Mantle - Very good

In mollusks and brachiopods, this refers to the membranous body surface that spreads over the visceral pouch. It is also called the mantle, and the shell is secreted from it. The difference in the structure of the mantle gives each species its unique shell shape. In primitive polyplacophora (chitons) among mollusks, it forms a single elliptical plate that is located on the dorsal side of the visceral pouch and secretes eight shell plates. In bivalves (axopods), two mantle membranes on the left and right secrete two shells to encase the body. In gastropods, it spreads cylindrically from the surface of the visceral pouch to the dorsal side of the foot, and creates a spiral shell according to the twist of the visceral pouch. In scutellar shells (horn shells), a cone-shaped shell with an open top is formed, and a horn-shaped shell is formed. Cephalopods have one mantle, which is cone-shaped in squids and pouch-shaped in octopuses, and the shell (also called the pen) is secreted into a pouch in the invaginated epidermis. On the other hand, brachiopods such as the oyster shell have one mantle extending dorsally and one ventrally, which secretes the shell, making them different from bivalves.

[Takeo Machida]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

軟体動物および腕足動物において、内臓嚢(のう)を覆うように広がった膜状の体表をいう。外套ともいい、これから貝殻が分泌される。外套膜の構造の違いにより、それぞれの種に特有の貝殻の形ができる。軟体動物のなかでも原始的な多板類（ヒザラガイ類）では、1枚の楕円(だえん)板状をなし、内臓嚢の背側にあって8枚の殻板を分泌する。二枚貝類（斧足(おのあし)類）では、左右2枚の外套膜が2枚の貝殻を分泌し体を包む。腹足類では内臓嚢の表面から足の背方に円筒状に広がり、内臓嚢のねじれに従って螺旋(らせん)状の貝殻をつくる。掘足(くっそく)類（ツノガイ類）では上端の開いた円錐(えんすい)形で、角笛状の貝殻ができる。頭足類の外套膜は1枚で、イカ類では円錐形、タコ類では嚢状をなし、表皮の陥入した嚢中に甲（ペンともいう）が分泌される。一方、シャミセンガイなど腕足動物では、背腹に1枚ずつの外套膜が伸び、貝殻を分泌するので、貝殻は背腹2枚となり、二枚貝類とは異なる。

［町田武生］

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