Book lung

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, who lived 410 million years ago and had book lungs already fully adapted to a terrestrial existence.