CHAPTER XXI 



SUB-PHYLUM IV. CRANIATA 



DIVISION II. GNATHOSTOMATA 



SUB-DIVISION I. ANAMNIA 



Class II. AMPHIBIA 



THE class Amphibia includes the familiar Frogs and Toads, the 

 less known Newts and Salamanders, and some very 

 curious worm-like tropical forms which burrow in 

 the earth. The name means double life (Gr. d/i^f, 

 double ; fitos, manner of living), and refers to the fact that all the 

 typical members of the class commence their lives as fish-like larvae, 

 breathing by gills, and afterwards become converted into land 

 animals, breathing by lungs. This strongly marked larval type of 

 development is one of the great distinctions between the Amphibia 

 and the only other class of Vertebrata with which they could be 

 confounded, viz., the Reptiles. In the Reptiles, as in the Birds, a 

 large egg abundantly provided with nutritive material is produced, 

 and the young animal practically completes its development within 

 the egg-shell and is born in a condition differing from the adult 

 chiefly in size. 



It might at first sight be thought that the fact that Amphibia 

 breathe air in their later life and live on land would be sufficient to 

 mark them off from the fish. But we have already seen that the 

 older orders of Bony Fish use their air-bladders as lungs to assist 

 in respiration, and on the other hand some Amphibia retain gills 

 throughout life and rarely if ever leave the water. 



The unbridged gap between true fish and Amphibia is to be 

 found not in the breathing organ but in the structure of the limb. 

 Fish possess fins median and paired which are in both cases 

 supported by horny rays, as well as an internal skeleton ; and the 

 paired fins have an internal skeleton which has the form of a jointed 

 axis bearing similar rays on one or both sides (Figs. 224 and 249). 



