52 PRACTICAL ZOOLOGY. 



these holes into the cavities within the gill; the 

 water from each compartment of the gill passes up 

 into the chamber leading to the anal siphon. 



Beginning at the upper edge of the anal siphon, in 

 the middle line, cut carefully forward just above the 

 intestine as far as the umbo. This lays bare the cav- 

 ity in which the heart lies, the pericardial cavity. 

 Carefully cut away the thin covering of this cavity 

 and make out the following parts : — 



a. The large yellowish ventricle in the anterior part 

 of the cavity; time its pulsations; observe that 

 the intestine runs directly through the ventricle, 

 though it has no more communication with the 

 ventricle than a stove pipe has with a room it 

 passes through ; an artery runs forward from the 

 ventricle along the upper surface of the intestine; 

 another artery runs from the ventricle backward 

 under the intestine. Again pull the mantle ven- 

 tral-ward to show b. 



b. A thin sac, triangular as seen from the side, with 

 its apex joining the ventricle, and its base 

 attached just above the upper edge of the gills ; 

 this is the left auricle. Each auricle receives 

 the blood from the gills of the corresponding side. 



11. Just in front of the posterior adductor muscle is the 

 dark kidney. 



12. Above the kidney, and in front of the posterior 

 adductor, js a small muscle, which extends backward 

 from the side of the body to join the valve near the 

 posterior adductor. This muscle pulls the foot back- 

 ward. 



