Gill cover

…In cartilaginous fishes, there are 5-7 pairs of gill slits, but the gill septum between the two rows of gill flaps on each gill arch extends long enough to reach the surface of the body, so each gill slit opens separately to the surface of the body through an external gill opening. In bony fishes, there are usually 5 pairs of gill slits, but the gill septum retracts and becomes detached from the surface of the body, and a gill cover develops on the surface of the body to protect the gills, so all the gill slits open to the outside of the body through a single common external gill opening. The gill flaps are thin and leaf-like, and each gill flap has numerous fold-like projections (secondary gill flaps) on both sides. …

*Some of the terminology that mentions "gill cover" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…軟骨魚類では鰓裂は5～7対あるが，各鰓弓に2列に並ぶ鰓弁の間に介在する鰓隔膜が長く延長して体表に達するため，各鰓裂は別々に外鰓孔によって体表へ開口する。硬骨魚類では鰓裂はふつう5対あるが，鰓隔膜が退縮して体表から遊離し，体表にはえらを保護する鰓蓋gill coverが発達するので，全鰓裂は1個の共通の外鰓孔を通し体外へ開く。鰓弁は葉状で薄く，それぞれの鰓弁は両側に多数のひだ状の突起(二次鰓弁)を備える。…

※「gill cover」について言及している用語解説の一部を掲載しています。

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