THE CIRCULATORY SYSTEM. 103 



3. The arteries. 



There are two main^arteries, one arising from 

 each end of the ventricle, which convey the aerated 

 blood from the heart to all parts of the body. Their 

 main branches may be followed by injecting them 

 from the ventricle. 



a. The anterior aorta is a continuation of the anterior 



end of the ventricle : it runs forwards along the 

 dorsal surface of the rectum, enters the visceral 

 mass, and divides into visceral and pedal 

 arteries, which supply the anterior two-thirds 

 of the animal. 



b. The posterior aorta arises from the posterior end 



of the ventricle, and runs backwards beneath the 

 rectum: it divides into right and left branches, 

 which supply the hinder part of the body. 



4. The veins, except the largest ones, cannot be dissected 



without injecting them, and even then they are 

 difficult to follow. They may, however, be identified 

 in sections of hardened specimens (p. 112). 



a. The vena cava is a median longitudinal vessel lying 



beneath the floor of the pericardium : it receives, 

 in front, large veins returning blood from the 

 foot and visceral mass ; and behind, much 

 smaller vessels from the posterior part of the 

 body. 



b. The afferent branchial veins are a pair of large 



vessels, running along the lines of attachment of 

 the outer and inner gills to each other. Vessels 

 arise from them which carry blood to the gills. 



c. The efferent branchial veins run along the bases 



of the outer lamellae of the outer gills : they 

 return to the auricles the blood from the gills, 

 and also receive at their anterior and posterior 

 ends large vessels returning aerated blood from 

 the mantle-lobes. 



