x. 6 



CIRCULATION 



277 



there is a more elaborate arrangement. The second and third gill- 

 arches bear no lamellae and their afferent and efferent branchial vessels 

 are directly continuous, so that blood flows from the ventral to the 

 dorsal aorta and carotids. The pulmonary artery springs from the 

 dorsal aorta. Blood returns in a special pulmonary vein to the partly 



Fig. 167. Branchial circulation of A Neoceratodus and b Protopterus. 



abr. anterior efferent vessel; ac. anterior carotid, a/ 3 " 6 , four afferent vessels (corresponding 

 to the original arches 4-6) ; ah. afferent hyoidean ; c. conus ; cl. coeliac artery ; da. dorsal aorta ; 

 dl. dorso-lateral aorta; eb-~*. second and fourth epibranchial arteries; eg. external gills; 

 eh. efferent hyoidean; ht. heart; hyp. median hypobranchial; L. air-bladder; la. lingual 

 artery; mes. mesenteric artery; pa. pulmonary artery; pc. posterior carotid; pv. pulmonary 

 vein; 5. position of closed spiracle; va. ventral artery; vx. vena cava inferior; I-V five 

 branchial slits. 



The gills are represented on the hyoid and next four branchial arches. (From Goodrich.) 



separated left side of the sinus venosus. The auricle is partly divided 

 into two and the ventricle is almost completely divided by a ridge 

 and a series of muscular trabeculae. The ventral aorta is shortened 

 into a spirally twisted truncus arteriosus, provided with a system of 

 valves such that the blood from the left side of the auricle is directed 

 mostly into the first two branchial arches, that from the right side 

 into the last two. In this way some separation of pulmonary and 

 systemic circulations is achieved. Indeed, although there is every 

 reason to believe that the mechanism has been in existence for nearly 

 300 million years, it shows us most clearly a possible intermediate 

 stage between aquatic and pulmonary respiration. It seems likely that 

 the earliest amphibia employed a similar system. There is a coronary 

 artery arising from the anterior efferent branchial arches (Fig. 167). 



