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 
Fic. 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 foetal 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 
Fic. 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. 
