xii. 1-2 AMPHIBIA 297 



drawn and the effects of gravity become insistent. At the same time 

 the skin must be changed to resist desiccation, the respiratory system 

 adapted to use gaseous oxygen, the receptors to signal the strange new 

 configurations of stimuli. It is not surprising that these new conditions 

 produced greater changes in vertebrate organization than had occurred 

 in tens of millions of years previously. Nevertheless, so slow is the 

 pace of evolution, the only known Devonian amphibia, and many of 

 the Carboniferous ones too, still looked and presumably behaved very 

 like fishes. Animals of this sort (e.g. *Eogyrinus) floundered about on 

 land for 30 million years or more before producing definitely terrestrial 

 types such as the Permian *Eryops. 



Of all the features that arose at this time in connexion with the new 

 life on land the presence of pentadactyl limbs is perhaps the most 

 conspicuous. It is appropriate that this should be marked in zoological 

 nomenclature: the amphibia are the first of the great group of land 

 vertebrates, the Tetrapoda. 



All existing amphibia have been much modified since their Devonian 

 ancestry, yet they retain many features that show how the transition 

 from water to land was produced. These modern forms are by 

 no means a precariously existing remnant but are quite numerous 

 and successful in the ecological niches that they occupy; they form 

 an important element in many food chains. There are some 2,000 

 species at present recognized, placed in 250 genera. However, con- 

 trasting this with the numerous species of teleosts, of birds, and of 

 mammals we shall see that the amphibians, though well adapted for 

 certain situations, do not succeed in maintaining themselves in many 

 different types of habitat. Broadly speaking they are unable to survive 

 for long except in the proximity of water. There are desert toads, such 

 as Chiroleptes of Australia, but these survive by burrowing and by 

 special abilities, such as the power to hold large amounts of water, 

 associated with loss of the glomeruli of the kidneys. 



Modern amphibia belong to three sharply separated subclasses. 

 Urodela (newts and salamanders) retain the original long-bodied, 

 partly fish-like form. The Anura (frogs and toads) have lost the tail 

 and become specialized as jumpers. The Apoda are limbless, blind, 

 burrowing animals found in the tropics. The urodeles and anurans 

 are found as fossils back to the Cretaceous and Trias respectively, but 

 we have only scanty information about their connexion with the 

 earlier amphibians, which are grouped loosely together as the Steno- 

 cephalia. These are found in rocks about 275-160 million years old, 

 that is to say from the late Devonian to the Trias (Fig. 211). 



