AMPHIBIA. 375 



amples. What forms have gills only? What evidence is there that the 

 environment has much to do with hastening or retarding the change from 

 gills to lungs? Give the natural history of the Mexican axolotl as far as 

 respiration is concerned. Are any amphibia hatched with lungs at the 

 outset? 



395. Circulation. In the gill-breathing larvae the circula- 

 tion is quite similar to that in fishes ( 378; Fig. 178). When 

 the gills are lost and lungs developed, the arterial arches (Fig. 

 167) which supply the gills change their course, or suffer de- 

 struction. This is an interesting instance of the modification 

 of old structures to meet new demands. Coupled with these 

 changes we find the separation of the auricle into two cham- 

 bers right and left. The veins from the lungs empty into 

 the left, and the systemic veins into the right auricle. While 

 there is only one ventricle into which both the pure blood from 

 the lungs and the venous blood from the system go, it is so 

 arranged that the venous blood is chiefly returned to the lungs 

 and the purest blood goes to the head and to the systemic 

 circulation. The venous circulation is modified in general 

 accordance with the changes in the heart and arteries. 



396. Supplementary Exercise. Compare the arterial vessels in the 

 adult frog with those in the fish and the tadpole stage of the frog, and 

 find what, in the opinion of the authors, is the fate of each of the arterial 

 arches. See Figs. 164-167. What are the most important differences in 

 the venous circulation in fishes and in adult amphibians ? 



397. Locomotion. In the lower amphibia, in which the 

 appendages are poorly or not at all developed, the muscles of 

 the body show the segmental arrangement seen in fishes, and 

 locomotion is effected by a serpentine or eel-like action of the 

 body. In the higher forms, especially the Anura, the limbs 

 are well developed ; and the body muscles lose something of the 

 regularity of their arrangement and become more as we find 

 them in the higher vertebrates. The Anura (frogs and toads) 

 are especially adapted to leaping and swimming by the great 

 muscular development of the hind legs. 



398. Exercise. Are there any special advantages in the leaping habit 

 of motion either in the capture of prey or in escape from enemies? Verify 



