^w^ Gill openings on each side of the head control the flow of water through the gill filaments and provide the basic structure for removing oxygen and expelling carbon
dioxide.
^w^ Fish have bony plates that cover the gills. Sharks have naked gills, which means there is only a skin-like flap covering them.
^w^ Most sharks favor ram ventilation: As they swim, the water flows naturally into the mouth and out through the gills. For some very active oceanic sharks this is the only way they can breathe, so they have to keep swimming to get oxygen.
^w^ Bottom dwellers, like Swell Sharks, pump water through the gills by lowering the floor of the mouth to increase the volume of water the mouth can hold. Then they close the mouth and open the gills, capturing the oxygen from the water as it flows past the gills.
^w^ Most sharks have the ability to switch between the two methods. They open their mouth and use ram ventilation when swimming, and the pump method when lying along the bottom.
^w^ The spiracle, the small round opening in the head of rays and some sharks, is also a gill. This separate small gill allows water into the mouth and provides oxygen directly to the
