Book Gills
Scientists think book lungs evolved from book gills. Although they have a similar book-like structure, they are 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 effect 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 are kept moist, the horseshoe crab can live on land for many hours.
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Famous quotes containing the words book and/or gills:
“If the book is good, is about something that you know, and is truly written, and reading it over you see that this is so, you can let the boys yip and the noise will have that pleasant sound coyotes make on a very cold night when they are out in the snow and you are in your own cabin that you have built or paid for with your work.”
—Ernest Hemingway (1899–1961)
“his gills were breathing in
the terrible oxygen”
—Elizabeth Bishop (1911–1979)