90 THE PRESHWATEE MUSSEL 



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. 



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 giUs : 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. 



5. The course of the circulation. 



The ventricle receives arterial blood from the 

 auricles, and drives it through the arteries all over 

 the body. The blood from the foot and viscera is re- 

 turned to the vena cava, from which it passes through 

 the kidneys to the gills. Prom the gills it is returned 

 by the efferent branchial veins to the auricles. The 

 blood from the mantle-lobes is returned direct to 

 the auricles, and does not pass through either the 

 kidneys or the gills. 



The circulation is in great part carried on through 

 irregular channels or lacunae. 



