Cover plate

…The seventh limb is small and located behind the mouth, and is called the labial limb. Of the six pairs of leaf-like abdominal limbs, the first is fused to form a single plate, which covers all the rear abdominal limbs and is called the operculum. A pair of genital pores is opened on the dorsal surface of its base. The remaining five pairs of abdominal limbs are two-branched swimming legs, and on the rear surface of the appendages is a respiratory organ made of many overlapping thin plates. This respiratory organ is called the gill book and is very similar in structure to the book lungs of spiders. …

*Some of the terminology explanations that mention "cover plate" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…第7肢は口の後にあり，小さく，唇様肢（しんようし）と呼ばれる。6対の葉状の腹肢のうち，第1腹肢は左右が融合して1枚となり，後方の全腹肢を覆うので蓋板（がいばん）と呼ばれ，その基部背面に1対の生殖孔が開いている。残りの5対の腹肢は2枝型の遊泳脚となっているが，その外肢後面に薄い板を多数重ねた呼吸器があり，この呼吸器官は鰓書（さいしよ）と呼ばれ，クモ類の書肺に構造がよく似ている。…

※「蓋板」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