440 CHORDATA. 
in some, only in the larval stages in others. The paired 
limbs are pentadactyle. Lungs are present in the adult, 
and the nasal sacs have internal nares through which air is 
supplied to the lungs. The heart is three-cchambered and 
there is a postcaval vein replacing the cardinals. The 
skeleton is partly cartilaginous. There is always a cloaca. 
The eggs are fertilised externally and there is usually a 
metamorphosis. 
The order Anwura includes the frogs and toads, with no 
tail and with no gills in the adult. The Urode/a, such as the 
newts and salamanders, retain their tail and aquatic habits 
throughout life ; whilst others, such as Proteus (a blind form 
found in the subterranean caves of Austria), retain also their 
gills. Hence the gilled Urodela, the Urodela which lose 
their gills and the Aura form a complete series illustrating 
the changes from an aquatic to a terrestrial life. 
There is also a small third order of Gymnophiona with 
no limbs. 
Cxiass IV.—REPTILIA. 
The Reptilia are a class of animals very definitely marked 
off by structural features at the present day, but the fossil 
forms show a gradation into Amphibia and Mammalia; and 
some of these even exhibit characters approximating to those 
of birds. During the secondary epoch, especially in the 
Trias, the reptilian was the dominant vertebrate type, 
and, as such, exhibited as wide adaptive modification as 
do the dominant mammals of the present day. Large rep- 
tiles ruled the sea, the land and the air, and some attained 
an enormous size. Since then the efzi/ia have declined in 
numbers and in size, and only five comparatively small 
orders remain. 
These all differ from the Amphzbia in never at any time 
in their life possessing gills, fins, or lateral sense-organs, in 
having an embryonic development involving internal fer- 
tilisation and usually an oviparous habit. The embryo is 
enveloped in a foetal membrane called the amnion, and has 
also a large excretory and respiratory organ, the allantois. 
(These foetal membranes, as well as the yolk-sac already 
