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book lung,terrestrial respiratory organ characteristic of arachnids such as scorpions and primitive spiders. Each book lung consists of hollow flat plates. Air bathes the outer surface of the plates and blood circulates within them, facilitating the exchange of gases. In most species, adequate gas exchange occurs without any muscular movement to ventilate the lung.
book lung[′bu̇k ‚ləŋ]
A saccular respiratory organ in many arachnids consisting of numerous membranous folds arranged like the pages of a book.