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The '''mollusks''' or '''molluscs''' are the large and diverse ] '''Mollusca''', which includes a variety of familiar ]s well-known for their decorative shells or as ]. These range from tiny ]s, ]s, and ] to ], ] and the ] (which is considered the most intelligent ]). There are some 112,000 ] within this phylum.
{{Taxobox
| color = pink
| name = Mollusks
| image = Caribbean reef squid.jpg
| image_width = 200px
| image_caption = ], ''Sepioteuthis sepioidea''
| regnum = ]ia
| phylum = '''Mollusca'''
| phylum_authority = ], 1758
| subdivision_ranks = ]es
| subdivision =
]<br/>
]<br/>
]<br/>
]<br/>
]<br/>
]<br/>
]<br/>
]<br/>
† ]<br/>
† ]
}}
The '''mollusks''' or '''molluscs''' are the large and diverse ] '''Mollusca''', which includes a variety of familiar ]s well-known for their decorative shells or as ]. These range from tiny ]s, ]s, and ] to ], ] and the ] (which is considered the most intelligent ]). There are some 112,000 ] within this phylum.<ref>Feldkamp, Susan (2002). Modern Biology. United States: Holt, Rinehart, and Winston. pp. 725</ref>

The ], which until recently had not been observed alive in its adult form, is the largest invertebrate; although it is possible that the ] is even larger. The scientific study of mollusks is called '''malacology'''.

==Anatomy==
Mollusks are ] ]s. The principal ] is a blood-filled ]. They have a true ] (eucoelom); any coelomic cavities have been reduced to vestiges around the ]s, ]s, and ] (]-like organs). The body is often divided into a head, with eyes or tentacles, a muscular foot and a ] mass housing the organs.

Mollusks have a mantle, which is a fold of the outer skin lining the shell, and a muscular foot that is used for motion. Many mollusks have their mantle produce a ] external shell and their ] extracts ] from the water and disposes waste. All species of the phylum Mollusca have a complete ] that starts from the mouth to the ]. Many have a feeding structure, the ], mostly composed of ]. Radulae are diverse within the Mollusca, ranging from structures used to scrape ] off rocks, to the harpoon-like structures of ]s. ] (], ], ]) also possess a chitinous beak. Unlike the closely related ]s, mollusks lack body segmentation.

Development passes through one or two ] stages, one of which (the ]) is unique to the group. These suggest a close relationship between the mollusks and various other protostomes, notably the ]s.

Mollusk fossils are some of the best known and are found from the ] onwards. The oldest fossil seems to be ''] omalus'', found in the ]. It lived about 500 millions years ago.

==Classification==
There are nine ] of mollusks, eight still living and one known only from fossils. These classes make up the 250,000 and more species of mollusk:

* Class ] (deep-sea wormlike creatures; 70 known species); now generally recognized as a subclass of Aplacophora.
* Class ] (solenogasters, deep-sea wormlike creatures; 250 species)
* Class ] (chitons; 600 species, rocky marine shorelines)
* Class ] (deep-sea limpet-like creatures; 11 living species)
* Class ] (also Pelecypoda) (]s, ]s, ]s, ]s; 8,000 species)
* Class ] (tusk shells; 350 species, all marine)
* Class ] (]s, ]s and ]s, ]s, ]s; ], ], ]; estimated 40,000 - 150,000 species)
* Class ]a (], ], ], ]; 786 species, all marine)
* Class † ] (fossils; probably more than 1,000 species; probable ancestors of bivalves)
* Class † ] (fossils; snail-like creatures such as '']'')

Main article: '']''

