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book lung
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Illustration showing a cross section of a spider's book lung, with layered lamellae for gas exchange, blood flow between the layers, and air entering through a spiracle opening. Illustration showing different animals and their respiratory organs: a spider with book lungs, a chicken with lungs, a sponge with labeled inflow and outflow pores, a fish with gills, and a caterpillar with spiracles.
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Book lung
Book lung Diagrammatic section through a book lung.

book lung

anatomy
Also known as: book trachea
Written and fact-checked by
Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
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History

book lung, form of respiratory organ found in certain air-breathing arachnid arthropods (scorpions and most spiders). Each book lung consists of a series of thin plates that are highly vascular (i.e., richly supplied with hemolymph) and are arranged in relation to each other like the pages of a book. These plates extend into an internal pouch formed by the external skeleton that opens to the exterior by a small slit. This provides an extensive surface for the exchange of oxygen and carbon dioxide with the surrounding air.

There are four pairs in scorpions and up to two in mygalomorph spiders such as tarantulas and trapdoor spiders. Most araneomorph spiders, such as orb weavers and jumping spiders, have only a single, front pair of book lungs and also make use of tracheal tubes for gas exchange.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Melissa Petruzzello.