A book lung is a type of respiration organ used for atmospheric gas exchange that is present in many

arachnid Arachnida () is a class of joint-legged invertebrate animals ( arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and ...

s, such as

scorpion Scorpions are predatory arachnids of the order Scorpiones. They have eight legs, and are easily recognized by a pair of grasping pincers and a narrow, segmented tail, often carried in a characteristic forward curve over the back and always endi ...

s and

spider Spiders (order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species d ...

s. Each of these organs is located inside an open ventral abdominal, air-filled cavity (atrium) and connects with the surroundings through a small opening for the purpose of respiration.

Structure and function

Book lungs are not related to the

lungs The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of ...

of modern land-dwelling

vertebrate Vertebrates () comprise all animal taxon, taxa within the subphylum Vertebrata () (chordates with vertebral column, backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the ...

s. Their name describes their structure and purpose. Stacks of alternating air pockets and tissue filled with

hemolymph Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...

give them an appearance similar to a "folded" book. 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

Sometimes, book lungs can be absent, and gas exchange is performed by the thin walls inside the cavity instead, with their surface area increased by branching into the body as thin tubes called

tracheae The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air- breathing animals with lungs. The trachea extends from the ...

. These tracheae may possibly have evolved directly from the book lungs because the tracheae in some spiders have a small number of greatly elongated chambers. Many

s, such as

mite Mites are small arachnids (eight-legged arthropods). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic analysis does not show clear evid ...

s and

harvestmen The Opiliones (formerly Phalangida) are an Order (biology), order of arachnids Common name, colloquially known as harvestmen, harvesters, harvest spiders, or daddy longlegs. , over 6,650 species of harvestmen have been discovered worldwide, alth ...

, have no traces of book lungs and breathe through tracheae or through their body-surfaces only.

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 Thelyphonida is an arachnid order comprising invertebrates commonly known as whip scorpions or vinegaroons (also spelled vinegarroons and vinegarones). They are often called uropygids in the scientific community based on an alternative name for ...

s, Schizomida, Amblypygi, and

s) and

s ; The a-pulmonate arachnids : book lungs absent;

microwhip scorpion Palpigrades, commonly known as microwhip scorpions, are arachnids belonging to the order Palpigradi. Description Palpigrades belong to the arachnid class. They are the sister group to Solifugae, no more than in length, and averaging . They have ...

, Acarina,

pseudoscorpion Pseudoscorpions, also known as false scorpions or book scorpions, are small, scorpion-like arachnids belonging to the order Pseudoscorpiones, also known as Pseudoscorpionida or Chelonethida. Pseudoscorpions are generally beneficial to humans sin ...

s, Ricinulei and 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. The pulmonate arachnids also appears to be the only members of Arachnida where the respiratory pigment

hemocyanin Hemocyanins (also spelled haemocyanins and abbreviated Hc) are proteins that transport oxygen throughout the bodies of some invertebrate animals. These metalloproteins contain two copper atoms that reversibly bind a single oxygen molecule (O2 ...

is present in their blood. 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, 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 million-year-old Rhynie chert of Scotland. These

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, w ...

fossil lungs are almost indistinguishable from the lungs of modern arachnids, fully adapted to a terrestrial existence.

Book gills

It is believed that book lungs evolved from book

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...

s. Although they have a similar book-like structure, book gills are external, while book lungs are internal. Both are considered appendages because book lungs develop from limb buds before the buds flatten into segmented lamellae. Book gills are still present in the marine arthropod '' Limulus'' ( 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.

Footnotes

References

{{DEFAULTSORT:Book Lung Arachnid anatomy Invertebrate respiratory system Spider anatomy