168 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



mandibular gill arch; the section of the arch should be identified in the cut 

 surface. On the floor of the mouth back of the teeth is the tongue; it forms a 

 flat, slight projection, which is practically immovable. It is supported by the 

 second or hyoid gill arch which should be felt within it and identified in section 

 at the cut surface. (The tongue is absent in the skate, owing to the reduction 

 of the hyoid arch.) 



The posterior and greater part of the cavityunder consideration is thepharynx. 

 Its wall is pierced by six internal gill slits. The first of these, the spiracle, is a 

 rounded opening in the roof of the mouth immediately posterior to the mandibular 

 arch. The remaining five gill slits are elongated. The internal gill slits communi- 

 cate with large cavities — the visceral pouches — which in turn open to the exterior 

 by way of the external gill slits. The tissue between successive visceral pouches is 

 a visceral arch. The parts of a visceral arch should be examined in section on the 

 left side where the arches have been cut through. The center of the section of 

 each visceral arch is formed by theinterbranchial septum which extends to the outer 

 surface of the body, where the spaces between successive septa form the external 

 gill slits. On each face of the interbranchial septum is borne a series of low, thin 

 folds or plates, the branchial or gill filaments. The set of filaments on one face 

 of the septum constitutes a half-gill or demibranch and the demibranchs on the 

 two sides of a septum together constitute a whole branchia or gill. The gills 

 are outgrowths of the walls of the visceral pouches and are covered with entoderm. 

 By examining all of the septa determine how many demibranchs are present 

 and where they are missing. In the inner end of the section of each visceral 

 arch locate the cross-section of the cartilaginous gill arch and external to this, 

 lying in the septum, the cartilages of the gill rays. Just external to the section 

 of the gill arch is the section of a blood vessel — the afferent branchial vessel — 

 which brings venous blood to the gills. At each side of the gill arch is a section 

 of another vessel, which is injected with a colored solution ; these are the efferent 

 branchial vessels, which carry the aerated blood away from the gills. Note the 

 fine branches of these vessels in the gill filaments. The gills are the respiratory 

 mechanism of the animal in which the blood obtains oxygen and gives off carbon- 

 dioxide. Water is kept flowing over the gills by movements of the gill arches. 



Draw the mouth cavity and pharynx. Draw one visceral arch and all of 

 its parts in cross-section. 



D. THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS OF NECTURUS 



Obtain a specimen and place in a wax-bottomed dissecting pan, fastening 

 it ventral side up by pins through the legs. 



i. The viscera of the pleuroperitoneal cavity. — Make a longitudinal incision 

 through the body wall a little to the left of the median ventral line from the left 

 side of the anus through the pelvic girdle to the pectoral girdle. Spread apart 

 the two flaps of the body wall and look within. The large cavity is, as in fishes, 



