What is the function of a book gill & how does it work?

Book gillls are found in horseshoe crabs. They are
external and there are five pairs of them. They are flap-like appendages and their
function is to absorb oxygen from the water. They can also be used for locomotion in
young.The book gills resemble a book because each appendage contains several thin
leaf-like membranes called lamellae, which are like pages in a book. These thin
membranes are for respiration and gas exchange. As long as they remain moist, the
horseshoe crab can venture out of the water for periods of time and still be able to
survive.