<div align="center"><center>
{| border="0" cellpadding="2" cellspacing="0"
|-
|bgcolor="#BBBBBB"|&nbsp;
|bgcolor="#AAAAAA"|&nbsp;
|bgcolor="#AAAAAA"|&nbsp;
|bgcolor="#AAAAAA"|&nbsp;
|bgcolor="#AAAAAA"|&nbsp;&nbsp;
|align="right" bgcolor="#AAAAAA"|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Caudofoveata (?)
|-
|bgcolor="#BBBBBB"|&nbsp;
|bgcolor="#EEEEEE"|&nbsp;
|bgcolor="#EEEEEE"|&nbsp;
|bgcolor="#EEEEEE"|&nbsp;
|bgcolor="#EEEEEE"|&nbsp;&nbsp;
|align="right" bgcolor="#EEEEEE"|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Aplacophora
|-
|bgcolor="#BBBBBB"|hypothetical
|bgcolor="#CCCCCC"|&nbsp; &nbsp; &nbsp;&nbsp;
|bgcolor="#AAAAAA"|&nbsp;
|bgcolor="#AAAAAA"|&nbsp;
|bgcolor="#AAAAAA"|&nbsp;&nbsp;
|align="right" bgcolor="#AAAAAA"|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Polyplacophora
|-
|bgcolor="#BBBBBB"|ancestral
|bgcolor="#CCCCCC"|&nbsp;
|bgcolor="#DDDDDD"|&nbsp;
|bgcolor="#DDDDDD"|&nbsp;
|bgcolor="#EEEEEE"|&nbsp;&nbsp;
|align="right" bgcolor="#EEEEEE"|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Monoplacophora
|-
|bgcolor="#BBBBBB"|mollusk
|bgcolor="#CCCCCC"|&nbsp;
|bgcolor="#DDDDDD"|&nbsp;
|bgcolor="#DDDDDD"|&nbsp;
|bgcolor="#CCCCCC"|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|align="right" bgcolor="#AAAAAA"|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Gastropoda
|-
|bgcolor="#BBBBBB"|&nbsp;
|bgcolor="#CCCCCC"|&nbsp;
|bgcolor="#DDDDDD"|&nbsp;
|bgcolor="#DDDDDD"|&nbsp;
|bgcolor="#CCCCCC"|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|align="right" bgcolor="#EEEEEE"|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Cephalopoda
|-
|bgcolor="#BBBBBB"|&nbsp;
|bgcolor="#CCCCCC"|&nbsp;
|bgcolor="#DDDDDD"|&nbsp;
|bgcolor="#DDDDDD"|&nbsp;
|bgcolor="#E8E8E8"|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|align="right" bgcolor="#AAAAAA"|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Bivalvia
|-
|bgcolor="#BBBBBB"|&nbsp;
|bgcolor="#CCCCCC"|&nbsp;
|bgcolor="#DDDDDD"|&nbsp;
|bgcolor="#DDDDDD"|&nbsp;
|bgcolor="#E8E8E8"|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
|align="right" bgcolor="#EEEEEE"|&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Scaphopoda
|}
</center>
</div>

Brusca & Brusca (1990) suggest that the bivalves and scaphopods are sister groups, as are the gastropods and cephalopods, so indicated in the relationship diagram above.

In this phylum's level of organization, organ systems from all three primary ]s can be found:

# Nervous System (with brain).
# Excretory System (] or nephridia).
# Circulatory System (open circulatory system).
# Respiratory System (]s or ]s).

All major molluscan groups possess a skeleton, though it has been lost evolutionarily in some members of the phylum. It is probable that the ] ancestor of the mollusks had calcium carbonate spicules embedded in its mantle and outer tissues, as is the case in some modern members. The skeleton, if present, is primarily external and composed of calcium carbonate (aragonite or calcite). The snail or gastropod shell is perhaps the best known molluscan shell, but many pulmonate and opistrobranch snails have internalized or altogether lost the shell secondarily. The bivalve or clam shell consists of two pieces (valves), articulated by muscles and an elastic hinge. The cephalopod shell was ancestrally external and chambered, as exemplified by the ammonites and nautiloids, and still possessed by ''Nautilus'' today. Other cephalopods, such as cuttlefish, have internalized the shell, the squid have mostly organic chitinous internal shells, and the octopods have lost the shell altogether.

==See also==
{{sisterlinks|Mollusk}}
{{Wikispecies|Mollusca}}
{{Wikibookspar|Dichotomous Key|Mollusca}}
* ]

==References==
<references/>
* {{cite book|author = Brusca & Brusca|year = 1990|title = Invertebrates|location= Sunderland, Mass. | publisher=Sinauer Associates}}
* {{cite book|author = Starr & Taggart|year = 2002|title = Biology: The Unity and Diversity of Life|location= Pacific Grove, California | publisher=Thomson Learning}}

==External links==
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Revision as of 00:56, 7 September 2006

The mollusks or molluscs are the large and diverse phylum Mollusca, which includes a variety of familiar animals well-known for their decorative shells or as seafood. These range from tiny snails, clams, and abalone to squid, cuttlefish and the octopus (which is considered the most intelligent invertebrate). There are some 112,000 species within this phylum.