Gastropods - Gastropoda

Japanese: 腹足類 - ふくそくるい（英語表記）Gastropoda

A general term for shells in the class Gastropoda of the phylum Mollusca. Traditionally, they were classified into three subclasses, Prosopagniformes, Opisthobranchia, and Pulmonata, based on the position and presence of gills, but in recent years, Opisthobranchia and Pulmonata have been combined and called Heterogniformes. The term refers to so-called gastropod shells, which usually have shells that roll up on their backs (such as abalone, turban shells, and turbans), but also includes limpets with cap-shaped shells, sea slugs that have lost their shells (such as the blue sea slug and the spotted sea slug), and slugs. Most shells are right-handed, but some, such as the clausilis and the left-handed snail, are left-handed. The head has a pair of antennae and eyes, and a mouth at the front end. The visceral sac is inside the shell, and the mantle cavity has a pair of gills, but pulmonates lack gills and the inner wall of the mantle acts as a lung and replaces the gills. The nerves are twisted and cross in a figure eight shape. This is because the body twists 180 degrees during development, a striking feature of gastropods, but in sea slugs, the twist is reversed. The feet are large with flat soles adapted for crawling. There is a flap on the rear dorsal side of the foot that covers the opening of the shell when the body is retracted into the shell, but this is lacking in pulmonates. Most are marine, but they also live on land and in freshwater. There are approximately 35,000 extant species worldwide, with 4,000 species in Japan.

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

Japanese:

軟体動物門腹足綱の貝の総称。従来は鰓の位置・有無から前鰓類，後鰓類，有肺類の3亜綱に分類されていたが，近年後鰓類と有肺類を合せて異鰓類と呼ぶ傾向がある。いわゆる巻貝類で，通常背上に巻いた殻をもつもの (アワビ，サザエ，バイなど) をいうが，笠形の殻をもつカサガイや，殻を失ったウミウシ類 (アオウミウシ，マダラウミウシなど) ，ナメクジなども含まれる。殻はほとんどのものは右巻きであるが，キセルガイ，ヒダリマキマイマイのように左巻きのものもある。頭には1対の触角と眼があり，前端に口がある。内臓嚢は殻の中にあり，外套腔には1対の鰓があるが，有肺類では鰓を欠き，外套膜の内壁が肺の作用をし，鰓に取替っている。神経はねじれて8字形に交差している。これは発生途中で体が 180°ねじれるためで，腹足類の著しい特徴となっているが，ウミウシの仲間などではねじれが戻っている。足は大きく，足裏が平らではうのに適している。足の後方背側に，体を殻内に縮めたとき殻の口をふさぐふたがあるが，有肺類ではこれを欠く。多くは海産であるが，陸，淡水にもすむ。現生種は世界でおよそ3万 5000種，日本に 4000種を産する。

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