Just behind the teeth, across the front of both the upper and lower jaw, find the oral valves . These are transverse membranes that prevent the outflow of water during respiration. An inflexible tongue is supported by the hyoid bone. Explore the spacious pharynx, nothing the size and arrangement of the gill bars and gill slits. The gills separate the oral cavity from the opercular cavity .
Valves guard the opercular clefts, preventing the backflow of water.
The action of the two pumps creates a pressure differential that maintains a smooth flow of water across the gills throughout nearly the entire breathing cycle. Water movement across the gills is actually much smoother and less pulsatile than it appears from watching a fish respire. Now on the animal s left body wall, make a transverse cut, extending dorsally from the vental area. ON the right side, make similar transversal cuts so that the right wall can be laid back, but do not remove it. You have now exposed the abdominal cavity.