells are composed mainly ofaragonite,those

gastropods that lay eggs with a hard shell usecalcite(sometimes with traces of aragonite) to construct

the eggshells.[32]

The shell consists of three layers: the outer layer (theperiostracum)made of organic matter, a middle

layer made of columnarcalcite,and an inner layer consisting of laminated calcite, oftennacreous.[6]

Foot[edit]The underside consists of a muscular foot, which has adapted to different purposes in different

classes.[33]:4

The foot carries a pair ofstatocysts,which act as balance sensors. In gastropods, it

secretesmucusas a lubricant to aid movement. In forms having only a top shell, such as limpets,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 .[8]

In
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bivalves, the foot is adapted for burrowing into the sediment;[33]:4

in cephalopods it is used for jet

propulsion,[33]:4

and the tentacles and arms are derived from the foot.[34]

Circulation[edit]

Molluscs'circulatory systemsare mainlyopen.Although molluscs arecoelomates,theircoelomsare

reduced to fairly small spaces enclosing theheartand gonads. The main body cavity is

ahemocoelthrough whichbloodand coelomic fluid circulate and which encloses most of the other internal

organs. These hemocoelic spaces act as an efficienthydrostaticskeleton.[6]

The blood contains

therespiratory pigmenthemocyaninas anoxygen-carrier. The heart consists of one or more pairs of atria

(auricles), which receive oxygenated blood from the gills and pump it to the ventricle,which pumps it into

theaorta(mainartery), which is fairly short and opens into the hemocoel.[8]

The atria of the heart also function as part of the excretory systemby filtering waste products out of the

blood and dumping it into the coleom asurine.A pair ofnephridia("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.[8]

Respiration[edit]

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 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 possiblysedimententering 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.[8]

Eating, digestion, and excretion[edit]

Most molluscs have muscular mouths withradulae,"tongues", bearing many rows of chitinous teeth, which are replaced

from the rear as they wear out. The radula primarily functions to scrapebacteriaandalgaeoff rocks, and is associated with

theodontophore,a cartilaginous supporting organ.[6]The radula is unique to the molluscs and has no equivalent in any other

animal.

Molluscs' mouths also containglandsthat secrete slimymucus,to which the food sticks. Beatingcilia(tiny "hairs") drive the

mucus towards the stomach, so the mucus forms a long string called a "food string" .[8]

At the tapered rear end of the stomach and projecting slightly into the hindgut is the prostyle, a backward -pointing cone

offecesand mucus, which is rotated by further cilia so 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 .[8]

The particles are sorted by yet another group of cilia, which send the smaller particles, mainly minerals, to the prostyle so

eventually they are excreted, while the larger ones, mainly food, are sent to the stomach's cecum(a pouch with no otherexit) to be digested. The sorting process is by no means perfect.

[8]

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, in the part of the mantle cavity, is swept by the outgoing "lane" of the current created by

the gills. Carnivorous molluscs usually have simpler digestive systems.[8]

As the head has largely disappeared in bivalves, the mouth has been equipped with labial palps (two on each side of the

mouth) to collect thedetritusfrom its mucus.[6]
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ikipedia.org/wiki/Mucushttp://en.wikipedia.org/wiki/Mucushttp://en.wikipedia.org/wiki/Ciliahttp://en.wikipedia.org/wiki/Ciliahttp://en.wikipedia.org/wiki/Ciliahttp://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Feceshttp://en.wikipedia.org/wiki/Feceshttp://en.wikipedia.org/wiki/Feceshttp://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Cecumhttp://en.wikipedia.org/wiki/Cecumhttp://en.wikipedia.org/wiki/Cecumhttp://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Detritushttp://en.wikipedia.org/wiki/Detritushttp://en.wikipedia.org/wiki/Detritushttp://en.wikipedia.org/wiki/Mollusca#cite_note-Handbook-7http://en.wikipedia.org/wiki/Mollusca#cite_note-Handbook-7http://en.wikipedia.org/wiki/Mollusca#cite_note-Handbook-7http://en.wikipedia.org/wiki/Mollusca#cite_note-Handbook-7http://en.wikipedia.org/wiki/Detritushttp://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Cecumhttp://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Feceshttp://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Ciliahttp://en.wikipedia.org/wiki/Mucushttp://en.wikipedia.org/wiki/Glandhttp://en.wikipedia.org/wiki/Mollusca#cite_note-Handbook-7http://en.wikipedia.org/wiki/Odontophorehttp://en.wikipedia.org/wiki/Algaehttp://en.wikipedia.org/wiki/Bacteriahttp://en.wikipedia.org/wiki/Radulaehttp://en.wikipedia.org/w/index.php?title=Mollusca&action=edit&section=10http://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Sedimenthttp://en.wikipedia.org/w/index.php?title=Mollusca&action=edit&section=9http://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Nephridiahttp://en.wikipedia.org/wiki/Urinehttp://en.wikipedia.org/wiki/Excretory_systemhttp://en.wikipedia.org/wiki/Mollusca#cite_note-RuppertFoxBarnes2004MolluscaGen-9http://en.wikipedia.org/wiki/Arteryhttp://en.wikipedia.org/wiki/Aortahttp://en.wikipedia.org/wiki/Ventricle_(heart)http://en.wikipedia.org/wiki/Atrium_(heart)http://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Hemocyaninhttp://en.wikipedia.org/wiki/Respiratory_pigmenthttp://en.wikipedia.org/wiki/Mollusca#cite_note-Handbook-7http://en.wikipedia.org/wiki/Hydrostatichttp://en.wikipedia.org/wiki/Bloodhttp://en.wikipedia.org/wiki/Hemocoelhttp://en.wikipedia.org/wiki/Hearthttp://en.wikipedia.org/wiki/Coelomhttp://en.wikipedia.org/wiki/Coelomatehttp://en.wikipedia.org/wiki/Open_circulatory_systemhttp://en.wikipedia.org/wiki/Circulatory_systemhttp://en.wikipedia.org/w/index.php?title=Mollusca&action=edit&section=8http://en.wikipedia.org/wiki/Mollusca#cite_note-35http://en.wikipedia.org/wiki/Mollusca#cite_note-mollusca11-34http://en.wikipedia.org/wiki/Mollusca#cite_note-mollusca11-34


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