242 ELEMENTARY ZOOLOGY 



The cavity thus exposed is the pericardium. Note 

 within the pericardium a long tube extending through it. 

 This is a portion of the alimentary canal, the rectum, 

 which opens posteriorly through the anus into the supra- 

 branchial chamber. Note a muscular sac about the 

 rectum midway of its course through the pericardium. 

 This is the unpaired ventricle of the heart. Attached to 

 each side of the ventricle are thin-walled sacs, the right 

 and left auricles, which are entered by fine blood-vessels, 

 the efferent branchial veins, from the right and left gills. 

 The blood brought through these blood-vessels from the 

 gills flows into the auricles and from them into the un- 

 paired muscular ventricle, from which it is forced anteriorly 

 and posteriorly through two main arteries, the anterior 

 and posterior aortas, to all parts of the body. After 

 bathing the body-tissues the blood is collected into a 

 median longitudinal vein beneath the pericardium called 

 the vena cava. From the vena cava the blood passes 

 through the kidneys and gills to be returned at last to the 

 heart. The mantle acts as an organ for the aeration of 

 the blood, and the blood it receives or at least part of it 

 passes directly back to the heart without passing through 

 the kidneys and gills. 



Note the delicate membranous dark-colored sac on the 

 floor of the pericardium, the kidneys or nepJiridia. These 

 are paired structures which appear as two U-shaped tubes 

 lying side by side. Each consists of a lower portion with 

 thick folded walls, the kidney proper, and an upper thin- 

 walled portion, the ureter. The kidneys open internally 

 through a pair of reno-pericardial openings into the peri- 

 cardium, while the ureters communicate with the mantle- 

 cavity by an opening on the side of the body beneath the 

 gills as already mentioned. The kidneys are profusely 

 supplied with fine blood-vessels and carry off the waste 

 matter from the blood. 



