A book lung is a type of respiration organ used for atmospheric gas exchange and is found in arachnids, such as scorpions and spiders, and horseshoe crabs. Each of these organs is found inside a ventral abdominal cavity and connects with the surroundings through a small opening. Book lungs are not related to the lungs of modern land-dwelling vertebrates.
Their name describes their structure. Stacks of alternating air pockets and hemolymph-filled tissue give them an appearance similar to a “folded” book. Their number varies from just one pair in most spiders to four pairs in scorpions. Sometimes the book lungs can be absent and the gas exchange is performed by the thin walls inside the cavity instead, with its surface area increased by branching into the body as thin tubes called tracheae. It is possible that the tracheae have evolved directly from the book lungs, because in some spiders the tracheae have a small number of greatly elongated chambers.
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image from: The Spider Book (1920 ed.) by John Henry Comstock