AMPHIBIA 5^(^ 



= Triton) in the British Isles, and the salamander [Salamandra) 

 of Europe, and the mud-puppy (Nectiiriis) of North America 

 are commonly dissected. On land they walk on their limbs, in 

 water they swim in a fish-like manner, most of the propulsion 

 presumably coming from the compressed tail, but while some 

 species are mainly terrestrial, others are entirely aquatic. The 

 Anura are a highly aberrant sub-class. The tail is lost, the 

 hind limbs are elongated, and there are many changes, mainly 

 in the direction of loss or reduction, in the skeleton. The frog is 

 an example, and there are also to be found in Great Britain a 

 few individuals of the edible frog, Rana esciilenta (mainly in 

 East Anglia) and two species of toad, Bufo hufo, the common 

 toad, and B. calamita, the natterjack. Both Anura and Urodela 

 have skins which are unprotected by scales. The members of the 

 last sub-class, the Gymnophiona (=Apoda), have lost tail, girdles 

 and limbs, but have small scales in the dermis. The embryos are 

 not free-living. Most of the adults are terrestrial and burrowing ; 

 there are no British species. 



AMNIOTA 



The remaining three classes of vertebrates are all primarily 

 land-living, and all air-breathing whatever their habitat. Their 

 embryos use as a respiratory and excretory organ the allantois, 

 an outgrowth from the hind gut apparently homologous with the 

 cloacal bladder of Amphibia. There is also another outgrowth 

 from the embryo, a double fold of tissue called the amnion, 

 which is presumably protective. The kidney is a metanephros. 

 They are collectively called Amniota. 



CLASS V — R E P T I L I A 



The reptiles can only be characterised as amniotes which are 

 not mammals or birds. Their skeleton and their circulatory system 

 show various degrees of development towards the avian or 

 mammalian condition ; thus the ventricle of the heart shows a 

 complete range of subdivision from the lizard, in which it is 

 single as in the frog, to the crocodile in which it is completely 

 divided by a septum except for a small foramen. Both systemic 

 arches are present, but one is usually reduced. A complete scaly 

 covering is almost universal, and all reptiles are cold-blooded. 

 They are divided into a number of main groups by the structure 



