xiii CLASSIFICATION 219 



Anura, the newts and salamanders in the order 

 Urodela. 



The Urodela and Anura, although differing from one 

 another in many important respects, agree, e.g., in possess- 

 ing gills during part or the whole of their existence, and 

 in nearly always possessing lungs. They usually pass 

 through a metamorphosis, the young being hatched in the form 

 of gilled larvae; their skin is soft and glandular, and the toes 

 are in nearly all cases without claws. These and numerous 

 other structural characters separate them from reptiles, in 

 which gills are never developed, and the young do not pass 

 through a metamorphosis, while the skin is provided with 

 scales and the toes have claws. The differences here are 

 considerably more important than those between the orders 

 referred to above, and are expressed by placing the latter 

 in the class Amphibia, while reptiles constitute the class 

 Reptilia. In the same way the finned, water-breathing 

 fishes form the class Pisces, the feathered birds the class 

 Aves, and the hairy animals which suckle their young the 

 class Mammalia. 



Mammals, Birds, Reptiles, Amphibians, and Fishes all 

 agree with one another in the possession of red blood and 

 an internal skeleton, an important part of which in the 

 embryo is the notochord (p. 203), which is nearly always 

 replaced in the adult by a backbone or vertebral column ; 

 and in never having more than two pairs of limbs. They 

 thus differ in some of the most fundamental features of 

 their organisation from such animals as Crayfishes, Insects, 

 Scorpions, and Centipedes, which have colourless blood, 

 a jointed external skeleton, and numerous limbs. These 

 differences — far greater than those between classes — are 

 expressed by placing the back- boned animals in the phylum 

 or sub-kingdom Vertebrata, the many-legged armoured forms 



