280 CLASSIFICATION OF VERTEBRATES. 



are typically pentadactyl, with primitively a simple carpus and 

 tarsus. In the anura there is a fusion of ulna and radius, while 

 in the hind foot the proximal elements of the tarsus (astragalus 

 and calcaneum) become greatly elongate. 



In the heart, which, except in the gymnophiona, is far ante- 

 rior, there is always a single ventricle. In the perennibranchs 

 and lungless salamanders the auricles are incompletely sepa- 

 rated, but in the other amphibia two distinct auricles occur. 

 The right auricle receives venous blood, while, when the lungs 

 are functional, the left receives arterial blood. In the lungless 

 forms the pulmonary vein is absent. In the gymnophiona two 

 rows of valves occur in the conus, but elsewhere this region is 

 reduced to a single circle of semilunar valves. The bulbus is 

 well developed, and in the anura contains a longitudinal valve 

 which, by changes in position, directs the first blood to leave the 

 ventricle (arterial blood) into the carotids and the general cir- 

 culation, while the venous blood which follows it is sent into the 

 pulmonary artery, and thence to the lungs. 



Four pairs of aortic arches appear in the later larvae, the 

 blood at first passing through them to the gills, and thence to 

 the dorsal aorta. With the metamorphosis the branchial circu- 



FiG. 280. Diagram of venous circulation in an amphibian, av, anterior 

 abdominal vein; cJ, caudal vein; cv, posterior cardinal veins; k, hepatic veins; ht, 

 heart ; ?', interrenal vein ; iv, iliac vein ; /, jugular vein ; <, mesonephros ; /, portal 

 vein ; pc, postcava ; s, subclavian vein. 



lation is lost ; but in the urodeles all four arches persist, the 

 first supplying the carotids, the second and third forming the 

 radices aortae, while the fourth go to the lungs. In the gymno- 

 phiona and anura the third of these disappears. In the venous 

 system the most marked feature is the appearance of a hepatic- 

 portal system (p. 192) lacking in the other ichthyopsida. 



The pronephros is a transitory organ. It is confined to two 



