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:
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Ay, thou poor ghost, whiles memory holds a seat
In this distracted globe. Remember thee?
Yea, from the table of my memory
I’ll wipe away all trivial fond records,
All saws of books, all forms, all pressures past
That youth and observation copied there,
And thy commandment all alone shall live
Within the book and volume of my brain,”
—William Shakespeare (1564–1616)
“his gills were breathing in
the terrible oxygen”
—Elizabeth Bishop (1911–1979)