xm CLASSIFICATION 219 



above, and are emphasised by placing the frogs and 

 toads in the order Amira, 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 

 possessing 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 larvje ; their skin is soft 

 and glandular, and the toes are in nearly all cases with- 

 out 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 con- 

 siderably 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 fishes, which 

 possess fins and gills, 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 fundamen- 

 tal 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 



