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book gill:see horseshoe crabhorseshoe crab,
large, primitive marine arthropod of the family Limulidae, related to the spider and scorpion and sometimes called a king crab (a name also used for the largest of the edible true crabs).
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book gill[′bu̇k ‚gil]
A type of gill in king crabs consisting of folds of membranous tissue arranged like the leaves of a book.