In the event of danger, the fish inflates itself by filling its extremely elastic stomach with water (or air when outside of the water) until the fish is almost spherical (hence the name blowfish or pufferfish). A series of things happen when a pufferfish inflates: First, the pufferfish fills its mouth with water. Then, it seals its mouth using a special valve at the bottom of the mouth. This valve flaps upward and covers the entire mouth of the fish. Next, a branchiostegal ray (a modified gill arch) pushes the water down the esophagus into the stomach. The extremely elastic stomach then expands. Depending on the species the fugu can achieve an almost perfect spherical shape.