Book lung
From Free net encyclopedia
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 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.
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.
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.