Editing Book lung (section)

==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.