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ON THE GENERAL MORPHOLOGY OF THE MYXTNOID FISHES. 297 



and also that the latter cavity communicates with the body cavity on this, the right, 

 side by a large pericardio-peritoneal foramen. This is the only one present, the 

 pericardium being imperforate on the left side. There are, however, two foramina in a 

 the Ammocoete, but none in the adult Petromi/zov . In a 34-cm. Hag the pericardio- i^^^'Cr**" '/^^ 

 peritoneal foramen was an elongated aperture 9 mm. long when slightly stretched, and / f^^^'^JTe 

 was directed obliquely forwards from right to left. Its right wall is formed by the i_ c**"^ ' 

 serosa of the gut, and here the very large supra-intestinal or portal vein (fig. 4,^.1;., 

 and cp. also Part II., fig. 1), and in some cases even the portal heart {x>Ji.), projects 

 boldly into the foramen. It is usually stated that the portal vein passes through the 

 foramen on its way to the portal heart, but the vein lies morphologically outside the 

 foramen, and is in fact only accidentally related to it. On the other hand, the common 

 portal vein {c.p.v.) is neither apparently nor really associated with the foramen. The 

 left wall of the latter is formed by a special duplicature of the peritoneum having a 

 well-defined and slightly thickened free border. This structure, doubtless, corresponds 

 to the Selachian pericardio-peritoneal septum. The presence of such a large pericardio- 

 peritoneal foramen in Myxine is possibly correlated with the existence of an important 

 pericardial pronephros. 



A little behind the heart the intestine, which has so far been suspended by the 

 median dorsal mesentery, takes an upward turn, and therefore the two sheets of the 

 peritoneum, instead of becoming opposed to form the median mesentery, pass at once 

 separately on to the gut, and from the gut they immediately reach the anterior lobe of 

 the liver, instead of forming a median ventral hepatic ligament such as suspends the 

 posterior lobe of the liver to the intestine. It is from these lateral sheets of peritoneum 

 that the pericardium is derived. There is thus no discontinuity in the mesentery at the 

 anterior end of the body cavity as there is at the rectum, apart, of course, from the 

 pericardio-peritoneal foramen itself. 



When the intestine is displaced to one side, four large vessels are seen attached to 

 the body wall at the mid-dorsal line. The outermost pair are the segmental or 

 pronephric ducts ; the innermost, the two posterior cardinal veins. It will be noticed 

 that of the two cardinals the right is perceptibly smaller than the left, as in Bdellostoma 

 and also in preserved specimens the left is the more usually blocked with blood. The 

 dorsal aorta is hidden from view (except anteriorly) between, and dorsal to, the two 

 cardinals, and is, as a rule, only exposed by dissection (but this varies). The arteries 

 from the aorta to the alimentary canal and gonad (cp. Part II., fig. 1) are seen emero-ing 

 between the two cardinals and apparently from them. 



Almost at the mid-dorsal line of the whole length of the gut, and to the right of 

 the attachment of the dorsal mesentery, courses the very large portal vein (Part II. 

 fig. 1, p.v.). 



The anterior region of the body cavity, with the opening from the abdominal into 

 the pericardial coeloin is figured by Goodrich (24, p. 44) in Myxine glutiviosa. 



