BOOK LUNG

form of respiratory organ found in certain air-breathing arachnid arthropods (scorpions and a few spiders). Each book lung consists of a series of plates that are highly vascular (i.e., richly supplied with blood vessels), parallel, and leaflike. They provide 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 spiders, each lung being enclosed in a cavity that opens to the exterior by a small slit.