78 PHYLUM MOLLUSCA 



ing into the pericardial cavity. Slit open the ureter and kidney- 

 proper in clean water and observe their inner structure. 



Exercise 8. Draw a diagram representing the pericardial cavity and 

 the kidney, showing the relation of the two structures to each 

 other. Draw the heart in the pericardial cavity, showing the rela- 

 tion of the auricle to the gills. 



The Digestive System. The stomach and intestine are embedded 

 in the visceral mass and are difficult to dissect out of it. With 

 care and patience, however, it can be done. Find the mouth be- 

 tween the two pairs of palps ; note the upper and the lower lips, 

 which connect the upper and the lower pairs of palps respec- 

 tively. The mouth is seen to the greatest advantage in a specimen 

 which has been deprived of both valves of the shell. Trace the 

 rectum from the anus to the place of its entrance into the visceral 

 mass. Carefully remove with forceps and knife the tough white 

 integument which covers the left side of the visceral mass, taking 

 care not to disturb the organs beneath. The soft cream-colored 

 mass just above the foot is the reproductive gland ; the light- 

 greenish mass lying just above this is the liver. Embedded in these 

 masses lies the alimentary tract, a narrow, delicate tube, which 

 will be injured in the dissection unless the greatest care is taken. 

 Gently scrape away the soft mass which surrounds the alimen- 

 tary tract, laying it entirely bare. The water in the dissecting 

 pan must be frequently renewed to keep it clear, and great care 

 must be taken not to break the tract. The course of the whole 

 digestive tract is the following : The mouth opens into the short 

 oesophagus, after which the canal dilates to form the stomach. 

 The dark-colored liver surrounds the stomach. Back of the 

 stomach is the intestine, a narrow tube which runs backward and 

 downward to the hinder end of the visceral mass ; it then turns 

 upward and runs forward to a point above the stomach, where it 

 turns downward to the lower side of the visceral mass ; it then 

 bends dorsally again and runs to the point where it leaves the 

 visceral mass. Here the rectum begins and passes through the 

 heart and above the posterior adductor muscle to the anus in 

 the cloacal chamber. 



