External gills

Auxiliary respiratory organs found in amphibian larvae and some bony fish larvae. They are tufted or feathery gills that protrude from the body. In contrast, gills that are inside the gill slits and not exposed to the outside of the body, like the gills of normal fish, are called internal gills. External gills are found in fish such as the African Polypterus and the lungfish of Africa and South America, and appear red because they are rich in blood vessels. Caudata amphibians have three pairs of external gills, but some species remain as larvae with external gills throughout their lives. Anuran larvae initially have external gills, but as the gill cover forms, they are enclosed in the gill cavity and eventually degenerate.

[Sumio Takahashi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

両生類の幼生、一部の硬骨魚類の幼生にみられる補助的な呼吸器官。体外に突出した房状、羽状のえらのことをいう。これに対し、通常の魚類のえらのように、鰓裂内にあり体外に露出しないものを内鰓ともいう。外鰓は、魚類ではアフリカ産のポリプテルスPolypterusや、アフリカや南アメリカの肺魚にみられ、血管に富むために赤くみえる。両生類の有尾類は3対の外鰓をもつが、一生の間、外鰓をもった幼生のままの姿で過ごす種類もある。無尾類の幼生は、初め外鰓をもつが、鰓蓋(さいがい)の形成につれて鰓腔(さいこう)内に包み込まれ、やがて退化してしまう。

［高橋純夫］

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