A book lung is a type of respiration organ used for atmospheric gas exhange 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 evolutionarily related to mammalian lungs. Their name describes their structure, as they are "folded" like a 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 book lung cavity instead, with its surface area increased by branching into the body as thin tubes called trachea. Or it could be that the tracheae have evolved direcetly from the book lungs as they in some spiders have a small number of greatly elongated chambers. Many arachnids, like mites, have no traces of book lungs and breathe through trachea or through their body surface only. Orders See text. ... Superfamilies Pseudochactoidea Buthoidea Chaeriloidea Chactoidea Iuroidea Scorpionoidea See the classification section for families. ... Suborders Araneomorphae Mesothelae Mygalomorphae See the taxonomy section for families Spiders are invertebrate animal(s) that produce silk, have eight legs and no wings. ... A speculative phylogenetic tree of all living things, based on rRNA gene data, showing the separation of the three domains, bacteria, archaea, and eukaryotes. ... Orders Subclass Multituberculata (extinct) Plagiaulacida Cimolodonta Subclass Palaeoryctoides (extinct) Subclass Triconodonta (extinct) Subclass Eutheria (includes extinct ancestors)/Placentalia (excludes extinct ancestors) Afrosoricida Artiodactyla Carnivora Cetacea Chiroptera Cimolesta (extinct) Creodonta (extinct) Condylarthra (extinct) Dermoptera Desmostylia (extinct) Embrithopoda (extinct) Hyracoidea Insectivora Lagomorpha Litopterna (extinct) Macroscelidea Mesonychia (extinct) Notoungulata (extinct) Perissodactyla Pholidota Plesiadapiformes... Trachea (IPA /treikiÉ™/) is a common biological term for an airway through which respiratory gas transport takes place in organisms. ... Orders See text. ... Families Tetranychidae - Spider mites Eriophyidae - Gall mites Sarcoptidae - Sarcoptic Mange mites The mites and ticks, order Acarina or Acari, belong to the Arachnida and are among the most diverse and successful of all the invertebrate groups, although some way behind the insects. ...

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 the chances of gas exchange with the environment. In most species, no motion of the plates is required to facilitate this kind of respiration. Hemolymph (or haemolymph) is the blood analogue used by all arthropods and most mollusks that have an open circulatory system. ... Subphyla and Classes Subphylum Trilobitomorpha Trilobita - trilobites (extinct) Subphylum Chelicerata Arachnida - spiders, scorpions, etc. ... Red blood cells (erythrocytes) are present in the blood and help carry oxygen to the rest of the cells in the body Blood is a circulating tissue composed of fluid plasma and cells (red blood cells, white blood cells, platelets). ... Look up air in Wiktionary, the free dictionary. ... A gas is one of the four main phases of matter (after solid and liquid, and followed by plasma), that subsequently appear as a solid material is subjected to increasingly higher temperatures. ... In biology, a species is the basic unit of biodiversity. ... It has been suggested that Gas exchange be merged into this article or section. ...

Book lungs have evolved from book gills, still found in Horseshoe crabs which have six pairs of them, though the first pair (called the operculum) serves as a cover for the other five. Book gills are flaplike appendages that are designed for gas exhange 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 are kept moist, the horseshoe crab can live on land for many hours. Binomial name Limulus polyphemus Linnaeus, 1758 The horseshoe crab (Limulus polyphemus) is an arthropod that is more closely related to spiders than crabs. ...

External links

• http://www.xs4all.nl/~ednieuw/Spiders/InfoNed/blood.html