Book gills

A book lung is a type of respiration organ used for atmospheric gas exchange that is found in many arachnids, such as scorpions and spiders. Each of these organs is found inside an open ventral abdominal, air-filled cavity (atrium) and connects with the surroundings through a small opening for the purpose of respiration. The oldest book lung found to date belongs to a member of the extinct Trigonotarbida, which lived 410 million years ago and had book lungs already fully adapted to a terrestrial existence.

Book lungs are not related to the lungs of modern land-dwelling vertebrates. Their name describes their structure. Stacks of alternating air pockets and tissue filled with hemolymph (the arthropod equivalent of blood) give them an appearance similar to a "folded" book. Their number varies from just one pair in most spiders to four pairs in scorpions. The unfolded "pages" (plates) of the book lung are filled with hemolymph. 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.

Sometimes, book lungs can be absent, and gas exchange is performed by the thin walls inside the cavity instead, with their surface area increased by branching into the body as thin tubes called tracheae. The tracheae possibly have evolved directly from the book lungs because, in some spiders, the tracheae have a small number of greatly elongated chambers. Many arachnids, such as mites and harvestmen (Opiliones), have no traces of book lungs and breathe through tracheae or through their body surfaces only. 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 from book gills just once in a common arachnid ancestor, or whether it evolved in multiple groups of arachnids in parallel as they came onto land.

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