Abook lung is a type ofrespiration organ used for atmospheric gas-exchange that is present in manyarachnids, such asscorpions andspiders. 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 ofrespiration.
Book lungs are not related to thelungs of modern land-dwellingvertebrates. Their name instead describes their structure and purpose as a case ofconvergent evolution. Stacks of alternating air pockets and tissue filled withhemolymph[a] give them an appearance similar to a "folded" book.[1]
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 toair, and thereby maximize the amount ofgas exchanged with the environment. In most species, no motion of the plates is needed to facilitate this kind of respiration.
Manyarachnids, such asmites andharvestmen, have no traces of book lungs and breathe through their body-surfaces only or throughtracheae. 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]
The absence or presence of book lungs divides the Arachnida into two main groups:
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.[2]
The pulmonate arachnids also appears to be the only members of Arachnida where the respiratory pigmenthemocyanin is present in their blood.[3]
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,[4] 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 extincttrigonotarbid arachnids preserved in the 410 million-year-oldRhynie chert of Scotland. TheseDevonian fossil lungs are almost indistinguishable from the lungs of modern arachnids, fully adapted to a terrestrial existence.[5]
Book lungs are thought to have evolved from bookgills, water-breathing structures among marinechelicerates. Although they have a similar book-like structure, book gills are external, while book lungs are internal.[6] Both are considered appendages rather than conventional internal organs, as they develop from limb buds before the buds flatten into segmentedlamellae.[7]
Book gills are still present in the marine arthropodLimulus (horseshoe crabs) 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 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.
book lung.