Horseshoe crabs are like fish and some other animals that live in the water in the way that they breathe. They breathe with gills, not lungs. But the horseshoe crabs have different kinds of gills than other animals. On the underside of a horseshoe crab's abdomen there is a leatherlike flap that covers 5 pairs of gills. Each of the 10 gills has about 100 sheets of tissue that look like the pages of a book. To breathe the horseshoe crabs flaps its book gills. The water is forced to go past its gills. As the water passes through the gills, oxygen is passed into its blood. The horseshoe crab's gills must always be wet or it will die. If a horseshoe crab is stranded out of water, it must dig into the wet sand to keep the gills moist. It can stay there for a few days, provided the gills stay damp.
