TERRESTRIAL VERTEBRATES 



367 



change. Gills are still possessed by the larvae but these are lost during 

 metamorphosis. 



The third adaptation, which is connected with the development of 

 the lungs, involves changes in the circulatory system which result in 

 a double circulation, the blood being sent from the heart to the lungs 

 for aeration, returned again to the heart, and then sent out over the 

 body. This change includes a remodeling of the branchial circulation 

 possessed by fishes which is still retained by larval amphibians (Fig. 258). 

 Of the four pairs of branchial arches characteristic of the fish, the first 

 of each side becomes in lunged amphibians the basal portion of a common 

 carotid artery, which supplies the corresponding side of the head; the 



Propferygium 



Bony rays 



Horny rays 



Me sop f^rygiunfy-^^ 



Me ftrp ferygium 



"\\\\\\\v»- 



^ Three parts 



of pterygium 



Ulna 



Pectoral arch 



Humerus 



Radius 

 Radius Metapferygium 



Fig. 259. — -Diagrams to illustrate the theoretical change of a fin into a limb. A, 

 pectoral fin of Ceratodus, an extinct dipnoan, representing a primitive fin. B, pectoral fin 

 of Polypterus, a lobe-finned ganoid, or crossopterygian, now living. C, D, E, hypothetical 

 changes from A to B. F, pectoral fin of a pro-amphibian, a hypothetical form intermediate 

 between Polypterus and an amphibian. G. condition in amphibian. {A, B, F, and G from 

 Wiedersheim, " Vergleichenden Anatomie der Wirheltiere" ; C to E. from Kingsley, "Outlines 

 of Comparative Anatomy of Vertebrates," by the courtesy of P. Blakiston's Son & Co.) 



second becomes an aortic arch, through which the blood passes to the 

 trunk and tail; the third disappears; and the fourth, in part, becomes 

 the basal portion of the pulmonary artery leading to the lung. A 

 cutaneous branch of this fourth arch also becomes developed in an 

 amphibian when it breathes through the skin. The heart becomes three- 

 chambered, the single auricle of the fish being divided in the amphibian 

 into a right auricle which receives the blood from over the body and a 

 left auricle w^hich receives the blood returned from the lungs. The 

 ventricle remains single and serves to send out blood both to the systemic 

 and to the pulmonary circulation, but its walls become thrown into 

 folds which prevent a complete mixing of the arterial and venous blood. 

 A fourth change is incident to the use of the limbs to support the 

 body and to serve as locomotor appendages on land. Paddle-like 



