146 MOLLUSKS. 



and the inner layers, connecting the arms of the V. 

 Observe also that the inner layer of the outer gill is 

 continuous with the outer layer of the inner gill. 



Having already observed that the water passes in at 

 the branchial and out at the cloacal siphon, give now an 

 explanation of the course it takes, and of the manner in 

 which it is propelled onward in its course. 



5. The blood vessels. Turn the free lobe of the mantle 

 down again, and loosen the dorsal portion from both 

 valves. Then pull downward gently on the free lobe so 

 as to bring the dorsal aspect of the body better into view. 

 The dorsal portion of the mantle is very thin and trans- 

 parent, and through it may be seen, in the part that was 

 directly beneath the hinge, an oval or elongated trans- 

 parent vessel regularly but slowly pulsating. This is the 

 ventricle. Pinch up the mantle with forceps, and cut it 

 away from this region, and the parts will be better seen. 

 The blood comes to the ventricle from a pair of auricles 

 located one on either side. Each auricle lies directly 

 between the gills and the ventricle, and is somewhat coni- 

 cal in form, with the apex of the cone toward the ventri- 

 cle. A branchial vein brings the blood from the gills to 

 the lower end of the auricle. The ventricle is emptied 

 principally through a pedal artery which curves forward 

 and downward toward the foot. 



Observe that the dark-colored intestine runs directly 

 through the ventricle, like a draught pipe through a loco- 

 motive boiler, there being no open communication between 

 the two vessels. 



Observe the pulsations of the heart. 1 First the auricle 



1 A very satisfactory demonstration of the action of the heart may be 

 made by cutting away with bone snips the portion of the shell which 

 immediately surrounds the hinge, and covers the heart, leaving the ani- 

 mal intact within. A thin-shelled specimen should be selected for this. 

 If kept under water, or, better, under f-per-cent salt solution, the pulsa- 

 tions may be observed for a long time. 



