A platypus skull is broad and flat. It has a flexible bill covered with a thin membrane of skin that contains electro-sensitive cells. Their eyes are covered, like many birds and reptiles, by a semi-clear extra eyelid, called a nictitating membrane, which allows them to open their eyes underwater yet still be protected from injury. Their ears and nostrils are all provisioned with small valves to keep out the water when they are in, or under it. Their brain case is small and open, which is a very primitive characteristic, but the whole of a platypus's sensory organs taken together would blow even an advanced submarine's electronic equipment right out of the water.