Editing Book lung (section)

==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>