CHAPTER III 

 THE RESPIRATORY SYSTEM 



The skate obtains its oxygen for respiration from the 

 water. A muscular expansion of the mouth and pharynx 

 causes water to enter the oral cavity through the spiracle 

 and the mouth. The spiracle is located dorsally in the skate 

 and so is free from the sand and mud of the bottom. Con- 

 traction of the muscular expansion of the mouth and 

 pharynx causes the water to be expelled through the gill 

 slits. In so doing the water passes over the gill filaments 

 where the oxygen is taken on by the blood. 



1. The Mouth and Pharyngeal Cavity. 



Make the following incisions very carefully as success 

 in much of the future work depends upon this initial dis- 

 section. Work a blade from a pair of heavy scissors or a 

 knife into the left (skate's left) corner of the mouth and cut 

 the jaw in the direction of the gill-slits; continue cutting 

 the entire tissue between the mouth cavity and the exterior 

 towards the gill-slits; extend the cut along the median 

 margin of the gill-slits and continue it posteriorly to the 

 heavy cartilaginous bar of the pectoral girdle ; cut through 

 this bar with the scalpel; then turn the 7-shaped flap 

 formed over to the side. This exposes as a continuous 

 cavity the mouth and pharynx. In very rare cases the 

 stomach is accidentally everted into the cavity and care 

 should be taken not to injure it. 



The anterior part of this cavity, which is between the 



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