130 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 
