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"Clam" is an informal term used to refer to any molluscans within Class Bivalvia. Remains first appeared in Cambrian age rocks 510 million years ago.〔(【引用サイトリンク】title=Bivalves )〕 They presently live in both freshwater and marine habitats, and range in adult size from nearly microscopic to the giant clam, which can weigh . Some have life cycles of only one year, while at least one has been discovered that may be over 500 years old. They lack heads but most can react to changes in light and some, such as the scallops, have rudimentary eyes. Though a common food item, many are too small to be useful as food, and not all species are considered palatable. All clams have two calcareous shells or valves joined near a hinge structure with a flexible ligament, and all are filter feeders. == Terminology == 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,〔(【引用サイトリンク】title=Oxford Dictionaries – Dictionary, Thesaurus, & Grammar )〕 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. The word "clam" is used in the idiom "to clam up", meaning to refuse to talk or answer, based on the clam behaviour 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). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「clam」の詳細全文を読む スポンサード リンク
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