VEBTEBRATA. 
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 CercUodus from Australia, Pro - 
topterus from Africa, and Lejpidosiren 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 
(Fig. 174). They commence as water-breathing larvae, 
when they resemble Fishes in their respiration, circula¬ 
tion, and locomotion. In the lowest forms, the gills are 
retained through life; but all others have, when mature, 
lungs only, the gills disappearing. The cuticle is frequent¬ 
ly shed, the mode varying with the habits of the species. 161 
The common Frog, the type of this class, stands interme¬ 
diate between the two extremes of the vertebrate series; 
no fundamental part is excessively developed. 
There are about sev¬ 
en hundred living spe¬ 
cies, grouped in four 
orders: 
1. Proteida have a 
naked skin, a tail, and 
two or four limbs. 
Some retain their gills 
,, , n ,i Fig. 294.— Head and Gills of Xecturus. Cayuga 
through hie, as the Lake. 
