VASCULAR SYSTEM 43 



tube in the lampreys. It is supported by a special cartilaginous 

 skeleton, elaborately developed in the Myxinoids (Figs. 30, 34). 



The alimentary canal passes backwards in a straight course to 

 the anus. The stomach is scarcely marked, and the long intestine 

 has a slightly spiral " valve " in the Petromyzontia. 



The liver is a large bilobed organ, provided with a gall-bladder. 

 A pancreas, on the contrary, is scarcely differentiated, being appar- 

 ently represented by small glandular tubes embedded in the liver. 



The vascular system has advanced far beyond the condition 

 found in the Cephalochorda, but still shows primitive characters 

 (Miiller [306], Goette [168], Klinckowstrom [267], Jackson [235], 

 Vialleton [474]). Although large, asymmetrical, and three- 

 chambered, the heart is not as completely twisted as in the higher 

 vertebrates (Figs. 32, 34). The sinus venosus, passing across the 

 pericardium from the dorsal to the ventral side, opens by a narrow 

 neck into the large thin-walled atrium lying on the left side. This 

 chamber opens into the more ventral ventricle by an aperture 

 protected by two valves. Two valves are also placed at the 

 entrance of the thick-walled ventricle into the swollen base of the 

 ventral aorta, lying outside the pericardium. 



Afferent vessels carry blood to the gills, and it is collected again 

 into efferent vessels, which join a longitudinal dorsal aorta begin- 

 ning very far forwards. Segmental somatic arteries are regularly 

 supplied to the myotomes from the dorsal aorta ; and a correspond- 

 ing series of somatic veins empty into the cardinals. There is no 

 renal-portal system. The kidneys are supplied with veins from the 

 posterior cardinals and with arteries from the aorta (Figs. 18, 32). 



In the early larva of Petromyzon we find paired anterior and 

 posterior cardinals joining to paired ductus Cuvieri, paired inferior 

 jugular veins outside the branchial basket, and a complete sub- 

 intestinal vein (Goette [168], Cori [93], Julin, Dohrn). Soon this 

 latter vein breaks up in the liver into the hepatic portal capillary 

 system, its anterior portion forming the hepatic veins. The 

 inferior jugulars are replaced by a median inferior jugular below 

 the ventral aorta. This vein, and a ventral hepatic vein, are 

 peculiar to the Cyclostomes (Fig. 18). The two anterior and also 

 the posterior cardinals join above the oesophagus to large trunks, 

 which open to the right into the dorsal limb of the sinus venosus. 

 The ductus Cuvieri on the left side thus disappears, and all the 

 venous blood pours into the heart on the right side. In the 

 Myxinoidea, strangely enough, it is the left ductus which persists, 

 and the right ductus which is suppressed (Figs. 32, 34). 



The pronephros nearly disappears in the adult Lamprey, but 

 persists as an organ of considerable size in Myxinoids (Fig. 27). 

 It is, however, degenerate (Weldon [483], Semon [400], Kirkaldy 

 ), consisting of a few. branching tubules, opening on the one 



