I/O CIRCULATORY SYSTEM chap. 



As regards position, the heart is situated within the pericar- 

 dium, a separate chamber whicli 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 Opistho- 

 branchiata (whence their name), while in the Prosobranchiata 

 they are situated in front 0/ the ventricle. 



The number of auricles corresponds to the number of bran- 

 chiae. Thus there is only one auricle in the great majority of 

 Prosobranchiata (which are accordingly classified as Monotocar- 

 did)^ and also in the Opisthobranchiata, while the Pulmonata 

 have a single auricle corresponding to the pulmonary chamber. 

 There are two auricles in the Amphineura, in a small group of 

 Gasteropoda, 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 aotae, 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 subse- 

 quently 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 

 1 E.g. Kollmann, Zeit. iciss. Zool. xxvi. p. 87. 



