55 



The Systemic Circulation. 



The Heart. The heart is a lozenge-shaped , one-chambered , muscular sac , 

 which lies mesially in the coelom behind the intestine, below the stomach and 

 caecum, and above the nephridium. Five vessels cnmmunicate with it. They are: 

 the two large branchial veins which enter its rounded lateral angles; two large 

 median vessels, the anterior and posterior aortae , which leave the conical anterior 

 and posterior projections of the heart : and the small genital aorta which arises 

 from the ventral edge of the heart. The heart is not quite symmetrical : its 

 ventral portion and the anterior aorta which arises from it being displaced 

 toward the right by the intestine. The exterior of the heart is covered by the 

 peiitoneum or coelomic epithelium. Its interior is probably lined by an endothe- 

 lium and its wall is formed by interlacing muscle fibres between which the 

 bloud penetrates some distance. The venous blood is collected from the tissues 

 of the heart by two or three veins which are probably connected with capillaries. 

 The muscle fibres have the character of smooth muscle except that they exhibit 

 a faint striation. Marceau, in his beautiful work upon the structure of the heart 

 of mollu.scs , shows that the muscle fibres of the heart of Cephalopods are long, 

 cylindrical and branched. The branches of each fibre anastimose with those of 

 other fibres forming a complex mesh. The fibres , like those of the mantle , 

 consist of a sheath of flbiillae separated by sarcoplasm and a core of cytoplasm 

 containing the nucleus, but the flbrillae of the cardiac muscles are parallel to the 

 fibres and are not homogeneous since the striation is transverse, not oblique and 

 is due to the differentiation of the fibrillae into the structures which in trans- 

 verse rows form the light (isotropic) band divided by Krause's membrane and 

 the dark (anisotropic) band divided by Heksen's line. The heart has four valves: a 

 pair of semilunar flaps at the end of each branchial vein prevent the return of 

 blood into the gill : a single semilunar valve at the origin of each of the large 

 aortae checks the re-fiow of blood to the heart. 



The Anterior Aorta. The anterior aorta arises just inside the visceral wall 

 a little in front of the base of the gill. At first it hes in the right nephridial 

 wall , then having been joined by the oesophagus , it pierces the liver obliquely 

 between the hepatic ducts and emerges in a shallow groove on the upper 

 surface of the liver. The aorta with the oesophagus passes forward in this 

 groove until it ends near the skull in two terminal branches. Near its 

 origin the aorta gives off an artery (1') which divides into four branches and 



