68 VERTEBRATE ZOOLOGY 



arteriosus, the enlarged anterior portion of the truncus which is 

 the beginning of the ventral aorta. The muscular walls of the 

 bulbus exert a steady pressure on the blood passing through it, so 

 that the latter goes forward with a steady flow instead of by beats. 

 At its anterior end the bulbus continues forward as the ventral 

 aorta. This vessel soon splits into two pairs of arteries which carry 

 the blood to the gills ; the posterior pair goes to the second and 

 third pairs of gills, and the anterior pair to the first pair of gills. 



Exercise 5. Draw an outline of the animal, and in it draw outlines 

 of the internal organs as they lie in the abdominal and pericardial 

 cavities ; carefully label all. 



The Digestive System. This is composed of the mouth, phar- 

 ynx, oesophagus, stomach, small intestine, liver, pancreas, large 

 intestine, and cloaca. 



Observe the great length of the stomach. Its anterior or cardiac 

 end, and the short oesophagus, which joins it with the pharynx, 

 are concealed by the heart. Cut out the heart. The posterior or 

 pyloric end of the stomach is marked by a constriction and is 

 situated near the posterior end of the liver. 



The intestine extends from the stomach to the cloaca. Two 

 regions may be distinguished, the small intestine and the large 

 intestine. The former makes up the greater part of the intes- 

 tine; its anterior end, which may be called the duodenum, bends 

 forward so as to lie parallel with the stomach for a short dis- 

 tance, and in the angle thus formed lies an irregularly folded 

 organ, the pancreas. The large intestine is somewhat thicker 

 than the small intestine, and joins it with the cloaca at the hinder 

 end of the abdominal cavity. 



Extend the intestine as much as possible without cutting 

 the mesentery which joins the entire digestive tract with the 

 dorsal body wall. Observe the numerous veins which lie in this 

 mesentery ; they belong to the hepatic portal system and carry 

 blood from the wall of the intestine to the liver. The intestinal 

 veins will be seen running from the wall of the intestine to the 

 large mesenteric vein, a longitudinal vessel which extends from 

 the hinder part of the abdominal cavity to the liver. 



