AMPHIBIA 177 



form that the air-bladder comes to be greatly in demand. In the on- 

 togeny of modern Amphibia we find this sequence repeated, for the 

 young amphibian is purely aquatic, and air-breathing comes only 

 when the larva metamorphoses into the young adult. 



2. Circulatioii. — The fish type of circulation is built primarily 

 along lines laid down by branchial respiration. The heart is purely 

 venous in its blood content and pmnps blood forward and through 

 the branchial arches. This involves as many pairs of branchial arches 

 as there are paired functional afferent vessels carrying blood to the 

 gills, and efferent vessels carrying the aerated blood from the 

 gills to the dorsal aorta. In the evolution from the fish to the am- 

 phibian type the principal changes in the circulation have to do with 

 the branchial arches, which cease to have a value as such, and their 

 profound remodeUing into blood vessels that fit into an air-breathing 

 physiology. The branchial vessels of lobe-finned ganoids and of lar- 

 val amphibians consist of four pairs; the first pair becomes the ca- 

 rotid arteries that supply the head, the second becomes the systemic 

 arches that supply most of the body, the third disappears, and the 

 fourth becomes mainly the pulmonary arches. It is of interest to note 

 that in all lung-breathing fishes the lungs are supplied from a branch 

 of the fourth branchial arch. In most Amphibia a branch of the 

 fourth arch becomes cutaneous, for the skin respiration is almost as 

 important as the pulmonary. The heart becomes three chambered, 

 the auricle dividing into a systemic half and a pulmonary half. The 

 single ventricle receives both arterial and venous blood, but there is 

 very little admixture of the two. 



3. Locomotion. — In fishes the chief locomotor organs are the tail 

 and the median fins. The paired fins are used largely for balancing; 

 in lobe-finned ganoids they are used to support the head when rest- 

 ing on the bottom. Naturally then we should expect the most radi- 

 cal change to concern the loss of tail fins and other median fins, on 

 the one hand, and the development of feet, on the other. The median 

 and caudal fins appear in the amphibian larvae and persist in a re- 

 duced form in certain persistently aquatic amphibia, which are prob- 

 ably no more than permanent larvae (psedogenetic). When the fins 

 do appear they are mere soft folds of the skin without any true skel- 

 etal supports, and they never become regionally specialized, but re- 

 main in the ancient diphycercal form. In the land salamanders the 

 tail-fin is lost but the tail persists. In the Anura, as well as in the 



