170 CIRCULATORY SYSTEM chap. 



As regards position, the heart is situated within the pericardium, 

 a separate chamber which in the Pelecypoda, Cephalopoda, and 

 the bilaterally symmetrical Gasteropoda lies on the median line, 

 while in the asymmetrical Gasteropoda it is on one or other of the 

 sides of the body, usually the right. The veins connected with 

 the branchiae, and consequently the auricle into which they open, 

 are situated behind the ventricle in the Opisthobranchiata (whence 

 their name), while in the Prosobranchiata they are situated in 

 front of the ventricle. 



The number of aiiricles corresponds to the number of branchiae. 

 Thus there is only one auricle in the great majority of Proso- 

 branchiata (which are accordingly classified as Monotocardia), 

 and also in the Opisthobranchiata, while the Pulmonata have a 

 single am'icle corresponding to the pulmonary chamber. There 

 are two auricles in the Amphineura, in a small group of Gastero- 

 poda, hence known as Diotocardia, in all Pelecypoda, and in the 

 Dibranchiate Cephalopoda. In the Tetrabranchiate Cephalopoda 

 alone there are four auricles corresponding to the four branchiae. 



A single aorta occurs only in the Amphineura and in the 

 Tetrabranchiate Cephalopoda. In all the other groups there are 

 two aortae, leading out of the anterior and posterior ends of the 

 ventricle in Pelecypoda and Dibranchiate Cephalopoda, while a 

 single aorta leads out of the posterior end alone, and subsequently 

 bifurcates, in most of the Gasteropoda. One aorta, the cephalic, 

 supplies the front part of the body, the oesophagus, stomach, 

 mantle, etc. ; the other, the visceral aorta, supplies the posterior 

 part, the liver and sexual organs. 



The general circulatory system in the Mollusca has not yet 

 been thoroughly investigated. As a general rule, the blood driven 

 from the ventricle through the aorta into the arteries, passes, on 

 reaching the alimentary canal and other adjacent organs, into a 

 number of irregular spaces called lacunae. These in their turn 

 branch into sinuses, or narrow tubes covered with muscular tissue, 

 which penetrate the body in every direction. In the Dibranchiate 

 Cephalopoda true capillaries are said to occur, which in some cases 

 form a direct communication between the arteries and veins. 

 According to some authorities ^ capillaries and veins exist in 

 certain Pelecypoda in connexion with the intestinal lacunae, but 

 this again is regarded by others as not established. A similar 

 ^ E.g. Kollmann, Zeit. toiss. Zool. xxvi. p. 87. 



