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|Edible clams in the family Veneridae|
Clams, like most molluscs, also have open circulatory systems, which means that their organs are surrounded by watery blood that contains nutrients and oxygen. They feed on plankton by filter feeding. Clams filter feed by drawing in water containing food using an incurrent siphon. The food is then filtered out of the water by the gills and swept toward the mouth on a layer of mucus. The water is then expelled from the animal by an ex-current siphon.
In the United States, the word "clam" has several different meanings. First, it can generally refer to all bivalve molluscs. In the more limited sense, the term refers to the large subset of bivalves living as infauna, rather than those attached to a substrate (like oysters and mussels) or those that lie and move near the bottom or swim (like scallops). It can also refer to one or more kinds of commonly consumed marine bivalves, such as in the phrase clam chowder, which refers to shellfish soup. Many edible clams are roughly oval-shaped or triangular; however, razor clams have an elongated, parallel-sided shell, suggesting an old-fashioned straight razor.
In the United Kingdom, "clam" is one of the common names of various species of marine bivalve mollusc, but it is not used as a term covering either edible clams that burrow or bivalves in general.
Numerous edible marine bivalve species live buried in sand or mud and respire by means of siphons, which reach to the surface. In the United States, these clams are collected by "digging for clams" or clam digging.
In October 2007 an Arctica islandica clam, caught off the coast of Iceland, was found to be at least 405 years old and declared the world's oldest living animal by researchers from Bangor University. It was later named Ming. In 2013 more accurate measurements by the same researchers showed the age to be 507 years. 
Some species of bivalves are too small to be useful for food, and not all species are considered palatable.
The word "clam" is used in the metaphor "to clam up," meaning to refuse to talk or answer, based on the clam behavior of quickly closing the shell when threatened. A "clamshell" is the name given to a container or mobile phone consisting of two hinged halves that lock together. Clams have also inspired the phrase "happy as a clam," short for "happy as a clam at high tide" (when it can't easily be dug up and eaten).
A clam's shell consists of two (usually equal) halves, which are connected by a hinge joint and a ligament which can be external or internal.
In clams, two adductor muscles contract to close the shells. The clam has no head or eyes, though scallops are an exception of this rule. Clams do have kidneys, a heart, a mouth, and an anus.
Clams begin as a shellfish the size of a grain of sand when born. It has a natural glue on it that causes it to connect to other shells or things at the bottom of the river. Once a clam is secure, it feeds on the plankton, as stated, and moves with the tide. It takes a clam 24-30 months to become harvestable.
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In culinary use, within the eastern coast of the United States, the term "clam" most often refers to the hard clam Mercenaria mercenaria. It may also refer to a few other common edible species, such as the soft-shell clam, Mya arenaria, and the ocean quahog, Arctica islandica. Another species which is commercially exploited on the Atlantic Coast of the United States is the surf clam Spisula solidissima.
In Italy, clams are often an ingredient of mixed seafood dishes, or are eaten together with pasta. The more commonly used varieties of clams in Italian cooking are the Vongola (Venerupis decussata), the Cozza (Mytilus galloprovincialis) and the Tellina (Donax trunculus). Though Dattero di mare (Lithophaga lithophaga) was once eaten, overfishing drove it to the verge of extinction (it takes 15 to 35 years to reach adult size and could only be harvested by smashing the calcarean rocks that form its habitat) and the Italian government has declared it an endangered species since 1998 and its harvest and sale are forbidden.
Trinidad and Tobago
Clams or shellfish are locally called chipchip and local fishermen sell those in rural markets.
- Grooved carpet shell: Ruditapes decussatus
- Hard clam or Northern Quahog: Mercenaria mercenaria
- Manila clam: Venerupis philippinarum
- Soft clam: Mya arenaria
- Atlantic surf clam: Spisula solidissima
- Ocean quahog: Arctica islandica
- Pacific razor clam: Siliqua patula
- Pismo clam: Tivela stultorum (8 inch shell on display at the Pismo Beach Chamber of Commerce)
- Geoduck: Panopea abrupta or Panope generosa (largest burrowing clam in the world)
- Atlantic jackknife clam: Ensis directus
- lyrate Asiatic hard clam: Meretrix lyrata
Not usually considered edible:
- Ark clams, family Arcidae
- Nut clams or pointed nut clams, family Nuculidae
- Duck clams or trough shells, family Mactridae
- Marsh clams, family Corbiculidae
- File clams, family Limidae
- Giant clam: Tridacna gigas
- Asian or Asiatic clam: genus Corbicula
- Peppery furrow shell: Scrobicularia plana
- Pearl oysters: genus Pinctada
- Compact Oxford English Dictionary
- . CBS News http://www.cbsnews.com/8301-205_162-57612409/ming-the-clam-worlds-oldest-animal-was-actually-507-years-old/. Retrieved 15 November 2013. Missing or empty
- "clam up - Idioms - by the Free Dictionary, Thesaurus and Encyclopedia". The Free Dictionary. Farlex. Retrieved September 19, 2011.
- "happy as a clam - Idioms - by the Free Dictionary, Thesaurus and Encyclopedia". The Free Dictionary. Farlex. Retrieved September 19, 2011.
- Berrin, Katherine & Larco Museum. The Spirit of Ancient Peru:Treasures from the Museo Arqueológico Rafael Larco Herrera. New York: Thames and Hudson, 1997.
- Kurlansky, Mark (2006), The Big Oyster: History on the Half Shell, Penguin Group, pp. 16,30–31, ISBN 0-345-47638-7, OCLC 60550567.
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