208 COMPARATIVE ANATOMY CHAP. 



The posterior aorta leaves the ventricle posteriorly and runs along the lower side 

 of the hind-gut. It soon divides into two large lateral arteries, the posterior pallial 

 arteries. The principal branches of the anterior and posterior pallial arteries run 

 along the free edge of the mantle on each side and then unite, forming together the 

 arteries of the pallial edge. From the roots of the posterior pallial artery smaller 

 arteries spring, which supply with blood the hind-gut, the pericardium, the posterior 

 adductor, the retractors of the siphons, etc. The venous blood is collected out of 

 the lacunar system of the body through converging channels into one longitudinal 

 venous sinus ; this lies under the pericardium (Fig. 172). 



From this sinus, the greater part of the blood flows through the complicated 

 system of venous canals in the kidneys, after which it is collected on each side into 

 a branchial artery which runs along the base of the gills, and thence enters the 

 two branchial lamellae. It becomes arterial through respiration in the gills, flows as 

 arterial blood into a branchial vein parallel with the branchial artery, and thence 

 into the auricle. 



I 'art of the venous blood, however, passes by direct channels out of the venous 

 sinus into the branchial artery (passing by the kidneys), and part even flows direct 

 into the pericardium. In this way some venous blood comes to be mixed with the 

 arterial blood flowing through the heart from the gills. 



Not all Lamellibranchia have an anterior and a posterior aorta springing out 

 of the heart. In the lower groups of the Protobranchia and Filibranchia there are 

 numerous forms (Nuculu, *SWon>//*////, Anumia, Mytilidcc] in which only one anterior 

 aorta leaves the ventricle ; this soon, however, gives off the arteria visceralis, which 

 supplies blood to those parts which, in other LameUibntncMa, are fed by the aorta 

 posterior. In their possession of a single aorta rising from the ventricle, the above 

 lower Lamellibranchiates agree with Chiton and the Gastropoda. The rise of this 

 aorta from the posterior end of the ventricle in the Prosobranchia and in most 

 I'ti/nionala is a secondarily acquired arrangement, caused by the shifting forward 

 of the pallial complex. 



It must further be noted that in a very specialised bivalve, Teredo, the posterior 

 aorta fuses with the anterior, and thus the two leave the heart as one vessel. 



In those Lamellibranchiates which have siphons, a muscular and contractile 

 widening occurs in the posterior aorta near the point where it leaves the ventricle ; 

 this is called the bulbus arteriosus. Its special function is perhaps that of bringing 

 about pressure of blood, to assist in the extension of the siphons. The backward 

 flow of the blood into the ventricle in the contraction of the bulbus arteriosus 

 (systole) is prevented by a linguiform valve which projects from its anterior wall. 



.". Cephalopoda. 



Heart (Figs. 127, 168, pp. 147, 199, and 174). We must here again point out the 

 important fact that Nautilus has a heart with four auricles, while the Decapoda and 

 Octopoda a heart with only two auricles. This difference is connected with the 

 difference in the number of the ctenidia : four in Nautilus (Tetrabranchia], two 

 in the Decapoda and Octopoda (Dibranchia}. 



In Nautilus, the heart is an almost square sac drawn out to two points on each 

 side ; the four auricles which open into the four points of the ventricle are long 

 tubes, more like widened branchial veins than auricles. 



The strongly muscular ventricle of the Dibranchiii is almost always elongated 

 into a tube. In the Octopoda it lies transversely, the two auricles being in the same 

 plane with the ventricle. In the Ocgopsidce, the ventricle lies along the longitudinal 

 axis of the body, i.e. it is elongated dorso-ventrally, and the auricles are at right 



