AMPHIBIANS. 5 



Amphibians. The Amphibians, such as frogs and newts, 

 were once regarded e.g. by Cuvier as naked Reptiles, 

 but a more accurate classification has linked them rather to 

 the Fishes. Thus Huxley grouped Birds and Reptiles 

 together as Sauropsida ; Amphibians and Fishes together as 

 Ichthyopsida for reasons which will be afterwards stated. 

 Amphibians mark the transition from aquatic life, habitual 



FlG. 5. Salamander, an Amphibian. 



among Fishes, to terrestrial life, habitual among Reptiles \ 

 for while almost all Amphibians have gills in their youth 

 at least all the adults have lungs, and some retain the gills 

 as well. In having limbs which are fingered and toed, and 

 thus very different from fins,' they resemble Reptiles. But 

 the two fcetal membranes characteristic of the embryonic life 

 of higher Vertebrates are not present in Amphibian embryos, 

 and the general absence of an exoskeleton in modern forms 

 is noteworthy. 



Fishes. The members of this class are as markedly 

 adapted to life in the water as birds to life in the air. The 

 very muscular posterior region of the body usually forms 



FlG. 6. Queensland dipnoan (Ceratodus}. 



the locomotor organ, and we say that a fish swims by 

 bending and straightening its tail. The limbs have the 

 form of paired fins that is, they are limbs without digits. 

 There are also unpaired median fins supported by fin rays. 

 All have permanent gills borne by bony or gristly arches, 



