Ostracod - Ostracod

Japanese: 貝虫類 - かいむしるい（英語表記）ostracod

A general term for crustaceans classified as Ostracoda, subclass of the Gnathopoda class. Commonly known as ostracods. The shell length is generally around 1mm. It is surrounded by two calcified shells, one on the left and one on the right, and has a bivalve-like appearance. The left and right shells are connected by a ligament along the dorsal midline, and open and close by the action of the adductor muscle. The body, enclosed inside the shell, has no clear distinction between the head and thorax, no clear body segments, and a small abdomen. They are dioecious, but males are generally rare, and most freshwater species reproduce by parthenogenesis. Approximately 2,500 species are known, about two-thirds of which are freshwater. They are classified into five orders, mainly based on the shape of the second antennae. They have survived from the Ordovician period of the Paleozoic era to the present day, and are important microfossils for comparing geological strata and understanding paleoenvironments. (→ Gnathopoda, Arthropods)

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

顎脚綱貝虫亜綱 Ostracodaとしてまとめられる甲殻類の総称。通称カイミジンコ類。殻長は一般に 1mm内外。左右 2枚の石灰化した殻に包まれ，外形は二枚貝状。左右の殻は背側の正中線で靭帯によって結ばれており，閉殻筋の働きで開閉する。殻の内部に閉じ込められた体は，頭部と胸部の区別，体節が明らかでなく，また腹部が小さい。雌雄異体であるが，一般に雄は少なく，淡水産種の多くは単為生殖によって増える。約 2500種が知られており，その 3分の2ほどは淡水産。おもに第2触角の形態によって 5目に分類されている。古生代オルドビス紀から今日まで生存し，地層の対比や古環境を知るうえで重要な微化石である。（→顎脚類，節足動物）

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