VERTEBRATA. 
317 
long, ribbon-like pectoral and ventral fins, set far apart; 
two auricles, and one ventricle; and, besides gills, a cellu¬ 
lar air-bladder, which is used as a lung. 
The representatives are Ceratodus from Australia, Pro- 
topterus from Africa, and Lepidosiren from Brazil. 
Class II. —Amphibia. 
These cold-blooded Vertebrates are distinguished by 
having gills when young, and true lungs when adult. 
They have no fin-rays, and the limbs, when present, have 
the same divisions as those of higher animals. The skin 
is soft, and generally naked, and the skeleton is ossified. 
The skull is flat, and articulates with the spinal column 
by two condyles. There is no distinct neck; and the ribs 
are usually small or wanting. The heart consists of two 
auricles and one ventricle. All undergo metamorphosis 
upon leaving the egg, passing through the “ tadpole” state. 
They commence as water-breathing larvse, when they re¬ 
semble Fishes in their respiration, circulation, and locomo¬ 
tion. In the lowest forms, the gills are retained through 
life; but all others have, when mature, lungs only, the 
gills disappearing. The cuticle is frequently shed, the 
mode varying with the habits of the species. 161 The com¬ 
mon Frog, the type of this class, stands intermediate be¬ 
tween the two extremes of the vertebrate series; no fun¬ 
damental part is excessively developed. 
There are about four 
hundred and fifty liv¬ 
ing species, grouped 
in four orders: 
1. Urodela have a 
naked skin, a tail, and 
two or four limbs. 
Some retain their gills 
. . ..» ,, Fig. 294.— Head and Gills of Menobranchus. Cayuga 
through life, as the Lake. 
