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The chambered nautilus, ''Nautilus pompilius'' also called pearly nautilus, is the best-known species of nautilus. The shell, when cut away, reveals a lining of lustrous nacre and displays a nearly perfect equiangular spiral, although it is not a golden spiral. The shell exhibits countershading, being light on the bottom and dark on top. This is to help avoid predators, because when seen from above, it blends in with the darkness of the sea, and when seen from below, it blends in with the light coming from above. The eyes of the chambered nautilus are more primitive than those of some other cephalopods; the eye has no lens and thus is comparable to a pinhole camera. The species has about 90 tentacles with no suckers, which is also different from other cephalopods. Chambered nautiluses have a pair of rhinophores, which detect chemicals, and use olfaction and chemotaxis to find their food. The oldest fossils of the species are known from Early Pleistocene sediments deposited off the coast of Luzon in the Philippines. Recently, scientists have become alarmed at declining populations of nautilus resulting from overfishing, and are studying world populations to determine the need for protection under the United Nations Convention on International Trade in Endangered Species. ==Subspecies== Two subspecies of ''N. pompilius'' have been described: ''N. p. pompilius'' and ''N. p. suluensis'' ''N. p. pompilius'' is by far the most common and widespread of all nautiluses. It is sometimes called the emperor nautilus due to its large size. The distribution of ''N. p. pompilius'' covers the Andaman Sea east to Fiji and southern Japan south to the Great Barrier Reef. Exceptionally large specimens with shell diameters up to 〔(''Nautilus repertus'' ID:118764 ). Shell Encyclopedia, Conchology, Inc.〕〔Harasewych, M.G. & F. Moretzsohn (2010). ''The Book of Shells: A lifesize guide to identifying and classifying six hundred shells''. A & C Black Publishers, London.〕 have been recorded from Indonesia and northern Australia. This giant form was described as ''Nautilus repertus'', but most scientists do not consider it a separate species. ''N. p. suluensis'' is a much smaller animal, restricted to the Sulu Sea in the southwestern Philippines, after which it is named. The largest known specimen measured 160 mm in shell diameter.〔(''Nautilus pompilius suluensis'' ID:626793 ). Shell Encyclopedia, Conchology, Inc.〕 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「Chambered nautilus」の詳細全文を読む スポンサード リンク
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