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Crocodiles (subfamily Crocodylinae) or true crocodiles are large aquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. Crocodylinae, all of whose members are considered true crocodiles, is classified as a biological subfamily. A broader sense of the term crocodile, Crocodylidae that includes ''Tomistoma'', is not used in this article. The term crocodile here applies only to the species within the subfamily of Crocodylinae. The term is sometimes used even more loosely to include all extant members of the order Crocodilia, which includes ''Tomistoma'', the alligators and caimans (family Alligatoridae), the gharials (family Gavialidae), and all other living and fossil Crocodylomorpha. Although they appear to be similar to the untrained eye, crocodiles, alligators and the gharial belong to separate biological families. The gharial having a narrow snout is easier to distinguish, while morphological differences are more difficult to spot in crocodiles and alligators. The most obvious external differences are visible in the head with crocodiles having narrower and longer heads, with a more V-shaped than a U-shaped snout compared to alligators and caimans. Another obvious trait is the upper and lower jaws of the crocodiles are the same width, and teeth in the lower jaw fall along the edge or outside the upper jaw when the mouth is closed; therefore all teeth are visible unlike an alligator; which possesses small depressions in the upper jaw where the lower teeth fit into. Also when the crocodile's mouth is closed, the large fourth tooth in the lower jaw fits into a constriction in the upper jaw. For hard-to-distinguish specimens, the protruding tooth is the most reliable feature to define the family that the species belongs to.〔(【引用サイトリンク】title=Crocodilian Biology Database - FAQ - What's the difference between a crocodile and an alligator )〕 Crocodiles have more webbing on the toes of the hind feet and can better tolerate saltwater due to specialized salt glands for filtering out salt, which are present but non-functioning in alligators. Another trait that separates crocodiles from other crocodilians is their much higher levels of aggression. Crocodile size, morphology, behavior and ecology somewhat differs between species. However, they have many similarities in these areas as well. All crocodiles are semiaquatic and tend to congregate in freshwater habitats such as rivers, lakes, wetlands and sometimes in brackish water and saltwater. They are carnivorous animals, feeding mostly on vertebrates such as fish, reptiles, birds and mammals, and sometimes on invertebrates such as molluscs and crustaceans, depending on species and age. All crocodiles are tropical species that unlike alligators, are very sensitive to cold. They first separated from other crocodilians during the Eocene epoch, about 55 million years ago. Many species are at the risk of extinction, some being classified as critically endangered. ==Etymology== The word "crocodile" comes from the Ancient Greek κροκόδιλος (''crocodilos''), "lizard," used in the phrase ''ho krokódilos tou potamoú'', "the lizard of the (Nile) river". There are several variant Greek forms of the word attested, including the later form κροκόδειλος (''crocodeilos'')〔http://perseus.mpiwg-berlin.mpg.de/cgi-bin/vor?lookup=krokodeilos&lang=greek〕 found cited in many English reference works.〔 In the Koine Greek of Roman times, ''crocodilos'' and ''crocodeilos'' would have been pronounced identically, and either or both may be the source of the Latinized form ''crocodīlus'' used by the ancient Romans. ''Crocodilos'' or ''crocodeilos'' is a compound of ''krokè'' ("pebbles"), and ''drilos/dreilos'' ("worm"), although ''drilos'' is only attested as a colloquial term for "penis".〔(【引用サイトリンク】title=Crocodile | Define Crocodile at Dictionary.com )〕 It is ascribed to Herodotus, and supposedly describes the basking habits of the Egyptian crocodile.〔(【引用サイトリンク】title=Online Etymology Dictionary )〕 The form ''crocodrillus'' is attested in Medieval Latin.〔 It is not clear whether this is a medieval corruption or derives from alternate Greco-Latin forms (late Greek ''corcodrillos'' and ''corcodrillion'' are attested). A (further) corrupted form ''cocodrille'' is found in Old French and was borrowed into Middle English as ''cocodril(le)''. The Modern English form ''crocodile'' was adapted directly from the Classical Latin ''crocodīlus'' in the 16th century, replacing the earlier form. The use of -y- in the scientific name ''Crocodylus'' (and forms derived from it) is a corruption introduced by Laurenti (1768). 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「crocodile」の詳細全文を読む スポンサード リンク
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