8 VERTEBRATE ZOOLOGY 



slit and may contain a rudimentary gill. Slit it open and examine 

 its walls. Note the gill rakers, short processes on the inner walls 

 of the gill slits, which strain the respiratory water and prevent 

 solid particles from entering the gill slits with it. 



On the ventral side is the prominent tongue, supported by the 

 hyoid cartilages beneath the mucous membrane. Feel these carti- 

 lages with the fingers. 



Observe the character and arrangement of the teeth in both 

 upper and lower jaw. In the spiny dogfish they are sharp, as in the 

 majority of sharks; in the smooth dogfish they have lost their 

 sharp points and are reduced to flat plates. 



Exercise 7. Draw a semidiagrammatic sketch of both dorsal and 

 ventral walls of the mouth and pharynx. 



Cut two or three gill arches from the body. Note that each 

 gill is composed of two rows of gill filaments, an anterior and a 

 posterior row, and that each gill slit is bounded on the one side 

 by the posterior filaments of one gill and on the opposite side 

 by the anterior filaments of the gill next back of it. The gills in 

 fishes are outgrowths of the wall of the pharynx. 



Exercise 8. Draw a gill slit and the filaments bounding it. 



The Digestive System. This system consists of the mouth, 

 pharynx, oesophagus, stomach, intestine, cloaca, and the three 

 intestinal glands,— the liver, pancreas, and rectal gland. 



Cut the mesentery which attaches the spleen to the stomach 

 and remove the spleen from the body. Joining the median por- 

 tion of the liver with the anterior end of the intestine is a large 

 tubular vessel, the bile duct, alongside which are the hepatic 

 artery and the portal vein; they must not be cut. Note the 

 wide mesentery which joins the stomach and the anterior end 

 of the intestine with the dorsal body wall; also the mesentery 

 which joins the two limbs of the V-shaped stomach. 



Lift up the stomach and intestine and observe the whitish 

 pancreas; carefully note its shape and position. Find the pan- 

 creatic duct ; this is a small tube which leaves the pancreas near 

 its anterior end and enters the ventral wall of the intestine, which 

 it traverses a short distance. 



