Book lung

A book lung is a type of respiration organ used for atmospheric gas-exchange that is present in many arachnids, such as scorpions and spiders.

Stacks of alternating air pockets and tissue filled with hemolymph[a] give them an appearance similar to a "folded" book.

While the third abdominal segment in Tetrapulmonata have book lungs, the scorpions have a pair of sensory organs called pectines instead.

The oldest book lungs have been recovered from extinct trigonotarbid arachnids preserved in the 410 million-year-old Rhynie chert of Scotland.

On the inside of each appendage, over 100 thin page-like membranes, lamellae, appearing as pages in a book, are where gas exchange takes place.