AMPHIBIA 183 



dently Amphibia that have developed certain dry-land adaptations. 

 Good examples of this transitional group are Cacops (Fig. 105, C), 

 Eryops (Fig. 105, F), and Cricotus (Fig. 105, D). 



The Permian was the period in which the amphibians passed their 

 climax. From that time on the Amphibia have lived a hard life in 

 competition with the Reptilia which are better adapted for land 

 life, and with the fishes which are better adapted for the waters. 

 Only a few rather small groups have survived and these largely 

 through their retiring habits and inconspicuous appearance. The 

 group of Anura has recently gained a secondary dominance through 

 a remarkable adaptive radiation into various land habitats. 



PRESENT-DAY AMPHIBIA 

 SUB-CLASS II. Liss AMPHIBIA &- 



The recent Amphibia are the Csecilia or Gymnophiona, newts and 

 salamanders, frogs and toads. The Amphibia are the least numerous 

 of the vertebrate classes, except the Cyclostomata. In all there are 

 only about 1,000 species (nearly 900 of which are frogs and toads). 

 This is to be compared with the nearly 10,000 species of birds, nearly 

 8,000 species of fishes, about 3,500 species of reptiles, and about 2,700 

 mammals. As a class the Amphibia have always been relatively 

 unimportant numerically, possibly because it is essentially an " in- 

 between" group, as has been shown. 



THE CHAKACTERS OF THE AMPHIBIA (AFTER GADOW) 



1. The vertebrae are (a) acentrous, Kj\ pseudocentrous, or (<^noto- 

 centrous. 



2. The skull articulates with the atlas by two condyles which are 

 " ^formed by the lateral occipitals (exoccipitals). 



3. There is an auditory columellar apparatus fitting into the fenestra 



ovalis. 



4. The limbs are of the tetrapodous, pentadactyle type. 



__ie red-blood corpuscles are nucleated, bjconyex, and ovaL^ 



6. The heart is (a) divided into two afna? (auricles) and one ventricle, 



and (b) it has a conus provided with valves. 



7. The aortic arches are strictly symmetrical.' 



8. Gills are present at least during some early stage of development 



9. The kidneys are provided with persistent nephrpstomes. 



