SURVEY OF VERTEBRATES 117 



So much for the Fishes — a group that is of such great economic 

 importance that the governments of progressive countries spend 

 vast sums for their study, protection, and propagation. The im- 

 portant food and game Fishes are almost without exception repre- 

 sentatives of the higher Bony Fishes — relatively modern forms 

 that did not exist during the Age of Fishes, when Fishes were the 

 only Vertebrates, but appeared later during the Age of Reptiles. 

 (Fig. 232.) 



B. Amphibians 



The members of the class Amphibia, commonly represented 

 by the Frogs and Salamanders, made a great forward step in Verte- 

 brate evolution by adopting — if somewhat falteringly — the 

 land-habit. This opened up to them a vast environment closed to 

 Fishes and demanded lungs and supporting limbs. The limbs 

 apparently were derived from the paired fins of Fishes and 

 built on the plan that persists in all the higher Vertebrates. 

 (Figs. 102, 103.) 



True, most Amphibians are cold-blooded, slimy-skinned animals 

 that spend the early part of their life with fins, tail, and gills and 

 only substitute, or add, limbs and lungs when finally they emerge 

 on dry land. But they do make the change, and during this meta- 

 morphosis from the larval to the adult form they apparently re- 

 capitulate broadly their evolutionary history. 



Nearly all Amphibians return to water to breed, and many 

 spend the cold months in a dormant condition buried in mud at 

 the bottom of ponds and streams. During this hibernation period 

 the metabolic processes are greatly reduced, and the temperature 

 is little above the surroundings. However, Frogs cannot survive 

 being actually frozen although they may remain alive when em- 

 bedded in a solid block of ice. 



Those Amphibians that retain the tail throughout adult life 

 constitute the order Caudata, and those deprived of this struc- 

 ture during metamorphosis, the order Salientia. 



1. Salamanders and Newts 



The tailed Amphibians, or Caudata, such as the Salamanders 

 and Newts, though hatched as aquatic larvae, known as tadpoles, 

 undergo a relatively inconspicuous metamorphosis that varies 

 considerably in different species. Thus some retain their gills 



