ZOOLOGY 



lungs in the adult, respiration taking place wholly through the 

 body surfaces. The frog breathes through its nostrils. The 

 mouth cavity can be increased by muscular action, thus allowing 

 the entrance of air. The nasal openings are then closed by 

 flaps and the air is forced by muscular action into the lungs. 



401. Supplementary Exercises for the Library. Find as many different types 

 of respiration as possible among the amphibians, and cite examples. 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? 



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

 tion is quite similar to that in fishes (385; Fig. 181). When 

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

 169) 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 chambers 

 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. 



403. 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 various authors, is the fate of each of the arterial arches. See Figs. 

 166-169. What are the most important differences in the venous circulation in 

 fishes and in adult amphibians? 



404. 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 muscles that move the 



