Everything about Book Lung totally explained
A
book lung is a type of
respiration organ used for atmospheric gas exchange and is found in
arachnids, such as
scorpions and
spiders. Each of these organs is found inside a ventral abdominal cavity and connects with the surroundings through a small opening. Book lungs are not related to the lungs of modern land-dwelling
vertebrates by
evolution. Their name describes their structure. Stacks of alternating air pockets and hemolymph-filled tissue gives them an appearance similar to a "folded" book. Their number varies from just one pair in most spiders to four pairs in scorpions. Sometimes the book lungs can be absent and the gas exchange is performed by the thin walls inside the cavity instead, with its surface area increased by branching into the body as thin tubes called
tracheae. It is possible that the tracheae have evolved directly from the book lungs, because in some spiders the tracheae have a small number of greatly elongated chambers. Many
arachnids, like
mites and harvestmen (
Opiliones), have no traces of book lungs and breathe through tracheae or through their body surface only.
The unfolded "pages" (plates) of the book lung are filled with
hemolymph (the
arthropod blood). 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 required to facilitate this kind of
respiration.
The oldest book lungs have been recovered from extinct
trigonotarbid arachnids preserved the 410 million year old
Rhynie chert of Scotland. These
Devonian fossil lungs are almost indistinguishable from the lungs of modern arachnids.
The absence or presence of book lungs divides the Arachnida in two main groups, but says nothing about the relationships between them: the pulmonate arachnids (book lungs present;
scorpions,
whip scorpions,
Schizomida,
Amblypygi, and
spiders), and the apulmonate arachnids (book lungs absent;
microwhip scorpions,
harvestmen,
Acarina,
pseudoscorpions,
Ricinulei and
sunspiders). One of the long-running controversies in arachnid evolution is whether the book lung evolved once in the arachnid common ancestor, or whether it evolved in multiple groups of arachnids in parallel as they came onto land.
Book Gills
Book lungs have evolved from book
gills. Although they've a similar book-like structure, they're found in different locations. Book gills are found externally while book lungs are found internally. Book gills are still found in
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 are designed for gas exchange within water and seem to have their origin as modified legs. On the inside of each appendage there are attached over 100 thin leaf-like membranes called
lamellae which appear as pages in a book, and are the areas of the gill where gas exchange takes place. These appendages move with rhythmic movements 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're kept moist, the horseshoe crab can live on land for many hours.
Further Information
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