This is a close-up of the book gills of a horseshoe crab. These gills have the ability to breath in or out of water as long as they are kept moist.
As you can see there are five sets of gills. The reason they are referred to book gills is they tend to fan them like pages of a book, but in actualiy they can be filled up with water to look more like bellows. There is another picture on this site that shows that.