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Sharks are a group of fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the rays. However, the term "shark" has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as ''Cladoselache'' and ''Xenacanthus'', as well as other Chondrichthyes such as the holocephalid eugenedontidans. Under this broader definition, the earliest known sharks date back to more than 420 million years ago. Acanthodians are often referred to as "spiny sharks"; though they are not part of Chondrichthyes proper, they are a paraphyletic assemblage leading to cartilaginous fish as a whole. Since then, sharks have diversified into over 500 species. They range in size from the small dwarf lanternshark (''Etmopterus perryi''), a deep sea species of only in length, to the whale shark (''Rhincodon typus''), the largest fish in the world, which reaches approximately in length. Sharks are found in all seas and are common to depths of . They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can survive and be found in both seawater and freshwater. Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They have several sets of replaceable teeth. Well-known species such as the great white shark, tiger shark, blue shark, mako shark, and the hammerhead shark are apex predators—organisms at the top of their underwater food chain. Many shark populations are threatened by human activities. ==Etymology== Until the 16th century,〔(【引用サイトリンク】title=Online Etymology Dictionary )〕 sharks were known to mariners as "sea dogs". The etymology of the word "shark" is uncertain. One theory is that it derives from the Yucatec Maya word ''xok'', pronounced 'shok'. Evidence for this etymology comes from the Oxford English Dictionary, which notes ''shark'' first came into use after Sir John Hawkins' sailors exhibited one in London in 1569 and posted "''sharke''" to refer to the large sharks of the Caribbean Sea. However, the Middle English Dictionary records an isolated occurrence of the word ''shark'' (referring to a sea fish) in a letter written by Thomas Beckington in 1442, which rules out a New World etymology.〔(【引用サイトリンク】url=http://quod.lib.umich.edu/cgi/m/mec/med-idx?type=id&id=MED39794&egs=all&egdisplay=open )〕 An alternate etymology states that the original sense of the word was that of "predator, one who preys on others" from the Dutch ''schurk'', meaning "villain, scoundrel" (cf. ''card shark'', ''loan shark'', etc.), which was later applied to the fish due to its predatory behaviour.〔Online Etymology Dictionary, shark.〕 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「shark」の詳細全文を読む スポンサード リンク
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