PHYSIOLOGY OF THE INVERTEBRATA. 203 



ings are each provided with a pair of valves, which project 

 into the ventricle, and there meet in front of the openings. 

 These valves allow the blood to pass from the auricle to the 

 ventricle, but prevent its return from the ventricle to the 

 auricle. By the contraction of the auricles the blood is 

 forced into the ventricle. After the contraction of the 

 auricles have ceased, the ventricle contracts and forces the 

 blood forwards and backwards through the two aortse. The 

 blood passes through the ramifications of these vessels into 

 the system of lacunaa or sinuses situated in .the mantle and 

 between the viscera. From these lacunse the blood passes 



Fig. 45. — Circulatory System of Helix. 



a = heart. i = vessels carrying blood from lung to heart, c = lung. 

 d = aorta, e = gastric artery, f — " hepatic " artery. g = pedal 

 artery. h = abdominal cavity, supplying the place of a venous sinus. 

 i = irregular canal communicating with /«, and canying blood to lung. 



into a large median venous sinus termed the vena cava, 

 which extends between the anterior and posterior adductor 

 muscles. At the base of the branchiae are two lateral sinuses. 

 The main portion of the blood passes into the renal organ 

 (the organ of Bojanus) and ultimately to the branchiee, and 

 from thence is returned as arterial blood to the auricnlar 

 portion of the heart. 



(2) The Gasteropoda. — In Helix the .heart (Fig. 45) is 



