book lung

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A saccular respiratory organ found in some arachnids, such as scorpions and spiders, consisting of several parallel membranous folds arranged like the pages in a book.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology A lamellate respiratory organ found in arachnids.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun organ in many arachnids containing many thin folds of membrane resembling the leaves of a book