Editing Book lung

{{Short description|Type of lung commonly found in arachnids}}
[[Image:Spin vent diag.jpg|thumb|right|In this spider diagram, the position of the book lungs is labelled 1.]]
[[File:Comstock-book-lungs.png|thumb|right|Spider book lungs (cross section)]]
[[File:Spider internal anatomy-en.svg|thumb|Internal anatomy of a female spider, book lungs shown in pink]]

A '''book lung''' is a type of [[respiration organ]] used for atmospheric gas-exchange that is present in many [[arachnid]]s, such as [[scorpion]]s and [[spider]]s. Each of these organs is located inside an open, ventral-abdominal, air-filled cavity (atrium) and connects with its surroundings through a small opening for the purpose of [[respiration (physiology)|respiration]].

==Structure and function==
{{further information|Spider anatomy}}
Book lungs are not related to the [[lungs]] of modern land-dwelling [[vertebrate]]s. Their name instead describes their structure and purpose as a case of [[convergent evolution]]. Stacks of alternating air pockets and tissue filled with [[hemolymph]]{{efn|[[Hemolymph]] is the [[arthropod]] equivalent of blood.}} give them an appearance similar to a "folded" book.<ref name="rain">{{cite book |title=Biology of Spiders |last=Foelix |first=Rainer F. |url=https://archive.org/details/biologyofspiders00foel_0 |url-access=registration |quote=book lung. |year=1996 |publisher=Oxford University Press US |isbn=0-19-509594-4 |pages=[https://archive.org/details/biologyofspiders00foel_0/page/61 61]–64 }}</ref>

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 needed to facilitate this kind of respiration.

===Occasionally absent===
Many [[arachnid]]s, such as [[mite]]s and [[Opiliones|harvestmen]], have no traces of book lungs and breathe through their body-surfaces only or through [[invertebrate trachea|tracheae]]. Gas exchange is performed by the thin walls inside the cavity instead, or with their surface area increased by branching into the body as thin tubes, the tracheae. These tracheae may possibly have evolved directly from book lungs because the tracheae in some spiders have a small number of greatly elongated chambers.{{citation needed|date=April 2025}}

==Arachnid taxonomy==
The absence or presence of book lungs divides the Arachnida into two main groups:

; The pulmonate arachnids
: book lungs present; [[Tetrapulmonata]] ([[whip scorpion]]s, [[Schizomida]], [[Amblypygi]], and [[spider]]s) and [[scorpion]]s
; The a-pulmonate arachnids
: book lungs absent; [[microwhip scorpion]]s, [[Opiliones|harvestmen]], [[Acarina]], [[pseudoscorpion]]s, [[Ricinulei]], and [[Solifugae|sunspiders]]

Tetrapulmonata have two pairs of book lungs found on the second and third abdominal segments (Schizomida have lost a pair, and most advanced spiders have replaced at least one of the pairs with trachea). Scorpions have four pairs of book lungs, found on abdominal segments number three, four, five, and six.<ref>[https://www.european-arachnology.org/esa/wp-content/uploads/2015/08/009-016_Dunlop.pdf The origins of tetrapulmonate book lungs and their significance for chelicerate phylogeny]</ref>

The pulmonate arachnids also appears to be the only members of Arachnida where the respiratory pigment [[hemocyanin]] is present in their blood.<ref>[https://bmcecolevol.biomedcentral.com/articles/10.1186/1471-2148-12-19 The diversity and evolution of chelicerate hemocyanins]</ref>

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,<ref>{{cite journal |author1=Scholtz, Gerhard |author2=Kamenz, Carsten |year=2006 |title=The book lungs of Scorpiones and Tetrapulmonata (Chelicerata, Arachnida): Evidence for homology and a single terrestrialisation event of a common arachnid ancestor |journal=Zoology |volume=109 |issue=1 |pages=2–13 |doi=10.1016/j.zool.2005.06.003|pmid=16386884 }}</ref> or whether book lungs evolved separately in several groups of arachnids as they came onto land. 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&nbsp;million-year-old [[Rhynie chert]] of Scotland. These [[Devonian]] fossil lungs are almost indistinguishable from the lungs of modern arachnids, fully adapted to a terrestrial existence.<ref>{{cite journal |last1=Kamenz |first1=Carsten |first2=Jason A. |last2=Dunlop |first3=Gerhard |last3=Scholtz |first4=Hans |last4=Kerp
|first5=Hagen |last5=Hass |year=2008 |journal=Biology Letters |volume=4 |pages=212–215 |doi=10.1098/rsbl.2007.0597 |title=Microanatomy of early Devonian book lungs |issue=2 |publisher=Royal Society |pmid=18198139 |place=London, UK|pmc=2429929 }}</ref>

==Book gills==
[[Image:Tachypleus_tridentatus_Cat_ba_2.JPG|thumb|Underside of a female [[horseshoe crab]] showing the legs and book gills]]

Book lungs are thought to have evolved from book [[gill]]s, water-breathing structures among marine [[chelicerates]]. Although they have a similar book-like structure, book gills are external, while book lungs are internal.<ref>{{cite book |last1=Bhamrah |first1=H. S. |first2=Kavita |last2=Juneja |title=An Introduction to Arthropoda |url=https://books.google.com/books?id=b_gsiQXNhTUC&pg=PA316&dq=book+lung+book+gills |year=2002 |publisher=Anmol Publications Pvt. Ltd. |isbn=81-261-0673-5}}</ref> Both are considered appendages rather than conventional internal organs, as they develop from limb buds before the buds flatten into segmented [[lamella (surface anatomy)|lamellae]]. <ref>{{cite journal |last1=Pechmann |first1=Matthias |last2=Khadjeh |first2=Sara |last3=Sprenger |first3=Frederik |last4=Prpic |first4=Nikola-Michael |title=Patterning mechanisms and morphological diversity of spider appendages and their importance for spider evolution |journal=Arthropod Structure & Development |date=November 2010 |volume=39 |issue=6 |pages=453–67 |doi=10.1016/j.asd.2010.07.007 |pmid=20696272 |url=https://www.sciencedirect.com/science/article/pii/S1467803910000551 |access-date=20 August 2020|url-access=subscription }}</ref>

Book gills are still present in the marine arthropod ''[[Limulus]]'' ([[horseshoe crab]]s) which have five pairs of them, the flap in front of them being the genital operculum which lacks gills. Book gills are flap-like appendages that effect gas exchange within water and seem to have their origin as modified legs. On the inside of each appendage, over 100&nbsp;thin page-like membranes, ''lamellae'',  appearing as pages in a book, are where gas exchange takes place. These appendages move rhythmically to drive blood in and out of the lamellae and to circulate water over them. Respiration being their main purpose, they can also be used for swimming in young individuals. If they are kept moist, the horseshoe crab can live on land for many hours.

==Footnotes==
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==References==
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{{DEFAULTSORT:Book Lung}}
[[Category:Arachnid anatomy]]
[[Category:Invertebrate respiratory system]]
[[Category:Spider anatomy]]