Paper nautilus (Aoi-gai) - Paper nautilus (English spelling)

Japanese: アオイガイ(葵貝) - アオイガイ（英語表記）paper nautilus

An octopus of the family Malvaceae (illustration) of the class Cephalopoda. When referring to the animal body, it is also called Kaidako, meaning an octopus with a shell. Females can reach a body length of 30 cm, and have a wide membrane on their first arm, from which they secrete a thin, plastic-like, white, boat-shaped shell that rolls loosely in one plane and forms the shape of a mallow leaf when the two are put together. Large ones can reach a length of about 25 cm. Females lay eggs inside this shell and raise their young. Males, which are only about 1/10 the body length of the female, do not secrete a shell. The male's left third arm is long and is initially enclosed in a pouch, but during mating, this breaks off and remains inside the female's mantle cavity, carrying the spermatophore (a sac that contains sperm).

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

頭足綱アオイガイ科のタコ（イラスト）。動物体のほうを指すときは殻をもったタコの意でカイダコともいう。雌は体長30cmに及び，第1腕に広い膜をもっていて，これから一平面に緩く巻き，二つ合わせるとアオイの葉形になるプラスチック製のような白くて薄い舟形の貝殻を分泌する。大きなものでは25cmくらいになる。雌はこの貝殻の中で産卵し，子を育てる。体長が雌の1/10くらいしかない雄は貝殻を分泌しない。雄の左第3腕は長くて，はじめ袋に包まれているが，交接に際しては，これが精莢（せいきよう）(精子を包むふくろ)を担ったまま雌の外套（がいとう）腔内に切れて残る。

出典　株式会社平凡社世界大百科事典第２版について　情報

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