12 THE SKATE 



moutli and gill arches, is the oral or mouth cavity. It is 

 limited in the front by the upper and lower jaws which 

 bear the teeth. The tongue is absent in the skate. The 

 lower and upper jaws are the two halves of the first or 

 mandibular gill-arch. 



The posterior part of the cavity is the pharynx. It con- 

 verges at its posterior end into the esophagus, which con- 

 nects directly with the stomach. Probe into the esophagus. 

 Six internal gill-sUts break the lateral wall of the pharynx. 

 The first, which is much modified, is the spiracle, which is 

 an opening in the roof of the mouth just posterior to the 

 mandibular arch. Locate the other five gill-slits. 



The internal gill-slits communicate with large visceral 

 pouches which open to the exterior through the external 

 gill-slits. Kun a probe through the internal gill-slit of the 

 right side, pushing it into the visceral pouch and out the 

 external gill-slit. This is the course which the water takes 

 during respiration. The visceral arch is composed of the 

 tissue between two successive visceral pouches. 



Drawing 7. Draw the mouth cavity and the pharynx. 



2. Visceral Arches. 



Examine the parts of the visceral arches where they have 

 been cut. The interbranchial septum is the central portion 

 of the arch which extends to the outer surface. The spaces 

 between successive septa on the outer surface of the body 

 form the external gill-slits, and those on the inner surface 

 of the body the internal gill-slits. The septum bears plates 

 and folds on its sides, the branchial or gill filaments. 

 These extend into the visceral pouches. The filaments on 

 one face of the septum form a half -gill or demibranch. The 

 two demibranchs of a septum make up an entire gill or 

 branchia. Count the demibranchs and determine whether 



