A book lung is a type of respiration organ used for atmospheric gas exchange and is found in arachnids, such as scorpions and spiders. Each of these organs is found inside a ventral abdominal air-filled cavity (atrium) 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. Many arachnids, like mites and harvestmen (Opiliones), have no traces of book lungs and breathe through tracheae or through their body surface only.
The unfolded "pages" (plates) of the book lung are filled with hemolymph (the arthropod blood). The folds maximize the surface exposed to air, and thereby maximize the amount of gas exchanged with the environment. In most species, no motion of the plates is required to facilitate this kind of respiration.
The oldest book lungs have been recovered from extinct trigonotarbid arachnids preserved in the 410-million-year-old Rhynie chert of Scotland. These Devonian fossil lungs are almost indistinguishable from the lungs of modern arachnids.
The absence or presence of book lungs divides the Arachnida into two main groups, the pulmonate arachnids (book lungs present; scorpions and the Tetrapulmonata; whip scorpions, Schizomida, Amblypygi, and spiders), and the apulmonate arachnids (book lungs absent; microwhip scorpions, harvestmen, Acarina, pseudoscorpions, Ricinulei and sunspiders). One of the long-running controversies in arachnid evolution is whether the book lung evolved once in the arachnid common ancestor, or whether it evolved in multiple groups of arachnids in parallel as they came onto land.
Read more about Book Lung: Book Gills
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