why is the number of pharyngeal gill slits in sharks
Gill slit
Most sharks and rays have five pairs of gill slits , but a few species have 6 or 7 pairs . Shark gill slits lie in a row behind the head . The anterior edge of a gill slit is motile , moving outward to allow water to exit , but closing to prevent reverse flow . A modified slit , called a spiracle , lies just behind the eye , which assists the shark with taking in water during respiration and plays a major role in bottom -- dwelling sharks . Spiracles are reduced or missing in active pelagic sharks . While the shark is moving , water passes through the mouth and over the gills in a process known as `` ram ventilation '' . While at rest , most sharks pump water over their gills to ensure a constant supply of oxygenated water . A small number of species have lost the ability to pump water through their gills and must swim without rest . These species are obligate ram ventilators and would presumably asphyxiate if unable to move . Obligate ram ventilation is also true of some pelagic bony fish species . 
Most sharks and rays have five pairs of gill slits , but a few species have 6 or 7 pairs . Shark gill slits lie in a row behind the head . The anterior edge of a gill slit is motile , moving outward to allow water to exit , but closing to prevent reverse flow . A modified slit , called a spiracle , lies just behind the eye , which assists the shark with taking in water during respiration and plays a major role in bottom -- dwelling sharks . Spiracles are reduced or missing in active pelagic sharks . While the shark is moving , water passes through the mouth and over the gills in a process known as `` ram ventilation '' . While at rest , most sharks pump water over their gills to ensure a constant supply of oxygenated water . A small number of species have lost the ability to pump water through their gills and must swim without rest . These species are obligate ram ventilators and would presumably asphyxiate if unable to move . Obligate ram ventilation is also true of some pelagic bony fish species .
Most sharks and rays have five pairs of gill slits, but a few species have 6 or 7 pairs. The anterior edge of a gill slit is motile, moving outward to allow water to exit, but closing to prevent reverse flow.
Most sharks and rays have five pairs of gill slits , but a few species have 6 or 7 pairs . The anterior edge of a gill slit is motile , moving outward to allow water to exit , but closing to prevent reverse flow .
Most sharks and rays have five pairs of gill slits, but a few species have 6 or 7 pairs. The anterior edge of a gill slit is motile, moving outward to allow water to exit, but closing to prevent reverse flow.  While the shark is moving, water passes through the mouth and over the gills in a process known as `` ram ventilation ''.  While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water.
Most sharks and rays have five pairs of gill slits , but a few species have 6 or 7 pairs . The anterior edge of a gill slit is motile , moving outward to allow water to exit , but closing to prevent reverse flow . While the shark is moving , water passes through the mouth and over the gills in a process known as `` ram ventilation '' . While at rest , most sharks pump water over their gills to ensure a constant supply of oxygenated water .