334 COMPARATIVE ZOOLOGY OF CHORDATES 



reason accounts for the arrangement of the arterial arches in 

 the higher forms. In the air-breathing vertebrates, the gills 

 are reduced and there is a continuous vessel in each visceral 

 arch running from the truncus arteriosus (ventral aorta) to the 

 lateral dorsal aorta. In Salamandra all the vessels in the 3rd 

 to 6th visceral arches persist. The 3rd becomes the carotid, 

 the 4th and 5th become systemics, and the 6th is the pul- 

 monary. All these arterial arches place the truncus in com- 

 munication with the lateral dorsal aorta. The lateral dorsal 

 aortas are, however, interrupted between the dorsal ends of 

 the 3rd and the 4th arterial arches ; i.e. there is no ductus 

 caroticus. The conditions in Triton are similar except that 

 the 5th arterial arch has completely disappeared. In Lacerta 

 (as in all higher forms) the 3rd arch persists as the carotid, the 

 4th as the systemic, and the 6th as the pulmonary. In Lacerta, 

 the connexion between the dorsal ends of the arteries of the 

 3rd and 4th arches persists, forming the ductus caroticus. 

 The lateral dorsal aorta is here accordingly uninterrupted. 

 The ductus caroticus is absent in the adult of higher forms. 

 The connexion between the pulmonary arch and the lateral 

 dorsal aorta is the ductus arteriosus. This connexion is 

 important in the embryonic stages of Amniotes. It enables 

 the blood from the right side of the ventricle (or the right 

 ventricle, if it is separated off) to reach the lateral dorsal aorta 

 through the pulmonary arteries, instead of going to the lungs. 

 At these early stages of development the lungs are not yet 

 open. In the adult amniote, the ductus arteriosus usually 

 degenerates into a ligament, as, for example, in the mammal 

 (on the left side), or disappears. It persists, however, in some 

 turtles, and their case is interesting, for they are in the habit of 

 diving, and during the submerged period the lungs are not 

 working. The blood in the pulmonary artery can then escape 

 into the general circulation without going through the lungs. 

 The ductus arteriosus is also called the ductus Botalli. 



In the frog, there is neither ductus caroticus nor ductus 

 arteriosus in the adult. 



In the Sauropsidan reptiles, the right and left systemic 

 arteries of the 4th arch are separate right down to the base of 



