440 CHORD AT A. 



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-chambered 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 Anura includes the frogs and toads, with no 

 tail and with no gills in the adult. The Urodela^ 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 Anura 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. 



Class 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 Reptilia have declined in 

 numbers and in size, and only five comparatively small 

 orders remain. 



These all differ from the Amphibia 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 



