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.