120 INVERTEBRATE ZOOLOGY 



easily seen. Trace them as far as possible. Observe again 

 the mantle veins, which bring blood from the mantle to the 

 branchial hearts. 



The course of the blood is the following: it enters the bran- 

 chial hearts through the postcaval, precaval, and mantle veins ; 

 the contraction of these hearts sends it into the branchial 

 arteries which pass along the upper side of the gills ; it then 

 traverses the delicate transverse filaments of the gills and 

 becomes oxygenated, when it collects again in the branchial 

 veins on the opposite side of the gills ; through these it passes 

 to the systemic heart, whence it is sent through the anterior 

 and posterior aortas to the different parts of the body. 



Exercise 3. Make a diagrammatic drawing of the circulatory and 

 the respiratory systems. 



The digestive system. Remove the kidneys and precaval veins. 

 Beneath them will be seen a large glandular bilobed organ of 

 somewhat doubtful function, called the pancreas. At its forward 

 end a pair of cylindrical organs, the liver ducts, will be seen enter- 

 ing it from the liver. The pancreas is made up of anastomosing 

 glandular projections of the walls of these ducts. Remove the 

 gills, branchial hearts, systemic heart, and hinder arteries. The 

 delicate body-wall should be completely removed from the entire 

 visceral mass, and great care be taken not to injure the stomach 

 pouch beneath. This latter organ is a large bag with thin 

 transparent walls which extends to the extreme hinder end 

 of the body ; beneath it will be seen the large testis or ovary, 

 according to the sex of the animal. This pouch is not really a 

 part of the stomach, notwithstanding its name, but is a reser- 

 voir for the secretions of the liver, which communicates with it 

 through the liver ducts. Carefully loosen the stomach pouch 

 without separating it from the body and let it float in the water 

 of the dissecting pan. It communicates with the thick-walled 

 stomach, which lies just in front of it, but food substances are 



