Shell - Kaigara (English spelling) shell

Japanese: 貝殻 - かいがら（英語表記）shell

A hard structure secreted by mollusks from the mantle and used as their outer covering. It is mainly composed of calcium carbonate, but also contains unique proteins such as conchiolin (about 1%). From the outside, it is divided into three layers: the periosteal layer, the crustous layer (prism layer), and the sub-crustous layer (nacreous layer). The periosteal layer is chitinous and extremely thin. The crustous layer contains pigments and is the thickest. The sub-crustous layer is made of nacre with countless thin layers, and gives off a beautiful luster due to the interference of light. A pearl is formed when nacre is secreted around a foreign object such as a grain of sand that has entered the body. The shape of the shell varies widely; generally, bivalves have two dish-shaped shells on the left and right, gastropods have cup-shaped or spiral-shaped shells, and scutellar shells such as horn shells have horn-shaped shells with openings at both ends. Cephalopods also have equivalents to these, but they are invaginated into the body and are called carapaces or pens.

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

Japanese:

軟体動物が外套膜から分泌形成し，その外被としている硬い構造。炭酸カルシウムを主成分とするが，コンキオリンなど特有の蛋白質も含む (約1%) 。外表から順に殻皮層，殻質層 (角柱層) ，殻質下層 (真珠層) の3層に分けられる。殻皮層はキチン質できわめて薄い。殻質層は色素を含み，最も厚い。殻質下層は無数の薄層をもつ真珠質から成り，光線の干渉による美しい光沢を放つ。体内に侵入した砂粒などの異物のまわりに真珠質が分泌されると，真珠ができる。殻の形状は変化に富み，一般に二枚貝類では皿状の殻が左右2枚，腹足類では杯状または螺旋状，ツノガイなどの掘足類では両端で開口する角笛状となる。頭足類でもこれらに相当するものがあるが，体内に陥入して甲またはペンと呼ばれている。

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