CLASS AMPHIBIA 491 



The blood is expelled from here through the conus arteriosus into the 

 ventral aorta. As will likely be remembered, the theoretical, primitive, 

 and embryonic typical number of aortic arches in vertebrates is six. 

 This number is modified in most adult vertebrates, usually by reduc- 

 tion. Even teleost fish have only four branchial arches. In sala- 

 manders this number may be referred to as four but considerable 

 modification has occurred. The first (anterior), second, and third 

 original arches have been rearranged and combined to form the com- 

 mon external, and internal carotid arteries. The fourth and fifth 

 supply the external gills with the fourth becoming the systemic arches 

 which meet dorsally to form the dorsal aorta. The sixth arch is 

 modified to supply a large pulmonary artery from each side to the 

 respective lung. The portion of the sixth aortic arch which continues 

 on dorsally to join the aorta, from the point where the pulmonary 

 branches off, is known as the duct of BotaUus. 



The special modification of the veins is centered around the devel- 

 opment of the post cava which is formed posteriorly by the junction 

 of urinogenital veins. The pair of posterior cardinals, which are 

 characteristic of fish, are retained also but are greatly reduced. They 

 usually join the post cava some distance anterior to the kidneys and 

 parallel the aorta to the heart where they enter the ducts of Cuvier, 

 one on each side. Another modification is the pelvic-ventral abdomi- 

 nal complex which connects the renal portal arrangement with the 

 hepatic portal. A pelvic vein branches from the femoral on each side 

 before it joins the renal portal. The two pelvic veins pass ventrally 

 to meet each other at the midventral point of the pelvis and this 

 union forms the ventral abdominal vein which either enters the liver 

 or a branch of the hepatic portal before it enters the liver, thus pro- 

 viding a cut-off in the course of the venous circulation in going 

 anteriorly from the posterior limbs. The lateral veins of sharks form 

 a similar cut-off but enter the duct of Cuvier instead of the hepatic 

 portal system.* 



Respiratory System and Breathing 



The respiration may be divided into cutaneous, performed through 

 the wet skin ; branchial through the gills ; and pidmonary through the 

 lungs. There are several parts to the latter arrangement. The small 

 external nares lead by way of passages to the slitlike internal nares 

 which open into the mouth between the posterior ends of the two 



♦Helpful Illustrations of the circulatoi-y system of Necturus may be found in 

 Stuart: Anatomy of Necturus maculosus, Denoyer-Geppert Co., Chicago. 



