THE KIDNEYS. 105 



returned to the vena cava, from which it passes 

 through the kidneys to the gills. From 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. 



D. The Kidneys. 



The kidneys, or ' organs of Bojanus,' are paired, and lie 

 side by side just beneath the pericardium. 



Each kidney is a wide thin-walled tube, doubled on itself 

 so that its two ends are closely approximated. These ends 

 are anterior, and placed opposite the anterior end of the 

 pericardium, while the loop is posterior and lies against the 

 posterior adductor muscle. 



The two limbs of the loop are very different : the ventral 

 limb, or kidney proper, has spongy walls, and is lined through- 

 out by a darkly pigmented glandular epithelium, so that the 

 whole limb appears black. The dorsal limb, or ureter, is a 

 wide thin-walled tube which conveys the excretory products 

 forwards to the external opening. 



Dissect the right kidney as described below, examining 

 first the ureter, and then the glandular portion. 



1. The external opening of the ureter is on the side of the 

 body, between the two lamellae of the inner gill, and 

 a short distance in front of the point at which its 

 inner lamella becomes free from the body. 



Turn back the gills of the right side, and pass one blade 

 of a fine pair of scissors between the lamellce of the inner 

 gill, through the anterior end of the slit between the inner lamella 

 and the body. Cut forwards horizontally through the junction 

 of inner and outer gills parallel to their base, and about an eighth 

 of an inch above this. Separate the cut edges, and gently brush 

 the parts clean. 



